AI Trajectory

null

{
  "EnableConfigs": [
    "BPF_SYSCALL",
    "DEBUG_INFO_BTF"
  ],
  "FocusSymbols": [
    "check_special_kfunc",
    "btf_type_has_trailing_flex_array"
  ],
  "Reasoning": "The patch modifies the BPF verifier to reject `bpf_obj_new` calls that attempt to allocate structs with trailing flexible arrays. This is a functional change in the core BPF logic that should be fuzzed to ensure it correctly handles various BTF types and doesn't introduce regressions or bypasses.",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit 15cf7a0b109ce5ce5a037f8c6e3b2b149b414900\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Mon Jun 22 05:20:28 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c\nindex 2abc79dbf281c2..c3a574cc0d09fd 100644\n--- a/kernel/bpf/verifier.c\n+++ b/kernel/bpf/verifier.c\n@@ -10955,6 +10955,30 @@ static bool __btf_type_is_scalar_struct(struct bpf_verifier_env *env,\n \treturn true;\n }\n \n+static bool btf_type_has_trailing_flex_array(const struct btf *btf,\n+\t\t\t\t\t     const struct btf_type *t)\n+{\n+\tconst struct btf_type *member_type;\n+\tconst struct btf_member *member;\n+\tconst struct btf_array *array;\n+\tu32 vlen;\n+\n+\tif (!btf_type_is_struct(t))\n+\t\treturn false;\n+\n+\tvlen = btf_type_vlen(t);\n+\tif (!vlen)\n+\t\treturn false;\n+\n+\tmember = btf_type_member(t) + vlen - 1;\n+\tmember_type = btf_type_skip_modifiers(btf, member-\u003etype, NULL);\n+\tif (!btf_type_is_array(member_type))\n+\t\treturn false;\n+\n+\tarray = btf_array(member_type);\n+\treturn !array-\u003enelems;\n+}\n+\n enum kfunc_ptr_arg_type {\n \tKF_ARG_PTR_TO_CTX,\n \tKF_ARG_PTR_TO_ALLOC_BTF_ID,    /* Allocated object */\n@@ -12749,6 +12773,10 @@ static int check_special_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_ca\n \t\t\tverbose(env, \"bpf_obj_new/bpf_percpu_obj_new type ID argument must be of a struct\\n\");\n \t\t\treturn -EINVAL;\n \t\t}\n+\t\tif (btf_type_has_trailing_flex_array(ret_btf, ret_t)) {\n+\t\t\tverbose(env, \"bpf_obj_new type must not contain a flexible array\\n\");\n+\t\t\treturn -EINVAL;\n+\t\t}\n \n \t\tif (is_bpf_percpu_obj_new_kfunc(meta-\u003efunc_id)) {\n \t\t\tif (ret_t-\u003esize \u003e BPF_GLOBAL_PERCPU_MA_MAX_SIZE) {\ndiff --git a/tools/testing/selftests/bpf/prog_tests/linked_list.c b/tools/testing/selftests/bpf/prog_tests/linked_list.c\nindex 8defea0253ed9a..49c250132b9b0e 100644\n--- a/tools/testing/selftests/bpf/prog_tests/linked_list.c\n+++ b/tools/testing/selftests/bpf/prog_tests/linked_list.c\n@@ -68,6 +68,8 @@ static struct {\n \t{ \"obj_type_id_oor\", \"local type ID argument must be in range [0, U32_MAX]\" },\n \t{ \"obj_new_no_composite\", \"bpf_obj_new/bpf_percpu_obj_new type ID argument must be of a struct\" },\n \t{ \"obj_new_no_struct\", \"bpf_obj_new/bpf_percpu_obj_new type ID argument must be of a struct\" },\n+\t{ \"obj_new_flex_array\",\n+\t  \"bpf_obj_new type must not contain a flexible array\" },\n \t{ \"obj_drop_non_zero_off\", \"R1 must have zero offset when passed to release func\" },\n \t{ \"new_null_ret\", \"R0 invalid mem access 'ptr_or_null_'\" },\n \t{ \"obj_new_acq\", \"Unreleased reference id=\" },\ndiff --git a/tools/testing/selftests/bpf/progs/linked_list_fail.c b/tools/testing/selftests/bpf/progs/linked_list_fail.c\nindex ddd26d1a083f72..031e77a288eec0 100644\n--- a/tools/testing/selftests/bpf/progs/linked_list_fail.c\n+++ b/tools/testing/selftests/bpf/progs/linked_list_fail.c\n@@ -167,6 +167,16 @@ CHECK_OP(push_back);\n #undef CHECK_OP\n #undef INIT\n \n+struct obj_new_flex_elem {\n+\tint lo;\n+\tint hi;\n+};\n+\n+struct obj_new_flex {\n+\tint hdr;\n+\tstruct obj_new_flex_elem cells[];\n+};\n+\n SEC(\"?kprobe/xyz\")\n int map_compat_kprobe(void *ctx)\n {\n@@ -230,6 +240,19 @@ int obj_new_no_struct(void *ctx)\n \treturn 0;\n }\n \n+SEC(\"?tc\")\n+int obj_new_flex_array(void *ctx)\n+{\n+\tstruct obj_new_flex *p;\n+\n+\tp = bpf_obj_new_impl(bpf_core_type_id_local(struct obj_new_flex), NULL);\n+\tif (!p)\n+\t\treturn 0;\n+\tp-\u003ecells[0].hi = 42;\n+\tbpf_obj_drop_impl(p, NULL);\n+\treturn 0;\n+}\n+\n SEC(\"?tc\")\n int obj_drop_non_zero_off(void *ctx)\n {\n"
}

