AI Trajectory

null

{
  "EnableConfigs": [
    "NET_SCHED"
  ],
  "FocusSymbols": [
    "tc_modify_qdisc",
    "__tc_modify_qdisc"
  ],
  "Reasoning": "The patch adds a depth check for qdisc hierarchies in `__tc_modify_qdisc` to prevent excessively deep trees. This is a functional change in the core traffic control (tc) subsystem that handles `RTM_NEWQDISC` netlink messages.",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit edda7f2a1783c5f6b7ab4521ed26b50f70de1622\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Fri Jul 31 00:29:15 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/net/sched/sch_api.c b/net/sched/sch_api.c\nindex 668bcd60d183e..09d57ad76b895 100644\n--- a/net/sched/sch_api.c\n+++ b/net/sched/sch_api.c\n@@ -1467,6 +1467,24 @@ check_loop_fn(struct Qdisc *q, unsigned long cl, struct qdisc_walker *w)\n \treturn 0;\n }\n \n+static int qdisc_parent_depth(struct net_device *dev, struct Qdisc *qdisc)\n+{\n+\tint depth = 1;\n+\n+\twhile (qdisc-\u003eparent != TC_H_ROOT) {\n+\t\tif (qdisc-\u003eflags \u0026 TCQ_F_NOPARENT)\n+\t\t\tbreak;\n+\n+\t\tqdisc = qdisc_lookup(dev, TC_H_MAJ(qdisc-\u003eparent));\n+\t\tif (!qdisc)\n+\t\t\tbreak;\n+\n+\t\tdepth++;\n+\t}\n+\n+\treturn depth;\n+}\n+\n const struct nla_policy rtm_tca_policy[TCA_MAX + 1] = {\n \t[TCA_KIND]\t\t= { .type = NLA_STRING },\n \t[TCA_RATE]\t\t= { .type = NLA_BINARY,\n@@ -1749,6 +1767,12 @@ static int __tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n,\n \t\treturn -ENOENT;\n \t}\n create_n_graft2:\n+\tif (p \u0026\u0026 !(p-\u003eflags \u0026 TCQ_F_MQROOT) \u0026\u0026\n+\t    qdisc_parent_depth(dev, p) \u003e 7) {\n+\t\tNL_SET_ERR_MSG(extack, \"Qdisc hierarchy is too deep\");\n+\t\treturn -E2BIG;\n+\t}\n+\n \tif (clid == TC_H_INGRESS) {\n \t\tif (dev_ingress_queue(dev)) {\n \t\t\tq = qdisc_create(dev, dev_ingress_queue(dev),\n"
}

best-expensive

null

{
  "EnableConfigs": [
    "NET_SCHED"
  ],
  "FocusSymbols": [
    "tc_modify_qdisc",
    "__tc_modify_qdisc"
  ],
  "Reasoning": "The patch adds a depth check for qdisc hierarchies in `__tc_modify_qdisc` to prevent excessively deep trees. This is a functional change in the core traffic control (tc) subsystem that handles `RTM_NEWQDISC` netlink messages.",
  "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 edda7f2a1783c5f6b7ab4521ed26b50f70de1622
Author: syz-cluster <triage@syzkaller.com>
Date:   Fri Jul 31 00:29:15 2026 +0000

    syz-cluster: applied patch under review

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index 668bcd60d183e..09d57ad76b895 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -1467,6 +1467,24 @@ check_loop_fn(struct Qdisc *q, unsigned long cl, struct qdisc_walker *w)
 	return 0;
 }
 
+static int qdisc_parent_depth(struct net_device *dev, struct Qdisc *qdisc)
+{
+	int depth = 1;
+
+	while (qdisc->parent != TC_H_ROOT) {
+		if (qdisc->flags & TCQ_F_NOPARENT)
+			break;
+
+		qdisc = qdisc_lookup(dev, TC_H_MAJ(qdisc->parent));
+		if (!qdisc)
+			break;
+
+		depth++;
+	}
+
+	return depth;
+}
+
 const struct nla_policy rtm_tca_policy[TCA_MAX + 1] = {
 	[TCA_KIND]		= { .type = NLA_STRING },
 	[TCA_RATE]		= { .type = NLA_BINARY,
@@ -1749,6 +1767,12 @@ static int __tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n,
 		return -ENOENT;
 	}
 create_n_graft2:
+	if (p && !(p->flags & TCQ_F_MQROOT) &&
+	    qdisc_parent_depth(dev, p) > 7) {
+		NL_SET_ERR_MSG(extack, "Qdisc hierarchy is too deep");
+		return -E2BIG;
+	}
+
 	if (clid == TC_H_INGRESS) {
 		if (dev_ingress_queue(dev)) {
 			q = qdisc_create(dev, dev_ingress_queue(dev),

gemini-3.1-pro-preview

null

null

							input: 2377
							output: 29
							thoughts: 143
						

**Analyzing a Traffic Control Hierarchy Depth Limit**

Okay, so this patch introduces a check within `__tc_modify_qdisc` in `net/sched/sch_api.c`. It's a straightforward functional change to the traffic control system. Basically, it's adding a depth limit to the qdisc hierarchy, returning `-E2BIG` if the depth exceeds 7, which sounds reasonable for avoiding potential stack overflow or resource exhaustion issues.

