AI Trajectory

null

{
  "EnableConfigs": [
    "IO_URING"
  ],
  "FocusSymbols": [
    "zerocopy_fill_skb_from_iter",
    "__zerocopy_sg_from_iter",
    "zerocopy_sg_from_iter",
    "skb_zerocopy_iter_stream",
    "io_sendmsg_zc"
  ],
  "KMSANReasoning": "The patch simply changes the error code returned by `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem` from `-EFAULT` to `-EMSGSIZE` when there is a mismatch in fragment readability. This change in error code propagation does not introduce any uninitialized memory allocations, accesses, or kernel-to-user info-leaks. Any potential bugs related to this change would likely be logical errors or state inconsistencies, which are not specific to KMSAN's detection capabilities. Therefore, a dedicated KMSAN fuzzing session is not necessary.",
  "NeedsKMSAN": false,
  "Reasoning": "The patch changes the return value of zerocopy_fill_skb_from_iter and zerocopy_fill_skb_from_devmem from -EFAULT to -EMSGSIZE when skb_frags_readable(skb) is false. This is a functional change in the core networking datagram handling, specifically affecting zerocopy operations (e.g., MSG_ZEROCOPY) and io_uring zerocopy sends. The change alters the error code returned to userspace and handled by the kernel, which is worth fuzzing to ensure it doesn't introduce unexpected behavior or regressions in error handling paths.",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit 240149f79aa011b0389e301448d24afa4f773aa9\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Mon Aug 10 23:34:34 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/net/core/datagram.c b/net/core/datagram.c\nindex 173b5d97bd409..6f8ffd61bcab4 100644\n--- a/net/core/datagram.c\n+++ b/net/core/datagram.c\n@@ -638,7 +638,7 @@ int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\n \tint frag = skb_shinfo(skb)-\u003enr_frags;\n \n \tif (!skb_frags_readable(skb))\n-\t\treturn -EFAULT;\n+\t\treturn -EMSGSIZE;\n \n \twhile (length \u0026\u0026 iov_iter_count(from)) {\n \t\tstruct page *head, *last_head = NULL;\n@@ -713,7 +713,7 @@ zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,\n \tstruct net_iov *niov;\n \n \tif (i \u0026\u0026 skb_frags_readable(skb))\n-\t\treturn -EFAULT;\n+\t\treturn -EMSGSIZE;\n \n \t/* Devmem filling works by taking an IOVEC from the user where the\n \t * iov_addrs are interpreted as an offset in bytes into the dma-buf to\n"
}

best-expensive

null

{
  "EnableConfigs": [
    "IO_URING"
  ],
  "FocusSymbols": [
    "zerocopy_fill_skb_from_iter",
    "__zerocopy_sg_from_iter",
    "zerocopy_sg_from_iter",
    "skb_zerocopy_iter_stream",
    "io_sendmsg_zc"
  ],
  "Reasoning": "The patch changes the return value of zerocopy_fill_skb_from_iter and zerocopy_fill_skb_from_devmem from -EFAULT to -EMSGSIZE when skb_frags_readable(skb) is false. This is a functional change in the core networking datagram handling, specifically affecting zerocopy operations (e.g., MSG_ZEROCOPY) and io_uring zerocopy sends. The change alters the error code returned to userspace and handled by the kernel, which is worth fuzzing to ensure it doesn't introduce unexpected behavior or regressions in error handling paths.",
  "WorthFuzzing": true
}

You are an expert Linux kernel maintainer.
Your job is to review a provided patch series and determine
if it makes functional changes to the kernel that should be fuzzed.

IMPORTANT: The changes have ALREADY been applied and committed as the HEAD commit in
your workspace. Do NOT rely on your internal knowledge of the kernel. You must actively
use your code access tools to examine the actual source code and confirm any assumptions.

Return WorthFuzzing=false if the patch only contains:
- Modifications to Documentation/, Kconfig files, or code comments.
- Purely decorative changes, such as logging (e.g., pr_err, printk) or tracepoints.
- Changes to numeric constants or macros that do not functionally alter execution flow.
- Code paths that are impossible to reach in virtualized environments like GCE or QEMU,
  even when utilizing software-emulated hardware (e.g., usb gadget, mac80211_hwsim).
- Code in vendor-specific PCIe switch, SmartNIC, or GPU drivers (e.g., mlxsw, pds_core, qed,
  ionic, amdgpu) that require physical PCIe hardware cards not emulated in standard QEMU.
- Driver .remove, .shutdown, or pci_unregister_driver teardown callbacks (e.g., igb_remove)
  that are executed only during PCI hot-unplug or sysfs driver unbind operations.

If it modifies reachable core kernel logic, drivers, or architectures, use your code search
tools to verify the code can be executed, then return WorthFuzzing=true.

When returning WorthFuzzing=true, you MUST ALSO:
1. Extract any specific kernel functions that should be heavily fuzzed into FocusSymbols.
   Avoid listing generic hot-path functions to prevent skewed test distributions.
   Prefer non-static, non-inlined API entrypoint functions over internal static helper functions
   (which are inlined by the compiler and do not have distinct symbol addresses).
2. Identify any specific CONFIG_ options required to properly test this new/modified feature.
   Go and look into the Kconfig files and check for ifdefs around the code, do not make assumptions.
   Also check "depends on" lines in Kconfig to include any non-standard parent subsystem configs
   needed for Kbuild to compile the code statically into vmlinux. List them in the EnableConfigs
   output array, and DO NOT add a 'CONFIG_' prefix (e.g., return "NET_IPV4" instead of "CONFIG_NET_IPV4").
Prefer calling several tools at the same time to save round-trips.


Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.

For your convenience, here is the diff of the changes:
commit 240149f79aa011b0389e301448d24afa4f773aa9
Author: syz-cluster <triage@syzkaller.com>
Date:   Mon Aug 10 23:34:34 2026 +0000

    syz-cluster: applied patch under review

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 173b5d97bd409..6f8ffd61bcab4 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -638,7 +638,7 @@ int zerocopy_fill_skb_from_iter(struct sk_buff *skb,
 	int frag = skb_shinfo(skb)->nr_frags;
 
 	if (!skb_frags_readable(skb))
-		return -EFAULT;
+		return -EMSGSIZE;
 
 	while (length && iov_iter_count(from)) {
 		struct page *head, *last_head = NULL;
@@ -713,7 +713,7 @@ zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,
 	struct net_iov *niov;
 
 	if (i && skb_frags_readable(skb))
-		return -EFAULT;
+		return -EMSGSIZE;
 
 	/* Devmem filling works by taking an IOVEC from the user where the
 	 * iov_addrs are interpreted as an offset in bytes into the dma-buf to

gemini-3.1-pro-preview

null

null

							input: 2123
							output: 53
							thoughts: 133
						

Okay, here's my understanding, playing the role of an expert:

**Analyzing a Datagram Patch**

Alright, I'm looking at a patch for `net/core/datagram.c`. It looks like we're changing the error handling within `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem`. Specifically, when `skb_frags_readable(skb)` isn't true, or when `i && skb_frags_readable(skb)` is true, the patch modifies the error return value. Instead of returning `-EFAULT`, the code will now return `-EMSGSIZE`.

The key here is understanding *why* this change is being made. `-EFAULT` usually indicates a generic "bad address" or "memory access error". `-EMSGSIZE`, on the other hand, suggests that the message size is too big for the buffer, or a related size issue. This means the existing logic might not have the correct handling when fragments of the SKB aren't readable.

Therefore, the next step is to examine the specific contexts where these functions are used, and the implications of this change. Why are we changing the error code? Is it because the problem is related to message size, or some other size issue? I need to analyze how the calling functions handle `-EFAULT` versus how they'd handle `-EMSGSIZE` to determine the impact of this change. I need to understand whether this is a correct fix, or whether it might introduce regressions elsewhere. This change looks potentially significant.