gemini-3.1-pro-preview

null

{
  "EnableConfigs": [
    "BPF_SYSCALL",
    "DEBUG_INFO_BTF"
  ],
  "FocusSymbols": [
    "check_special_kfunc",
    "btf_type_has_trailing_flex_array"
  ],
  "Reasoning": "The patch modifies the BPF verifier to reject `bpf_obj_new` calls that attempt to allocate structs with trailing flexible arrays. This is a functional change in the core BPF logic that should be fuzzed to ensure it correctly handles various BTF types and doesn't introduce regressions or bypasses.",
  "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).

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.
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.
   Do not list too generic configs (we already have them enabled). Only list those that
   specifically cover the modified code. 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 15cf7a0b109ce5ce5a037f8c6e3b2b149b414900
Author: syz-cluster <triage@syzkaller.com>
Date:   Mon Jun 22 05:20:28 2026 +0000

    syz-cluster: applied patch under review

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2abc79dbf281c2..c3a574cc0d09fd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10955,6 +10955,30 @@ static bool __btf_type_is_scalar_struct(struct bpf_verifier_env *env,
 	return true;
 }
 
+static bool btf_type_has_trailing_flex_array(const struct btf *btf,
+					     const struct btf_type *t)
+{
+	const struct btf_type *member_type;
+	const struct btf_member *member;
+	const struct btf_array *array;
+	u32 vlen;
+
+	if (!btf_type_is_struct(t))
+		return false;
+
+	vlen = btf_type_vlen(t);
+	if (!vlen)
+		return false;
+
+	member = btf_type_member(t) + vlen - 1;
+	member_type = btf_type_skip_modifiers(btf, member->type, NULL);
+	if (!btf_type_is_array(member_type))
+		return false;
+
+	array = btf_array(member_type);
+	return !array->nelems;
+}
+
 enum kfunc_ptr_arg_type {
 	KF_ARG_PTR_TO_CTX,
 	KF_ARG_PTR_TO_ALLOC_BTF_ID,    /* Allocated object */
@@ -12749,6 +12773,10 @@ static int check_special_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_ca
 			verbose(env, "bpf_obj_new/bpf_percpu_obj_new type ID argument must be of a struct\n");
 			return -EINVAL;
 		}
+		if (btf_type_has_trailing_flex_array(ret_btf, ret_t)) {
+			verbose(env, "bpf_obj_new type must not contain a flexible array\n");
+			return -EINVAL;
+		}
 