Now, I need to understand the implications of this change. My immediate thought is, can this be triggered easily, and what configuration options come into play? Let's trace the call path to `__tc_modify_qdisc`. I know `net/sched/sch_api.c` is compiled conditionally based on `CONFIG_NET_SCHED`. So, the very first step is to verify whether that's set in this environment. I'll need to check the relevant Kconfig files or the kernel configuration to confirm that `CONFIG_NET_SCHED` is enabled. If it is, then the `__tc_modify_qdisc` function will definitely be part of the kernel image.

{
  "Expression": "NET_SCHED",
  "PathPrefix": "net/sched/Makefile"
}

{
  "Output": "net/sched/Makefile=8=obj-$(CONFIG_INET)\t\t+= sch_frag.o\nnet/sched/Makefile:9:obj-$(CONFIG_NET_SCHED)\t\t+= sch_api.o sch_blackhole.o\nnet/sched/Makefile-10-obj-$(CONFIG_NET_CLS)\t\t+= cls_api.o\n"
}

gemini-3.1-pro-preview

null

null

							input: 2884
							output: 35
							thoughts: 135
						

**Analyzing Qdisc Modification Functionality**

Okay, so I've confirmed that enabling `CONFIG_NET_SCHED` is essential, no surprise there. Looking into the core logic, I'm zeroing in on `__tc_modify_qdisc`. This is the heavy lifter for modifying traffic control disciplines, right? It's directly called by `tc_modify_qdisc`, which is, in turn, the rtnetlink handler for `RTM_NEWQDISC`. That nails down its reachability and confirms its role in the user space interaction.

Now, I need to make sure that there aren't any other gotchas in the kernel configuration. While `CONFIG_NET_SCHED` is the primary one, I need to double-check for any dependencies that I might be overlooking.

I'm focusing specifically on `__tc_modify_qdisc` and the rtnetlink handler, `tc_modify_qdisc`.
Let me dig into the handler and related structures to be thorough.

{
  "Expression": "tc_modify_qdisc",
  "PathPrefix": "net/sched/sch_api.c"
}

{
  "Output": "net/sched/sch_api.c=1610=static bool req_change(struct nlmsghdr *n)\n--\nnet/sched/sch_api.c-1616-\nnet/sched/sch_api.c:1617:static int __tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n,\nnet/sched/sch_api.c-1618-\t\t\t     struct netlink_ext_ack *extack,\n--\nnet/sched/sch_api.c=1813=static void request_qdisc_module(struct nlattr *kind)\n--\nnet/sched/sch_api.c-1836- */\nnet/sched/sch_api.c:1837:static int tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n,\nnet/sched/sch_api.c-1838-\t\t\t   struct netlink_ext_ack *extack)\n--\nnet/sched/sch_api.c-1858-\tnetdev_lock_ops(dev);\nnet/sched/sch_api.c:1859:\terr = __tc_modify_qdisc(skb, n, extack, dev, tca, tcm);\nnet/sched/sch_api.c-1860-\tnetdev_unlock_ops(dev);\n--\nnet/sched/sch_api.c=2538=static const struct rtnl_msg_handler psched_rtnl_msg_handlers[] __initconst = {\nnet/sched/sch_api.c:2539:\t{.msgtype = RTM_NEWQDISC, .doit = tc_modify_qdisc},\nnet/sched/sch_api.c-2540-\t{.msgtype = RTM_DELQDISC, .doit = tc_get_qdisc},\n"
}

gemini-3.1-pro-preview

null

null

							input: 3743
							output: 110
							thoughts: 187
						

**Fuzzing Target Assessment**

Okay, so this patch looks promising – it's functional and definitely worth a fuzzing effort. My initial thought is to zero in on `tc_modify_qdisc` as the primary focus symbol. While `__tc_modify_qdisc` is static, meaning it *could* be inlined, `tc_modify_qdisc` is the public entry point, and that's usually the safer bet. Though, I should probably include both, or just focus on the entrypoint as it would be more efficient.

Thinking about it, `__tc_modify_qdisc` is actually quite large. It's less likely to be aggressively inlined, making `tc_modify_qdisc` the more robust choice, but still, better to include both or test both at some point.

As for configuration requirements, the traffic control subsystem (`tc`) is involved, so `NET_SCHED` seems like the essential configuration option we need enabled.

The reasoning behind this target is the patch's core function: it introduces a depth check within `__tc_modify_qdisc`, specifically to restrict potentially excessively deep qdisc hierarchies. This falls squarely within the critical traffic control (tc) subsystem, and specifically, the handling of `RTM_NEWQDISC` netlink messages. That's a good place to be fuzzing.

Let's start there and get some results.

{
  "EnableConfigs": [
    "NET_SCHED"
  ],
  "FocusSymbols": [
    "tc_modify_qdisc",
    "__tc_modify_qdisc"
  ],
  "Reasoning": "The patch adds a depth check for qdisc hierarchies in `__tc_modify_qdisc` to prevent excessively deep trees. This is a functional change in the core traffic control (tc) subsystem that handles `RTM_NEWQDISC` netlink messages.",
  "WorthFuzzing": true
}

{
  "EnableConfigs": [
    "NET_SCHED"
  ],
  "FocusSymbols": [
    "tc_modify_qdisc",
    "__tc_modify_qdisc"
  ],
  "Reasoning": "The patch adds a depth check for qdisc hierarchies in `__tc_modify_qdisc` to prevent excessively deep trees. This is a functional change in the core traffic control (tc) subsystem that handles `RTM_NEWQDISC` netlink messages.",
  "WorthFuzzing": true
}