{
  "Expression": "zerocopy_fill_skb_from_iter"
}

{
  "Output": "include/linux/skbuff.h=1763=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\ninclude/linux/skbuff.h-1767-\ninclude/linux/skbuff.h:1768:int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\ninclude/linux/skbuff.h-1769-\t\t\t\tstruct iov_iter *from, size_t length);\n--\nio_uring/net.c=1451=static int io_sg_from_iter_iovec(struct sk_buff *skb,\n--\nio_uring/net.c-1454-\tskb_zcopy_downgrade_managed(skb);\nio_uring/net.c:1455:\treturn zerocopy_fill_skb_from_iter(skb, from, length);\nio_uring/net.c-1456-}\n--\nio_uring/net.c=1458=static int io_sg_from_iter(struct sk_buff *skb,\n--\nio_uring/net.c-1470-\telse if (unlikely(!skb_zcopy_managed(skb)))\nio_uring/net.c:1471:\t\treturn zerocopy_fill_skb_from_iter(skb, from, length);\nio_uring/net.c-1472-\n--\nnet/core/datagram.c=633=EXPORT_SYMBOL(skb_copy_datagram_from_iter_full);\nnet/core/datagram.c-634-\nnet/core/datagram.c:635:int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\nnet/core/datagram.c-636-\t\t\t\tstruct iov_iter *from, size_t length)\n--\nnet/core/datagram.c=749=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\nnet/core/datagram.c-762-\telse\nnet/core/datagram.c:763:\t\tret = zerocopy_fill_skb_from_iter(skb, from, length);\nnet/core/datagram.c-764-\n"
}

{
  "Expression": "zerocopy_fill_skb_from_devmem"
}

{
  "Output": "net/core/datagram.c=706=static int\nnet/core/datagram.c:707:zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,\nnet/core/datagram.c-708-\t\t\t      int length,\n--\nnet/core/datagram.c=749=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\nnet/core/datagram.c-760-\telse if (binding)\nnet/core/datagram.c:761:\t\tret = zerocopy_fill_skb_from_devmem(skb, from, length, binding);\nnet/core/datagram.c-762-\telse\n"
}

gemini-3.1-pro-preview

null

null

							input: 3448
							output: 23
							thoughts: 139
						

Okay, here's my take on those changes:

**Analyzing the Zero-Copy Patch**

Alright, so the core of this patch seems to be a subtle but potentially impactful change in error reporting. The functions `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem` are now returning `-EMSGSIZE` instead of `-EFAULT`. Functionally, this is a change, and that immediately piques my interest. I need to understand why this alteration was deemed necessary.

Now, let's trace this through. We know these two functions are called from `__zerocopy_sg_from_iter`, and that in turn is called from `io_sg_from_iter_iovec` / `io_sg_from_iter` and potentially other functions. The callstack is critical here, but it looks like `__zerocopy_sg_from_iter` is also called by `zerocopy_sg_from_iter`. I need to start at the top, so let's check `zerocopy_sg_from_iter` – that's the logical starting point. I'll need to see how that function uses the error codes to properly assess the scope of the potential impact. It's time to dig in and determine what situation warrants this code change.

{
  "Expression": "zerocopy_sg_from_iter"
}

{
  "Output": "drivers/net/tap.c=572=static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,\n--\ndrivers/net/tap.c-627-\tif (zerocopy)\ndrivers/net/tap.c:628:\t\terr = zerocopy_sg_from_iter(skb, from);\ndrivers/net/tap.c-629-\telse\n--\ndrivers/net/tun.c=1774=static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,\n--\ndrivers/net/tun.c-1867-\t\t\t * If zerocopy is enabled, then this layout will be\ndrivers/net/tun.c:1868:\t\t\t * overwritten by zerocopy_sg_from_iter().\ndrivers/net/tun.c-1869-\t\t\t */\n--\ndrivers/net/tun.c-1883-\t\tif (zerocopy)\ndrivers/net/tun.c:1884:\t\t\terr = zerocopy_sg_from_iter(skb, from);\ndrivers/net/tun.c-1885-\t\telse\n--\ninclude/linux/skbuff.h=1761=struct net_devmem_dmabuf_binding;\ninclude/linux/skbuff.h-1762-\ninclude/linux/skbuff.h:1763:int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\ninclude/linux/skbuff.h-1764-\t\t\t    struct sk_buff *skb, struct iov_iter *from,\n--\ninclude/linux/skbuff.h=1771=static inline int skb_zerocopy_iter_dgram(struct sk_buff *skb,\n--\ninclude/linux/skbuff.h-1773-{\ninclude/linux/skbuff.h:1774:\treturn __zerocopy_sg_from_iter(msg, skb-\u003esk, skb, \u0026msg-\u003emsg_iter, len,\ninclude/linux/skbuff.h-1775-\t\t\t\t       NULL);\n--\ninclude/linux/skbuff.h=4252=int skb_copy_datagram_from_iter_full(struct sk_buff *skb, int offset,\ninclude/linux/skbuff.h-4253-\t\t\t\t     struct iov_iter *from, int len);\ninclude/linux/skbuff.h:4254:int zerocopy_sg_from_iter(struct sk_buff *skb, struct iov_iter *frm);\ninclude/linux/skbuff.h-4255-void skb_free_datagram(struct sock *sk, struct sk_buff *skb);\n--\ninclude/net/dropreason-core.h=138=enum skb_drop_reason {\n--\ninclude/net/dropreason-core.h-412-\t * @SKB_DROP_REASON_SKB_UCOPY_FAULT: failed to copy data from user space,\ninclude/net/dropreason-core.h:413:\t * e.g., via zerocopy_sg_from_iter() or skb_orphan_frags_rx()\ninclude/net/dropreason-core.h-414-\t */\n--\nnet/core/datagram.c=707=zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,\n--\nnet/core/datagram.c-748-\nnet/core/datagram.c:749:int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\nnet/core/datagram.c-750-\t\t\t    struct sk_buff *skb, struct iov_iter *from,\n--\nnet/core/datagram.c-774-}\nnet/core/datagram.c:775:EXPORT_SYMBOL(__zerocopy_sg_from_iter);\nnet/core/datagram.c-776-\nnet/core/datagram.c-777-/**\nnet/core/datagram.c:778: *\tzerocopy_sg_from_iter - Build a zerocopy datagram from an iov_iter\nnet/core/datagram.c-779- *\t@skb: buffer to copy\n--\nnet/core/datagram.c-786- */\nnet/core/datagram.c:787:int zerocopy_sg_from_iter(struct sk_buff *skb, struct iov_iter *from)\nnet/core/datagram.c-788-{\n--\nnet/core/datagram.c-794-\nnet/core/datagram.c:795:\treturn __zerocopy_sg_from_iter(NULL, NULL, skb, from, ~0U, NULL);\nnet/core/datagram.c-796-}\nnet/core/datagram.c:797:EXPORT_SYMBOL(zerocopy_sg_from_iter);\nnet/core/datagram.c-798-\n--\nnet/core/skbuff.c=1912=int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,\n--\nnet/core/skbuff.c-1933-\nnet/core/skbuff.c:1934:\terr = __zerocopy_sg_from_iter(msg, sk, skb, \u0026msg-\u003emsg_iter, len,\nnet/core/skbuff.c-1935-\t\t\t\t      binding);\n--\nnet/vmw_vsock/virtio_transport_common.c=80=static int virtio_transport_fill_skb(struct sk_buff *skb,\n--\nnet/vmw_vsock/virtio_transport_common.c-87-\tif (zcopy)\nnet/vmw_vsock/virtio_transport_common.c:88:\t\treturn __zerocopy_sg_from_iter(msg, NULL, skb,\nnet/vmw_vsock/virtio_transport_common.c-89-\t\t\t\t\t       \u0026msg-\u003emsg_iter, len, NULL);\n--\nnet/vmw_vsock/virtio_transport_common.c=212=static struct sk_buff *virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,\n--\nnet/vmw_vsock/virtio_transport_common.c-239-\t/* If 'vsk' != NULL then payload is always present, so we\nnet/vmw_vsock/virtio_transport_common.c:240:\t * will never call '__zerocopy_sg_from_iter()' below without\nnet/vmw_vsock/virtio_transport_common.c-241-\t * setting skb owner in 'skb_set_owner_w()'. The only case\n--\nnet/vmw_vsock/virtio_transport_common.c-246-\nnet/vmw_vsock/virtio_transport_common.c:247:\t/* Set owner here, because '__zerocopy_sg_from_iter()' uses\nnet/vmw_vsock/virtio_transport_common.c-248-\t * owner of skb without check to update 'sk_wmem_alloc'.\n"
}

gemini-3.1-pro-preview

null

null

							input: 5729
							output: 25
							thoughts: 202
						

**Tracing the Zero-Copy Data Path**

Okay, so I'm mapping out the zero-copy data flow here. It looks like `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem` are the core functions getting called, but they're being invoked by `__zerocopy_sg_from_iter` and `io_sg_from_iter` / `io_sg_from_iter_iovec`.  `__zerocopy_sg_from_iter` is the critical upstream function, it's what ties everything together. I see it's called from `skb_zerocopy_iter_dgram`, `zerocopy_sg_from_iter`, `skb_zerocopy_iter_stream`, and even `virtio_transport_fill_skb`.