 		if (is_bpf_percpu_obj_new_kfunc(meta->func_id)) {
 			if (ret_t->size > BPF_GLOBAL_PERCPU_MA_MAX_SIZE) {
diff --git a/tools/testing/selftests/bpf/prog_tests/linked_list.c b/tools/testing/selftests/bpf/prog_tests/linked_list.c
index 8defea0253ed9a..49c250132b9b0e 100644
--- a/tools/testing/selftests/bpf/prog_tests/linked_list.c
+++ b/tools/testing/selftests/bpf/prog_tests/linked_list.c
@@ -68,6 +68,8 @@ static struct {
 	{ "obj_type_id_oor", "local type ID argument must be in range [0, U32_MAX]" },
 	{ "obj_new_no_composite", "bpf_obj_new/bpf_percpu_obj_new type ID argument must be of a struct" },
 	{ "obj_new_no_struct", "bpf_obj_new/bpf_percpu_obj_new type ID argument must be of a struct" },
+	{ "obj_new_flex_array",
+	  "bpf_obj_new type must not contain a flexible array" },
 	{ "obj_drop_non_zero_off", "R1 must have zero offset when passed to release func" },
 	{ "new_null_ret", "R0 invalid mem access 'ptr_or_null_'" },
 	{ "obj_new_acq", "Unreleased reference id=" },
diff --git a/tools/testing/selftests/bpf/progs/linked_list_fail.c b/tools/testing/selftests/bpf/progs/linked_list_fail.c
index ddd26d1a083f72..031e77a288eec0 100644
--- a/tools/testing/selftests/bpf/progs/linked_list_fail.c
+++ b/tools/testing/selftests/bpf/progs/linked_list_fail.c
@@ -167,6 +167,16 @@ CHECK_OP(push_back);
 #undef CHECK_OP
 #undef INIT
 
+struct obj_new_flex_elem {
+	int lo;
+	int hi;
+};
+
+struct obj_new_flex {
+	int hdr;
+	struct obj_new_flex_elem cells[];
+};
+
 SEC("?kprobe/xyz")
 int map_compat_kprobe(void *ctx)
 {
@@ -230,6 +240,19 @@ int obj_new_no_struct(void *ctx)
 	return 0;
 }
 
+SEC("?tc")
+int obj_new_flex_array(void *ctx)
+{
+	struct obj_new_flex *p;
+
+	p = bpf_obj_new_impl(bpf_core_type_id_local(struct obj_new_flex), NULL);
+	if (!p)
+		return 0;
+	p->cells[0].hi = 42;
+	bpf_obj_drop_impl(p, NULL);
+	return 0;
+}
+
 SEC("?tc")
 int obj_drop_non_zero_off(void *ctx)
 {

gemini-3.1-pro-preview

null

null

							input: 3081
							output: 34
							thoughts: 184
						

**Analyzing a New BPF Verifier Patch**

Okay, so I've got this patch – a pretty straightforward one, really. It seems to be preventing `bpf_obj_new` from allocating structs that have trailing flexible arrays. The core of this functionality lies within `kernel/bpf/verifier.c`. Specifically, they've added a function called `btf_type_has_trailing_flex_array` and incorporated it into the `check_special_kfunc` function. This is a functional change to the BPF verifier, which, in my experience, screams "needs fuzzing." Gotta make sure this addition doesn’t introduce any new vulnerabilities or unexpected behavior.

Now, let's zero in on the relevant configurations and symbols. `CONFIG_BPF_SYSCALL` is a given, of course; this change directly impacts the BPF syscall path. For the key symbols, I'm thinking `check_special_kfunc` and `btf_type_has_trailing_flex_array` are the main points to focus on. Wait a second, the second function is static. So, it may be better to just concentrate on `check_special_kfunc`.

Alright, I'll dive into `kernel/bpf/verifier.c` and take a closer look at the surrounding code for `check_special_kfunc`. See what other related areas need careful consideration.