Now, `zerocopy_sg_from_iter` itself is being used by `tap_get_user` and `tun_get_user`... interesting, those are both related to network interfaces. The `skb_zerocopy_iter_dgram` function is the one that's really intriguing me.  I know it must be involved in datagram-based communication, and I'm pretty sure it's involved in `udp_sendmsg` and `raw_sendmsg`, although I should confirm that latter point. Better double-check the code there, just to be absolutely certain.

{
  "Expression": "skb_zerocopy_iter_dgram"
}

{
  "Output": "include/linux/skbuff.h=1768=int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\n--\ninclude/linux/skbuff.h-1770-\ninclude/linux/skbuff.h:1771:static inline int skb_zerocopy_iter_dgram(struct sk_buff *skb,\ninclude/linux/skbuff.h-1772-\t\t\t\t\t  struct msghdr *msg, int len)\n--\nnet/ipv4/ip_output.c=949=static int __ip_append_data(struct sock *sk,\n--\nnet/ipv4/ip_output.c-1265-\t\t} else {\nnet/ipv4/ip_output.c:1266:\t\t\terr = skb_zerocopy_iter_dgram(skb, from, copy);\nnet/ipv4/ip_output.c-1267-\t\t\tif (err \u003c 0)\n--\nnet/ipv6/ip6_output.c=1449=static int __ip6_append_data(struct sock *sk,\n--\nnet/ipv6/ip6_output.c-1831-\t\t} else {\nnet/ipv6/ip6_output.c:1832:\t\t\terr = skb_zerocopy_iter_dgram(skb, from, copy);\nnet/ipv6/ip6_output.c-1833-\t\t\tif (err \u003c 0)\n"
}

gemini-3.1-pro-preview

null

null

							input: 6653
							output: 21
							thoughts: 129
						

**Analyzing a Kernel Change**

Okay, so `skb_zerocopy_iter_dgram` is definitely called from `__ip_append_data` and `__ip6_append_data`. That means this is core kernel logic we're dealing with, directly related to UDP and Raw sockets using `MSG_ZEROCOPY`. This is significant because it's in the critical path.

The change itself is the error code – switching from `-EFAULT` to `-EMSGSIZE`. Functionally, that's a pretty big deal. Different error codes are handled differently at the userspace and kernel level. It means the behavior of applications or the kernel itself could change depending on how it handles this specific error. Given its centrality and the change in error codes, this is absolutely something that warrants fuzzing. We need to explore potential side effects and unexpected behaviors.

Now, let's take a look at how this interacts with `io_sg_from_iter` and `io_sg_from_iter_iovec`. Understanding the surrounding context will provide further insights into the change's impact and inform our fuzzing strategy.

{
  "Expression": "io_sg_from_iter"
}

{
  "Output": "io_uring/net.c=112=struct io_recvzc {\n--\nio_uring/net.c-118-\nio_uring/net.c:119:static int io_sg_from_iter_iovec(struct sk_buff *skb,\nio_uring/net.c-120-\t\t\t\t struct iov_iter *from, size_t length);\nio_uring/net.c:121:static int io_sg_from_iter(struct sk_buff *skb,\nio_uring/net.c-122-\t\t\t   struct iov_iter *from, size_t length);\n--\nio_uring/net.c=1378=int io_send_zc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)\n--\nio_uring/net.c-1443-\tif (!(zc-\u003eflags \u0026 IORING_RECVSEND_FIXED_BUF)) {\nio_uring/net.c:1444:\t\tiomsg-\u003emsg.sg_from_iter = io_sg_from_iter_iovec;\nio_uring/net.c-1445-\t\treturn io_notif_account_mem(zc-\u003enotif, iomsg-\u003emsg.msg_iter.count);\nio_uring/net.c-1446-\t}\nio_uring/net.c:1447:\tiomsg-\u003emsg.sg_from_iter = io_sg_from_iter;\nio_uring/net.c-1448-\treturn 0;\n--\nio_uring/net.c-1450-\nio_uring/net.c:1451:static int io_sg_from_iter_iovec(struct sk_buff *skb,\nio_uring/net.c-1452-\t\t\t\t struct iov_iter *from, size_t length)\n--\nio_uring/net.c-1457-\nio_uring/net.c:1458:static int io_sg_from_iter(struct sk_buff *skb,\nio_uring/net.c-1459-\t\t\t   struct iov_iter *from, size_t length)\n"
}

gemini-3.1-pro-preview

null

null

							input: 7597
							output: 26
							thoughts: 213
						

**Focusing on Zero-Copy in io_uring sendmsg**

Okay, so I see that `io_sg_from_iter` and `io_sg_from_iter_iovec` are key for io_uring's `sendmsg` functionality when used with `MSG_ZEROCOPY`. The real meat of the changes must be within `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem`. Let me map out the relevant functions to dig into.

Alright, these seem to be the primary functions I need to focus on: `zerocopy_fill_skb_from_iter`, `zerocopy_fill_skb_from_devmem`, then the internals are `__zerocopy_sg_from_iter`, `zerocopy_sg_from_iter`, `skb_zerocopy_iter_dgram`, and `skb_zerocopy_iter_stream`.

Let's start by diving into `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem` as these are probably the highest level changes in the zero-copy datapath, and I'll find them within `net/core/datagram.c`.

{
  "Expression": "zerocopy_fill_skb_from_iter"
}

{
  "Output": "include/linux/skbuff.h=1763=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\ninclude/linux/skbuff.h-1767-\ninclude/linux/skbuff.h:1768:int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\ninclude/linux/skbuff.h-1769-\t\t\t\tstruct iov_iter *from, size_t length);\n--\nio_uring/net.c=1451=static int io_sg_from_iter_iovec(struct sk_buff *skb,\n--\nio_uring/net.c-1454-\tskb_zcopy_downgrade_managed(skb);\nio_uring/net.c:1455:\treturn zerocopy_fill_skb_from_iter(skb, from, length);\nio_uring/net.c-1456-}\n--\nio_uring/net.c=1458=static int io_sg_from_iter(struct sk_buff *skb,\n--\nio_uring/net.c-1470-\telse if (unlikely(!skb_zcopy_managed(skb)))\nio_uring/net.c:1471:\t\treturn zerocopy_fill_skb_from_iter(skb, from, length);\nio_uring/net.c-1472-\n--\nnet/core/datagram.c=633=EXPORT_SYMBOL(skb_copy_datagram_from_iter_full);\nnet/core/datagram.c-634-\nnet/core/datagram.c:635:int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\nnet/core/datagram.c-636-\t\t\t\tstruct iov_iter *from, size_t length)\n--\nnet/core/datagram.c=749=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\nnet/core/datagram.c-762-\telse\nnet/core/datagram.c:763:\t\tret = zerocopy_fill_skb_from_iter(skb, from, length);\nnet/core/datagram.c-764-\n"
}

gemini-3.1-pro-preview

null

null

							input: 8713
							output: 36
							thoughts: 31
						

Here's my take on the thought process, as requested:

**Investigating `zerocopy_fill_skb_from_iter` Export Status**

Okay, so `zerocopy_fill_skb_from_iter` seems like a pretty interesting function, and the name suggests some optimization potential for zero-copy operations. The question now is: is it actually exported, and therefore available for use outside of its own module? I need to quickly determine its visibility. My first instinct, given its datagram-related name, is to go digging through the kernel's network core. Specifically, I should check `net/core/datagram.c`. That's the obvious place to find out if it is an externally visible symbol or if it's restricted to internal use within that module. Let's get to the source!

{
  "File": "net/core/datagram.c",
  "FirstLine": 635,
  "LineCount": 10
}

{
  "Contents": " 635:\tint zerocopy_fill_skb_from_iter(struct sk_buff *skb,\n 636:\t\t\t\t\tstruct iov_iter *from, size_t length)\n 637:\t{\n 638:\t\tint frag = skb_shinfo(skb)-\u003enr_frags;\n 639:\t\n 640:\t\tif (!skb_frags_readable(skb))\n 641:\t\t\treturn -EMSGSIZE;\n 642:\t\n 643:\t\twhile (length \u0026\u0026 iov_iter_count(from)) {\n 644:\t\t\tstruct page *head, *last_head = NULL;\n"
}

gemini-3.1-pro-preview

null

null

							input: 9139
							output: 36
							thoughts: 4
						

Okay, here's the summary, expressed in the first person, assuming I'm an expert in whatever field is relevant, and designed to provide a "fulsome summary of the thought process":

**Initial Assessment: The Export Context**

Right, let's break this down. My immediate thought is: **Is it exported?** This is the crucial, foundational question. I need to know if we are even talking about something that's intended for, or susceptible to, being moved out of its original environment. This sets the stage for everything else. "Export" is a loaded term; it implies certain regulations, processes, potential costs, and, frankly, opportunities or issues with compliance. I need to understand the source and the potential destination. 

{
  "File": "net/core/datagram.c",
  "FirstLine": 695,
  "LineCount": 10
}

{
  "Contents": " 695:\t\t\t\t\tpage_ref_sub(last_head, refs);\n 696:\t\t\t\t\trefs = 0;\n 697:\t\t\t\t}\n 698:\t\t\t\tskb_fill_page_desc_noacc(skb, frag++, head, start, size);\n 699:\t\t\t}\n 700:\t\t\tif (refs)\n 701:\t\t\t\tpage_ref_sub(last_head, refs);\n 702:\t\t}\n 703:\t\treturn 0;\n 704:\t}\n"
}

gemini-3.1-pro-preview

null

null

							input: 9482
							output: 36
							thoughts: 47
						

Okay, let me think this through.

**Analyzing the Function's Scope**

Hmm, so it's *not* exported, that's clear. But just because it's not exported doesn't automatically mean it's static. Okay, let's back up a moment. If it's *not* static, then it's accessible globally, which suggests it is a global symbol.

Now, let's examine the function in question: `zerocopy_fill_skb_from_devmem`. Wait, I thought I remember reading that function as declared `static`. So, going back, it is `static` and thus local to the compilation unit.

{
  "File": "net/core/datagram.c",
  "FirstLine": 705,
  "LineCount": 10
}

{
  "Contents": " 705:\t\n 706:\tstatic int\n 707:\tzerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,\n 708:\t\t\t\t      int length,\n 709:\t\t\t\t      struct net_devmem_dmabuf_binding *binding)\n 710:\t{\n 711:\t\tint i = skb_shinfo(skb)-\u003enr_frags;\n 712:\t\tsize_t virt_addr, size, off;\n 713:\t\tstruct net_iov *niov;\n 714:\t\n"
}

gemini-3.1-pro-preview

null

null

							input: 9882
							output: 24
							thoughts: 165
						

**Focusing on Zero-Copy Data Transfer Functions**

Okay, so I'm thinking about the key functions involved in zero-copy data transfer, and I believe that `zerocopy_fill_skb_from_iter` is a great starting point for further investigation. And, yes, `__zerocopy_sg_from_iter` is definitely on the radar as a good focus point. Also, I'm thinking about including `zerocopy_sg_from_iter` to this list as well. So, let me add those to my mental notes, `zerocopy_fill_skb_from_iter`, `__zerocopy_sg_from_iter`, and `zerocopy_sg_from_iter`.

Hmm, a quick note, I should keep in mind that `io_sg_from_iter` and `io_sg_from_iter_iovec` are *static*. And `skb_zerocopy_iter_dgram` is a static inline function. And, to keep my bearings, `skb_zerocopy_iter_stream` *isn't* static.

{
  "Expression": "skb_zerocopy_iter_stream"
}

{
  "Output": "include/linux/skbuff.h=1771=static inline int skb_zerocopy_iter_dgram(struct sk_buff *skb,\n--\ninclude/linux/skbuff.h-1777-\ninclude/linux/skbuff.h:1778:int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,\ninclude/linux/skbuff.h-1779-\t\t\t     struct msghdr *msg, int len,\n--\nnet/core/skbuff.c=1910=EXPORT_SYMBOL_GPL(msg_zerocopy_ubuf_ops);\nnet/core/skbuff.c-1911-\nnet/core/skbuff.c:1912:int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,\nnet/core/skbuff.c-1913-\t\t\t     struct msghdr *msg, int len,\n--\nnet/core/skbuff.c-1949-}\nnet/core/skbuff.c:1950:EXPORT_SYMBOL_GPL(skb_zerocopy_iter_stream);\nnet/core/skbuff.c-1951-\n--\nnet/ipv4/tcp.c=1116=int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)\n--\nnet/ipv4/tcp.c-1338-\nnet/ipv4/tcp.c:1339:\t\t\terr = skb_zerocopy_iter_stream(sk, skb, msg, copy, uarg,\nnet/ipv4/tcp.c-1340-\t\t\t\t\t\t       binding);\n"
}

gemini-3.1-pro-preview

null

null

							input: 10751
							output: 26
							thoughts: 54
						

Okay, let's see. This `skb_zerocopy_iter_stream` looks like a promising avenue to explore. I need to understand this better.

**Zero-Copy and Configuration Requirements**

Alright, I'll need to check the configuration options associated with this particular method, or related methods. The standard `MSG_ZEROCOPY` feature is well-established, but it's likely there are more granular configurations or dependencies. Time to dig into the kernel configs and ensure that whatever support is required is enabled and available. Let me see which specific knobs and levers I can turn.

{
  "Expression": "zerocopy_fill_skb_from_iter"
}

{
  "Output": "include/linux/skbuff.h=1763=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\ninclude/linux/skbuff.h-1767-\ninclude/linux/skbuff.h:1768:int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\ninclude/linux/skbuff.h-1769-\t\t\t\tstruct iov_iter *from, size_t length);\n--\nio_uring/net.c=1451=static int io_sg_from_iter_iovec(struct sk_buff *skb,\n--\nio_uring/net.c-1454-\tskb_zcopy_downgrade_managed(skb);\nio_uring/net.c:1455:\treturn zerocopy_fill_skb_from_iter(skb, from, length);\nio_uring/net.c-1456-}\n--\nio_uring/net.c=1458=static int io_sg_from_iter(struct sk_buff *skb,\n--\nio_uring/net.c-1470-\telse if (unlikely(!skb_zcopy_managed(skb)))\nio_uring/net.c:1471:\t\treturn zerocopy_fill_skb_from_iter(skb, from, length);\nio_uring/net.c-1472-\n--\nnet/core/datagram.c=633=EXPORT_SYMBOL(skb_copy_datagram_from_iter_full);\nnet/core/datagram.c-634-\nnet/core/datagram.c:635:int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\nnet/core/datagram.c-636-\t\t\t\tstruct iov_iter *from, size_t length)\n--\nnet/core/datagram.c=749=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\nnet/core/datagram.c-762-\telse\nnet/core/datagram.c:763:\t\tret = zerocopy_fill_skb_from_iter(skb, from, length);\nnet/core/datagram.c-764-\n"
}

gemini-3.1-pro-preview

null

null

							input: 11548
							output: 31
							thoughts: 55
						

**Analyzing the Networking Codebase**

Okay, so I'm diving into this networking code, and it looks like the relevant stuff is all centralized in `net/core/datagram.c`. That's a solid place to start since it's always included as long as networking is up. Considering the context, I think it's also worth investigating this in light of `IO_URING`, as it tends to leverage core datagram functionalities. Let's see, now I'll quickly check the `net/core/Makefile` to confirm the build dependencies and to ensure there aren't any sneaky surprises.

{
  "File": "net/core/Makefile",
  "FirstLine": 1,
  "LineCount": 20
}

{
  "Contents": "   1:\t# SPDX-License-Identifier: GPL-2.0\n   2:\t#\n   3:\t# Makefile for the Linux networking core.\n   4:\t#\n   5:\t\n   6:\tobj-y := sock.o skbuff.o datagram.o stream.o scm.o \\\n   7:\t\t gen_stats.o gen_estimator.o net_namespace.o secure_seq.o \\\n   8:\t\t flow_dissector.o\n   9:\t\n  10:\tobj-$(CONFIG_SYSCTL) += sysctl_net_core.o\n  11:\t\n  12:\tobj-y\t\t     += dev.o dev_api.o dev_addr_lists.o dst.o netevent.o \\\n  13:\t\t\t\tneighbour.o rtnetlink.o utils.o link_watch.o filter.o \\\n  14:\t\t\t\tsock_diag.o dev_ioctl.o tso.o sock_reuseport.o \\\n  15:\t\t\t\tfib_notifier.o xdp.o flow_offload.o gro.o \\\n  16:\t\t\t\tnetdev-genl.o netdev-genl-gen.o netdev_work.o gso.o\n  17:\t\n  18:\tobj-$(CONFIG_NETDEV_ADDR_LIST_TEST) += dev_addr_lists_test.o\n  19:\t\n  20:\tobj-y += net-sysfs.o\n"
}

gemini-3.1-pro-preview

null

null

							input: 12125
							output: 27
							thoughts: 71
						

Here's my summary, as I see it:

**Considering the `datagram.o` Module**

Okay, so `datagram.o` is a consistently compiled component – good to know. This means I don't need to mess around with special configurations *unless* I specifically want to interact with it through `io_uring`. Even then, that's really just the one potential wrinkle for access.

Of course, even if I don't use `io_uring`, I should be able to get at it using a standard socket `sendmsg` call with `MSG_ZEROCOPY`. That's a solid fallback and potential performance path. Now, I want to take a closer look at `zerocopy_fill_skb_from_devmem` to understand the low-level details of how this zero-copy mechanism functions in this particular scenario.

{
  "Expression": "zerocopy_fill_skb_from_devmem"
}

{
  "Output": "net/core/datagram.c=706=static int\nnet/core/datagram.c:707:zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,\nnet/core/datagram.c-708-\t\t\t      int length,\n--\nnet/core/datagram.c=749=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\nnet/core/datagram.c-760-\telse if (binding)\nnet/core/datagram.c:761:\t\tret = zerocopy_fill_skb_from_devmem(skb, from, length, binding);\nnet/core/datagram.c-762-\telse\n"
}

gemini-3.1-pro-preview

null

null

							input: 12616
							output: 24
							thoughts: 34
						

Here's a breakdown of my thought process:

**Diving into `zerocopy_fill_skb_from_devmem`**

Okay, so I'm looking at `zerocopy_fill_skb_from_devmem`. The code seems to be explicitly tied to a situation where `binding` isn't a null pointer. So, the key question is: What does this `binding` thing *actually* represent? Where does this variable originate? It's crucial to understand the lifecycle and purpose of `binding` to fully grasp the execution path and rationale behind using this specific function. I need to trace back and figure out where the code is setting or passing a non-null `binding` to see why it necessitates this zero-copy approach. That's the first thing I need to investigate.

{
  "Expression": "__zerocopy_sg_from_iter"
}

{
  "Output": "include/linux/skbuff.h=1761=struct net_devmem_dmabuf_binding;\ninclude/linux/skbuff.h-1762-\ninclude/linux/skbuff.h:1763:int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\ninclude/linux/skbuff.h-1764-\t\t\t    struct sk_buff *skb, struct iov_iter *from,\n--\ninclude/linux/skbuff.h=1771=static inline int skb_zerocopy_iter_dgram(struct sk_buff *skb,\n--\ninclude/linux/skbuff.h-1773-{\ninclude/linux/skbuff.h:1774:\treturn __zerocopy_sg_from_iter(msg, skb-\u003esk, skb, \u0026msg-\u003emsg_iter, len,\ninclude/linux/skbuff.h-1775-\t\t\t\t       NULL);\n--\nnet/core/datagram.c=707=zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,\n--\nnet/core/datagram.c-748-\nnet/core/datagram.c:749:int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\nnet/core/datagram.c-750-\t\t\t    struct sk_buff *skb, struct iov_iter *from,\n--\nnet/core/datagram.c-774-}\nnet/core/datagram.c:775:EXPORT_SYMBOL(__zerocopy_sg_from_iter);\nnet/core/datagram.c-776-\n--\nnet/core/datagram.c=787=int zerocopy_sg_from_iter(struct sk_buff *skb, struct iov_iter *from)\n--\nnet/core/datagram.c-794-\nnet/core/datagram.c:795:\treturn __zerocopy_sg_from_iter(NULL, NULL, skb, from, ~0U, NULL);\nnet/core/datagram.c-796-}\n--\nnet/core/skbuff.c=1912=int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,\n--\nnet/core/skbuff.c-1933-\nnet/core/skbuff.c:1934:\terr = __zerocopy_sg_from_iter(msg, sk, skb, \u0026msg-\u003emsg_iter, len,\nnet/core/skbuff.c-1935-\t\t\t\t      binding);\n--\nnet/vmw_vsock/virtio_transport_common.c=80=static int virtio_transport_fill_skb(struct sk_buff *skb,\n--\nnet/vmw_vsock/virtio_transport_common.c-87-\tif (zcopy)\nnet/vmw_vsock/virtio_transport_common.c:88:\t\treturn __zerocopy_sg_from_iter(msg, NULL, skb,\nnet/vmw_vsock/virtio_transport_common.c-89-\t\t\t\t\t       \u0026msg-\u003emsg_iter, len, NULL);\n--\nnet/vmw_vsock/virtio_transport_common.c=212=static struct sk_buff *virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,\n--\nnet/vmw_vsock/virtio_transport_common.c-239-\t/* If 'vsk' != NULL then payload is always present, so we\nnet/vmw_vsock/virtio_transport_common.c:240:\t * will never call '__zerocopy_sg_from_iter()' below without\nnet/vmw_vsock/virtio_transport_common.c-241-\t * setting skb owner in 'skb_set_owner_w()'. The only case\n--\nnet/vmw_vsock/virtio_transport_common.c-246-\nnet/vmw_vsock/virtio_transport_common.c:247:\t/* Set owner here, because '__zerocopy_sg_from_iter()' uses\nnet/vmw_vsock/virtio_transport_common.c-248-\t * owner of skb without check to update 'sk_wmem_alloc'.\n"
}

gemini-3.1-pro-preview

null

null

							input: 13954
							output: 24
							thoughts: 15
						

Okay, here's my attempt at summarizing the thought process, tailored for an expert audience, and written in the first person:

**Zero-Copy Iteration Considerations**

Right, so I'm thinking about `skb_zerocopy_iter_stream`. This function is fundamental when dealing with zero-copy operations on skbs, particularly within stream-oriented networking. My immediate concern is how it handles the iteration process. I need to be absolutely sure that the underlying memory is handled correctly. Are we dealing with contiguous segments or are we dealing with fragmented skbs here? It's critical to avoid any unnecessary copying, because the whole point is efficiency and minimal overhead, so I need to understand if the iterating process could inadvertently cause copying to occur. I’m wondering about potential pitfalls, things that could cause issues. Is this the *right* way to go about it? Or am I missing something? How does it handle potential partial reads and writes? I need to quickly and comprehensively confirm its behaviour under various edge cases: buffer overflows, underflows, and partial writes/reads.

{
  "Expression": "skb_zerocopy_iter_stream"
}

{
  "Output": "include/linux/skbuff.h=1771=static inline int skb_zerocopy_iter_dgram(struct sk_buff *skb,\n--\ninclude/linux/skbuff.h-1777-\ninclude/linux/skbuff.h:1778:int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,\ninclude/linux/skbuff.h-1779-\t\t\t     struct msghdr *msg, int len,\n--\nnet/core/skbuff.c=1910=EXPORT_SYMBOL_GPL(msg_zerocopy_ubuf_ops);\nnet/core/skbuff.c-1911-\nnet/core/skbuff.c:1912:int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,\nnet/core/skbuff.c-1913-\t\t\t     struct msghdr *msg, int len,\n--\nnet/core/skbuff.c-1949-}\nnet/core/skbuff.c:1950:EXPORT_SYMBOL_GPL(skb_zerocopy_iter_stream);\nnet/core/skbuff.c-1951-\n--\nnet/ipv4/tcp.c=1116=int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)\n--\nnet/ipv4/tcp.c-1338-\nnet/ipv4/tcp.c:1339:\t\t\terr = skb_zerocopy_iter_stream(sk, skb, msg, copy, uarg,\nnet/ipv4/tcp.c-1340-\t\t\t\t\t\t       binding);\n"
}

gemini-3.1-pro-preview

null

null

							input: 14652
							output: 18
							thoughts: 47
						

Okay, here's my interpretation of those thoughts, framed as my own:

**Investigating a `tcp_sendmsg_locked` Binding**

Alright, let's see... The `binding` I'm seeing inside `tcp_sendmsg_locked` is pointing back to `sockc.devmem_binding`. Ah, right, that connects it to the TCP devmem functionality – device memory TCP. So, the question immediately is: is this specific TCP devmem implementation configurable? Are there any knobs or settings I can tweak related to TCP devmem that might impact this `binding` behavior? Need to dig into the kernel configs to see what options are exposed for this. That's the next step.

no matches

{
  "Expression": "devmem_binding"
}

{
  "error": "no matches"
}

gemini-3.1-pro-preview

null

null

							input: 14881
							output: 30
							thoughts: 14
						

Here's my attempt at summarizing the thought process, tailored for an expert audience and written in the first person:

**Diving into `tcp_sendmsg_locked`**

Okay, hold on a second. Before I proceed any further with this line of reasoning, I need to make sure I'm grounded. My immediate instinct is to look deeper into what's happening within `tcp_sendmsg_locked`. It's a critical function, and a lot hinges on its behavior. I know this is where the core work of sending TCP data happens, and any subtle missteps here could have ripple effects throughout the network stack. I need to refresh my understanding of its locking strategies, how it handles congestion control, and how it interacts with the socket buffer. I need to confirm the assumptions I’m making about its data flow and state management. Before I go down a rabbit hole, I want to confirm that I understand what is actually going on. This is where the rubber meets the road, so let's start there.

{
  "Expression": "devmem",
  "PathPrefix": "net/ipv4/tcp.c"
}

{
  "Output": "net/ipv4/tcp.c-290-\nnet/ipv4/tcp.c:291:#include \"../core/devmem.h\"\nnet/ipv4/tcp.c-292-\n--\nnet/ipv4/tcp.c=1116=int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)\nnet/ipv4/tcp.c-1117-{\nnet/ipv4/tcp.c:1118:\tstruct net_devmem_dmabuf_binding *binding = NULL;\nnet/ipv4/tcp.c-1119-\tstruct tcp_sock *tp = tcp_sk(sk);\n--\nnet/ipv4/tcp.c-1158-\t\t\tif (!sockc_err \u0026\u0026 sockc.dmabuf_id) {\nnet/ipv4/tcp.c:1159:\t\t\t\tbinding = net_devmem_get_binding(sk, sockc.dmabuf_id);\nnet/ipv4/tcp.c-1160-\t\t\t\tif (IS_ERR(binding)) {\n--\nnet/ipv4/tcp.c-1420-\tif (binding)\nnet/ipv4/tcp.c:1421:\t\tnet_devmem_dmabuf_binding_put(binding);\nnet/ipv4/tcp.c-1422-\treturn copied + copied_syn;\n--\nnet/ipv4/tcp.c-1439-\tif (binding)\nnet/ipv4/tcp.c:1440:\t\tnet_devmem_dmabuf_binding_put(binding);\nnet/ipv4/tcp.c-1441-\n--\nnet/ipv4/tcp.c=2499=static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,\n--\nnet/ipv4/tcp.c-2575-\t\t\tniov = skb_frag_net_iov(frag);\nnet/ipv4/tcp.c:2576:\t\t\tif (!net_is_devmem_iov(niov)) {\nnet/ipv4/tcp.c-2577-\t\t\t\terr = -ENODEV;\n--\nnet/ipv4/tcp.c-2598-\t\t\t\tdmabuf_cmsg.frag_token = tcp_xa_pool.tokens[tcp_xa_pool.idx];\nnet/ipv4/tcp.c:2599:\t\t\t\tdmabuf_cmsg.dmabuf_id = net_devmem_iov_binding_id(niov);\nnet/ipv4/tcp.c-2600-\n"
}

gemini-3.1-pro-preview

null

null

							input: 15792
							output: 34
							thoughts: 26
						

Here's my attempt at summarizing the thought process, tailored for an expert audience:

**Diving into `net_devmem_get_binding`**

Right, `net_devmem_get_binding`... I need to refresh my memory on the underlying data structures and intended behavior. Best to start by checking the definition. Let's pull up `net/core/devmem.h` and see what's in there. I'm primarily interested in the relevant structures, enumerations, and any inline documentation regarding the intent. Knowing the binding strategy and how it manages memory regions is key to understanding this function's role. I expect to see the typical memory mapping details, and perhaps some considerations around device-specific allocations.

{
  "File": "net/core/devmem.h",
  "FirstLine": 1,
  "LineCount": 20
}

{
  "Contents": "   1:\t/* SPDX-License-Identifier: GPL-2.0-or-later */\n   2:\t/*\n   3:\t * Device memory TCP support\n   4:\t *\n   5:\t * Authors:\tMina Almasry \u003calmasrymina@google.com\u003e\n   6:\t *\t\tWillem de Bruijn \u003cwillemb@google.com\u003e\n   7:\t *\t\tKaiyuan Zhang \u003ckaiyuanz@google.com\u003e\n   8:\t *\n   9:\t */\n  10:\t#ifndef _NET_DEVMEM_H\n  11:\t#define _NET_DEVMEM_H\n  12:\t\n  13:\t#include \u003cnet/netmem.h\u003e\n  14:\t#include \u003cnet/netdev_netlink.h\u003e\n  15:\t\n  16:\tstruct netlink_ext_ack;\n  17:\t\n  18:\tstruct net_devmem_dmabuf_binding {\n  19:\t\tstruct dma_buf *dmabuf;\n  20:\t\tstruct dma_buf_attachment *attachment;\n"
}

gemini-3.1-pro-preview

null

null

							input: 16290
							output: 19
							thoughts: 10
						

Here's my thought process, as if I were considering this myself:

**Investigating `devmem` Configuration**

Okay, so I'm wondering about `devmem`. Is there a configuration file somewhere that controls its behavior or default settings? I know it's a pretty straightforward utility for accessing memory directly, but surely there's *something* I can tweak to tailor its operation. Maybe something about error reporting, access permissions, or perhaps even a default address space it operates within. I'm thinking I've probably just missed something in the documentation, as that's usually the answer. Time to dig around the usual places - `/etc`, `/usr/local/etc`, the man page of course, and any relevant package documentation. I doubt it would be anything particularly complex, but I do tend to overlook the obvious!

{
  "Expression": "config.*DEVMEM"
}

{
  "Output": "drivers/char/Kconfig=249=config NSC_GPIO\n--\ndrivers/char/Kconfig-258-\ndrivers/char/Kconfig:259:config DEVMEM\ndrivers/char/Kconfig-260-\tbool \"/dev/mem virtual device support\"\n--\nlib/Kconfig=617=config GENERIC_LIB_UCMPDI2\n--\nlib/Kconfig-619-\nlib/Kconfig:620:config GENERIC_LIB_DEVMEM_IS_ALLOWED\nlib/Kconfig-621-\tbool\n--\nlib/Kconfig.debug=1957=source \"samples/Kconfig\"\nlib/Kconfig.debug-1958-\nlib/Kconfig.debug:1959:config ARCH_HAS_DEVMEM_IS_ALLOWED\nlib/Kconfig.debug-1960-\tbool\nlib/Kconfig.debug-1961-\nlib/Kconfig.debug:1962:config STRICT_DEVMEM\nlib/Kconfig.debug-1963-\tbool \"Filter access to /dev/mem\"\n--\nlib/Kconfig.debug-1981-\nlib/Kconfig.debug:1982:config IO_STRICT_DEVMEM\nlib/Kconfig.debug-1983-\tbool \"Filter I/O access to /dev/mem\"\n--\nnet/Kconfig=66=config SKB_EXTENSIONS\n--\nnet/Kconfig-68-\nnet/Kconfig:69:config NET_DEVMEM\nnet/Kconfig-70-\tdef_bool y\n"
}

gemini-3.1-pro-preview

null

null

							input: 16894
							output: 18
							thoughts: 409
						

**Analyzing Zero-Copy Networking Functions**

Okay, so `NET_DEVMEM` is a `def_bool y`, which means it's always on if networking is enabled. That simplifies things, no need to worry about any special configuration for it. Let's move on to identifying the key symbols we need to focus on.

Right, the initial list of focus symbols are: `zerocopy_fill_skb_from_iter`, `__zerocopy_sg_from_iter`, `zerocopy_sg_from_iter`, and `skb_zerocopy_iter_stream`.

Hold on, I recall that `skb_zerocopy_iter_stream` is defined in `net/core/skbuff.c`. Let me check if there are other associated functions while I'm here. It appears `zerocopy_fill_skb_from_iter` is the one that was recently modified. The static function `zerocopy_fill_skb_from_devmem` can't be used directly but is called from `__zerocopy_sg_from_iter`.

Let's double-check the definition of `zerocopy_fill_skb_from_iter`, it's defined in `net/core/datagram.c`. Okay, it's not static, which means it's globally visible. We can confirm that `__zerocopy_sg_from_iter` and `zerocopy_sg_from_iter` are both exported, and so is `skb_zerocopy_iter_stream`.

Alright, just to make sure we're on the same page, the focus symbols are:
*   `zerocopy_fill_skb_from_iter`
*   `__zerocopy_sg_from_iter`
*   `zerocopy_sg_from_iter`
*   `skb_zerocopy_iter_stream`

Now, let's think about relevant configuration options. Ah! Since `zerocopy_fill_skb_from_iter` is used directly by `io_sg_from_iter`, `IO_URING` is a good config to add to the list.

{
  "Expression": "config IO_URING"
}

{
  "Output": "init/Kconfig=1910=config AIO\n--\ninit/Kconfig-1917-\ninit/Kconfig:1918:config IO_URING\ninit/Kconfig-1919-\tbool \"Enable IO uring support\" if EXPERT\n--\ninit/Kconfig=1927=config GCOV_PROFILE_URING\n--\ninit/Kconfig-1939-\ninit/Kconfig:1940:config IO_URING_MOCK_FILE\ninit/Kconfig-1941-\ttristate \"Enable io_uring mock files (Experimental)\" if EXPERT\n--\nio_uring/Kconfig-5-\nio_uring/Kconfig:6:config IO_URING_ZCRX\nio_uring/Kconfig-7-\tdef_bool y\n--\nio_uring/Kconfig-12-\nio_uring/Kconfig:13:config IO_URING_BPF\nio_uring/Kconfig-14-\tdef_bool y\n--\nio_uring/Kconfig-17-\nio_uring/Kconfig:18:config IO_URING_BPF_OPS\nio_uring/Kconfig-19-\tdef_bool y\n"
}

gemini-3.1-pro-preview

null

null

							input: 18104
							output: 21
							thoughts: 60
						

Okay, here's my take on those config thoughts:

**Configuring the Kernel: Initial Checks and Considerations**

Alright, so the core of this seems to revolve around kernel configuration, particularly around the `IO_URING` feature. That's the main driver of this thought process. It makes sense to start there; that's the focal point.

I'm immediately prompted to check for other related configurations. It's a systematic approach, right? See what else is going on in the same neighborhood. `NET_DEVMEM` is showing `def_bool y`, and, well, `NET` itself is a given. These are pretty common; nothing groundbreaking there.

Now, let's circle back and double-check `io_sg_from_iter`. Seems like there's something specific here that warrants a second look. Maybe there's a dependency, a subtle interaction, or perhaps I'm trying to refine my understanding of its purpose within the broader context of `IO_URING`. Gotta make sure I'm fully up to speed on all the details.

{
  "Expression": "io_sg_from_iter"
}

{
  "Output": "io_uring/net.c=112=struct io_recvzc {\n--\nio_uring/net.c-118-\nio_uring/net.c:119:static int io_sg_from_iter_iovec(struct sk_buff *skb,\nio_uring/net.c-120-\t\t\t\t struct iov_iter *from, size_t length);\nio_uring/net.c:121:static int io_sg_from_iter(struct sk_buff *skb,\nio_uring/net.c-122-\t\t\t   struct iov_iter *from, size_t length);\n--\nio_uring/net.c=1378=int io_send_zc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)\n--\nio_uring/net.c-1443-\tif (!(zc-\u003eflags \u0026 IORING_RECVSEND_FIXED_BUF)) {\nio_uring/net.c:1444:\t\tiomsg-\u003emsg.sg_from_iter = io_sg_from_iter_iovec;\nio_uring/net.c-1445-\t\treturn io_notif_account_mem(zc-\u003enotif, iomsg-\u003emsg.msg_iter.count);\nio_uring/net.c-1446-\t}\nio_uring/net.c:1447:\tiomsg-\u003emsg.sg_from_iter = io_sg_from_iter;\nio_uring/net.c-1448-\treturn 0;\n--\nio_uring/net.c-1450-\nio_uring/net.c:1451:static int io_sg_from_iter_iovec(struct sk_buff *skb,\nio_uring/net.c-1452-\t\t\t\t struct iov_iter *from, size_t length)\n--\nio_uring/net.c-1457-\nio_uring/net.c:1458:static int io_sg_from_iter(struct sk_buff *skb,\nio_uring/net.c-1459-\t\t\t   struct iov_iter *from, size_t length)\n"
}

gemini-3.1-pro-preview

null

null

							input: 18968
							output: 21
							thoughts: 36
						

Okay, here's my interpretation of those thoughts, presented as a summary:

**Digging into Data Transfer Preparation**

Alright, I'm trying to trace the data flow here. The function `io_send_zc_prep` is the one that sets the `sg_from_iter` data structure, which is critical. Now, I need to know the scope of `io_send_zc_prep`. Is it exported, or is it a global function? Knowing that will influence how accessible and, by extension, how I can best utilize this function in my use case. It's a key piece of the puzzle for understanding how data transfer is set up.

{
  "Expression": "io_send_zc_prep"
}

{
  "Output": "io_uring/net.c=1361=void io_send_zc_cleanup(struct io_kiocb *req)\n--\nio_uring/net.c-1377-\nio_uring/net.c:1378:int io_send_zc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)\nio_uring/net.c-1379-{\n--\nio_uring/net.h=54=int io_sendmsg_zc(struct io_kiocb *req, unsigned int issue_flags);\nio_uring/net.h:55:int io_send_zc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);\nio_uring/net.h-56-void io_send_zc_cleanup(struct io_kiocb *req);\n--\nio_uring/opdef.c=54=const struct io_issue_def io_issue_defs[] = {\n--\nio_uring/opdef.c-433-\t\t.async_size\t\t= sizeof(struct io_async_msghdr),\nio_uring/opdef.c:434:\t\t.prep\t\t\t= io_send_zc_prep,\nio_uring/opdef.c-435-\t\t.issue\t\t\t= io_sendmsg_zc,\n--\nio_uring/opdef.c-446-\t\t.async_size\t\t= sizeof(struct io_async_msghdr),\nio_uring/opdef.c:447:\t\t.prep\t\t\t= io_send_zc_prep,\nio_uring/opdef.c-448-\t\t.issue\t\t\t= io_sendmsg_zc,\n"
}

gemini-3.1-pro-preview

null

null

							input: 19597
							output: 229
							thoughts: 273
						

**My Analysis of the Zerocopy Patch**

Okay, so I'm looking at this patch and immediately two global functions jump out at me: `io_send_zc_prep` and `io_sendmsg_zc`. Let's add that `io_sendmsg_zc` to the list of focus symbols. The key here is about how we're treating datagrams and how they are handled.

Right, so the important symbols for me to focus on are now:

*   `zerocopy_fill_skb_from_iter`
*   `__zerocopy_sg_from_iter`
*   `zerocopy_sg_from_iter`
*   `skb_zerocopy_iter_stream`
*   `io_sendmsg_zc`

This patch is interesting because of the change it makes in the core networking datagram handling. Specifically, it's tweaking the return value of `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem`. Now, when `!skb_frags_readable(skb)`, the return value is shifting from `-EFAULT` to `-EMSGSIZE`. The implications are clear. This is directly tied to MSG\_ZEROCOPY and io\_uring zerocopy sends, which, as we know, are critical areas of high performance networking, and can be easily impacted.

The functional change is subtle, but it's changing the error code that goes back to userspace, and more importantly, how the kernel itself handles the error. This is a classic area where fuzzing is going to be useful, as you need to make sure the expected results are as expected. We need to be careful with regression and unexpected behavior.

So, let's call set-results.

{
  "EnableConfigs": [
    "IO_URING"
  ],
  "FocusSymbols": [
    "zerocopy_fill_skb_from_iter",
    "__zerocopy_sg_from_iter",
    "zerocopy_sg_from_iter",
    "skb_zerocopy_iter_stream",
    "io_sendmsg_zc"
  ],
  "Reasoning": "The patch changes the return value of zerocopy_fill_skb_from_iter and zerocopy_fill_skb_from_devmem from -EFAULT to -EMSGSIZE when skb_frags_readable(skb) is false. This is a functional change in the core networking datagram handling, specifically affecting zerocopy operations (e.g., MSG_ZEROCOPY) and io_uring zerocopy sends. The change alters the error code returned to userspace and handled by the kernel, which is worth fuzzing to ensure it doesn't introduce unexpected behavior or regressions in error handling paths.",
  "WorthFuzzing": true
}

{
  "EnableConfigs": [
    "IO_URING"
  ],
  "FocusSymbols": [
    "zerocopy_fill_skb_from_iter",
    "__zerocopy_sg_from_iter",
    "zerocopy_sg_from_iter",
    "skb_zerocopy_iter_stream",
    "io_sendmsg_zc"
  ],
  "Reasoning": "The patch changes the return value of zerocopy_fill_skb_from_iter and zerocopy_fill_skb_from_devmem from -EFAULT to -EMSGSIZE when skb_frags_readable(skb) is false. This is a functional change in the core networking datagram handling, specifically affecting zerocopy operations (e.g., MSG_ZEROCOPY) and io_uring zerocopy sends. The change alters the error code returned to userspace and handled by the kernel, which is worth fuzzing to ensure it doesn't introduce unexpected behavior or regressions in error handling paths.",
  "WorthFuzzing": true
}

{
  "WorthFuzzing": true
}

null

best-expensive

null

{
  "KMSANReasoning": "The patch simply changes the error code returned by `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem` from `-EFAULT` to `-EMSGSIZE` when there is a mismatch in fragment readability. This change in error code propagation does not introduce any uninitialized memory allocations, accesses, or kernel-to-user info-leaks. Any potential bugs related to this change would likely be logical errors or state inconsistencies, which are not specific to KMSAN's detection capabilities. Therefore, a dedicated KMSAN fuzzing session is not necessary.",
  "NeedsKMSAN": false
}

You are an expert Linux kernel security engineer specializing in kernel memory
error detectors (KASAN and KMSAN). Your job is to review the provided patch series and
determine if the code changes justify spawning a dedicated KMSAN (KernelMemorySanitizer)
fuzzing session in addition to standard KASAN fuzzing.

CRITICAL DISTINCTION BETWEEN KASAN AND KMSAN:
- Standard KASAN kernel builds (upstream-apparmor-kasan.config) already enable
  a comprehensive suite of debugging tools and sanitizers, including KASAN
  (out-of-bounds accesses, use-after-free, double free, invalid free), LOCKDEP
  (locking bugs and deadlocks), UB-sanitizers, and memory corruption checks.
- KMSAN (KernelMemorySanitizer) detects reads of UNINITIALIZED memory (stack, heap,
  or page allocations) and kernel-to-user memory info-leaks.

Rule: THERE IS NO SENSE IN RUNNING A KMSAN SESSION IF A BUG CAN BE CAUGHT BY KASAN,
LOCKDEP, OR OTHER STANDARD BUG DETECTORS.
A dedicated KMSAN fuzzing session incurs significant resource costs. You must ONLY
set NeedsKMSAN=true if the code changes introduce or expose UNINITIALIZED MEMORY risks
that are detected ONLY by KMSAN.

Look holistically at the patch series and surrounding code. Even if no direct
uninitialized field accesses or new buffer allocations are added in the diff itself,
a patch may alter control flow, bounds checking, or data length calculations in ways
that change how the rest of the code operates on existing buffers (e.g. allowing
uninitialized stack/heap memory to be read, copied to user space, or used in control
flow). Do not hesitate to use your code access tools to inspect the surrounding code,
called functions, and callers.

Set NeedsKMSAN=true ONLY IF the patch introduces or modifies:
1. Kernel structures sent to user space (via copy_to_user, put_user, netlink skb
   attributes, ioctl output arguments, socket options, or BPF buffers) where fields
   or structure padding might not be fully initialized/zeroed.
2. Conditional logic or branching that depends on potentially uninitialized variables
   or struct fields.
3. Allocation or initialization of complex data structures where uninitialized fields
   could be read later in reachable code paths.
4. Bounds checks, lengths, or logic in a way that allows surrounding code to access
   uninitialized bytes of existing buffers.

Set NeedsKMSAN=false IF:
- The code changes primarily risk out-of-bounds access, array overflows, NULL pointer
  dereferences, locking deadlocks, or use-after-free bugs (these are already caught
  by KASAN, LOCKDEP, or standard bug detectors).
- All stack/heap structures touched or introduced by the patch are fully zeroed
  or initialized (e.g. using = {0}, memset, kzalloc) before being read or copied.
- The patch does not introduce any risk of uninitialized memory usage or info-leaks.

Use your code access tools to inspect the surrounding code if necessary, then provide
detailed KMSANReasoning contrasting KASAN vs KMSAN applicability for this patch.
Prefer calling several tools at the same time to save round-trips.


Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.

For your convenience, here is the diff of the changes:
commit 240149f79aa011b0389e301448d24afa4f773aa9
Author: syz-cluster <triage@syzkaller.com>
Date:   Mon Aug 10 23:34:34 2026 +0000

    syz-cluster: applied patch under review

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 173b5d97bd409..6f8ffd61bcab4 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -638,7 +638,7 @@ int zerocopy_fill_skb_from_iter(struct sk_buff *skb,
 	int frag = skb_shinfo(skb)->nr_frags;
 
 	if (!skb_frags_readable(skb))
-		return -EFAULT;
+		return -EMSGSIZE;
 
 	while (length && iov_iter_count(from)) {
 		struct page *head, *last_head = NULL;
@@ -713,7 +713,7 @@ zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,
 	struct net_iov *niov;
 
 	if (i && skb_frags_readable(skb))
-		return -EFAULT;
+		return -EMSGSIZE;
 
 	/* Devmem filling works by taking an IOVEC from the user where the
 	 * iov_addrs are interpreted as an offset in bytes into the dma-buf to

gemini-3.1-pro-preview

null

null

							input: 2120
							output: 32
							thoughts: 622
						

**Analyzing a Change in Error Code Handling**

Okay, so I'm looking at a patch that subtly changes the error return value from `-EFAULT` to `-EMSGSIZE` within the `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem` functions in `net/core/datagram.c`. The condition triggering this change seems to be when `skb_frags_readable(skb)` is false (for the first function) or true with an index 'i' (for the second). My initial thought is, "So what?". It's just a different error code, right? Not exactly a functional change, no new allocations or reads of uninitialized memory.

However, I need to be more thorough. Error codes, while seemingly simple, can have implications in how the calling code, particularly in user space, behaves. Specifically, I'm now thinking: Does this different error code potentially change how the caller handles uninitialized memory? The callers should just be propagating the errors upwards, right? 

Okay, let me check where these functions are called. The text mentions `__zerocopy_sg_from_iter` but I want to be certain, because I thought the second function was called by the first. I will perform a grep to confirm the direct call site of `zerocopy_fill_skb_from_iter`.

{
  "Expression": "zerocopy_fill_skb_from_iter",
  "PathPrefix": ""
}

{
  "Output": "include/linux/skbuff.h=1763=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\ninclude/linux/skbuff.h-1767-\ninclude/linux/skbuff.h:1768:int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\ninclude/linux/skbuff.h-1769-\t\t\t\tstruct iov_iter *from, size_t length);\n--\nio_uring/net.c=1451=static int io_sg_from_iter_iovec(struct sk_buff *skb,\n--\nio_uring/net.c-1454-\tskb_zcopy_downgrade_managed(skb);\nio_uring/net.c:1455:\treturn zerocopy_fill_skb_from_iter(skb, from, length);\nio_uring/net.c-1456-}\n--\nio_uring/net.c=1458=static int io_sg_from_iter(struct sk_buff *skb,\n--\nio_uring/net.c-1470-\telse if (unlikely(!skb_zcopy_managed(skb)))\nio_uring/net.c:1471:\t\treturn zerocopy_fill_skb_from_iter(skb, from, length);\nio_uring/net.c-1472-\n--\nnet/core/datagram.c=633=EXPORT_SYMBOL(skb_copy_datagram_from_iter_full);\nnet/core/datagram.c-634-\nnet/core/datagram.c:635:int zerocopy_fill_skb_from_iter(struct sk_buff *skb,\nnet/core/datagram.c-636-\t\t\t\tstruct iov_iter *from, size_t length)\n--\nnet/core/datagram.c=749=int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,\n--\nnet/core/datagram.c-762-\telse\nnet/core/datagram.c:763:\t\tret = zerocopy_fill_skb_from_iter(skb, from, length);\nnet/core/datagram.c-764-\n"
}

gemini-3.1-pro-preview

null

null

							input: 3668
							output: 154
							thoughts: 213
						

**My Assessment of the Error Code Change**

Okay, so this change boils down to a straightforward error code adjustment. Specifically, if `zerocopy_fill_skb_from_iter` returns `-EMSGSIZE` instead of `-EFAULT`, the call site propagates `-EMSGSIZE`. The important thing I'm assessing is whether this introduces any new memory issues. It doesn't look like it introduces any uninitialized memory usage because the core functionality remains the same; it's simply a change in how we signal a specific error condition related to fragment readability.

My reasoning is that the fundamental issue this patch touches isn't about memory safety itself. If there are bugs, they'd likely be logical or state-related, or a functional problem, and wouldn't be something KMSAN would readily catch. KMSAN is primarily designed for detecting memory-related issues, and since this is just an error code change I'm not expecting any KMSAN related findings. So, there is no need for a KMSAN session. I'll set NeedsKMSAN to false.

{
  "KMSANReasoning": "The patch simply changes the error code returned by `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem` from `-EFAULT` to `-EMSGSIZE` when there is a mismatch in fragment readability. This change in error code propagation does not introduce any uninitialized memory allocations, accesses, or kernel-to-user info-leaks. Any potential bugs related to this change would likely be logical errors or state inconsistencies, which are not specific to KMSAN's detection capabilities. Therefore, a dedicated KMSAN fuzzing session is not necessary.",
  "NeedsKMSAN": false
}

{
  "KMSANReasoning": "The patch simply changes the error code returned by `zerocopy_fill_skb_from_iter` and `zerocopy_fill_skb_from_devmem` from `-EFAULT` to `-EMSGSIZE` when there is a mismatch in fragment readability. This change in error code propagation does not introduce any uninitialized memory allocations, accesses, or kernel-to-user info-leaks. Any potential bugs related to this change would likely be logical errors or state inconsistencies, which are not specific to KMSAN's detection capabilities. Therefore, a dedicated KMSAN fuzzing session is not necessary.",
  "NeedsKMSAN": false
}