{
  "Expression": "check_special_kfunc",
  "PathPrefix": "kernel/bpf/verifier.c"
}

{
  "Output": "kernel/bpf/verifier.c=12674=s64 bpf_kfunc_stack_access_bytes(struct bpf_verifier_env *env, struct bpf_insn *insn,\n--\nkernel/bpf/verifier.c-12738- */\nkernel/bpf/verifier.c:12739:static int check_special_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_call_arg_meta *meta,\nkernel/bpf/verifier.c-12740-\t\t\t       struct bpf_reg_state *regs, struct bpf_insn_aux_data *insn_aux,\n--\nkernel/bpf/verifier.c=12914=static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,\n--\nkernel/bpf/verifier.c-13148-\t\tptr_type = btf_type_skip_modifiers(desc_btf, t-\u003etype, \u0026ptr_type_id);\nkernel/bpf/verifier.c:13149:\t\terr = check_special_kfunc(env, \u0026meta, regs, insn_aux, ptr_type, desc_btf);\nkernel/bpf/verifier.c-13150-\t\tif (err) {\n"
}

gemini-3.1-pro-preview

null

null

							input: 3940
							output: 30
							thoughts: 39
						

Okay, here's my interpretation of those thoughts, tailored for an expert audience, presented in the first person:

**Analyzing the Kernel Function Call Verification Flow**

Right, so I'm focusing on the interaction between `check_special_kfunc` and `check_kfunc_call`. It looks like the verification logic for special kernel functions is being invoked from the broader kernel function call verification process. My next step, as always, is to dive into the configuration and implementation. Specifically, I need to check the relevant code within `kernel/bpf/verifier.c`. This file almost certainly holds the key to understanding how the verifier handles these special kernel functions and how they're integrated into the overall BPF program verification strategy. I'll need to scrutinize the configurations related to these functions. This will help me understand the constraints, security checks, and limitations imposed during program verification. I need to be prepared for potentially complex interaction. Let's see how `check_special_kfunc` integrates with the overall flow.

{
  "Expression": "CONFIG_BPF",
  "PathPrefix": "kernel/bpf/Makefile"
}

{
  "Output": "kernel/bpf/Makefile=2=obj-y := core.o\nkernel/bpf/Makefile:3:ifneq ($(CONFIG_BPF_JIT_ALWAYS_ON),y)\nkernel/bpf/Makefile-4-# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details\n--\nkernel/bpf/Makefile=7=CFLAGS_core.o += -Wno-override-init $(cflags-nogcse-yy)\nkernel/bpf/Makefile-8-\nkernel/bpf/Makefile:9:obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o cnum.o log.o token.o liveness.o const_fold.o\nkernel/bpf/Makefile:10:obj-$(CONFIG_BPF_SYSCALL) += bpf_iter.o map_iter.o task_iter.o prog_iter.o link_iter.o\nkernel/bpf/Makefile:11:obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o\nkernel/bpf/Makefile:12:obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o bpf_insn_array.o\nkernel/bpf/Makefile:13:obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o\nkernel/bpf/Makefile:14:obj-$(CONFIG_BPF_SYSCALL) += fixups.o cfg.o states.o backtrack.o check_btf.o\nkernel/bpf/Makefile:15:obj-${CONFIG_BPF_LSM}\t  += bpf_inode_storage.o\nkernel/bpf/Makefile:16:obj-$(CONFIG_BPF_SYSCALL) += disasm.o mprog.o\nkernel/bpf/Makefile:17:obj-$(CONFIG_BPF_JIT) += trampoline.o\nkernel/bpf/Makefile:18:obj-$(CONFIG_BPF_SYSCALL) += btf.o memalloc.o rqspinlock.o stream.o\nkernel/bpf/Makefile-19-ifeq ($(CONFIG_MMU)$(CONFIG_64BIT),yy)\nkernel/bpf/Makefile:20:obj-$(CONFIG_BPF_SYSCALL) += arena.o range_tree.o\nkernel/bpf/Makefile-21-endif\nkernel/bpf/Makefile:22:obj-$(CONFIG_BPF_JIT) += dispatcher.o\nkernel/bpf/Makefile-23-ifeq ($(CONFIG_NET),y)\nkernel/bpf/Makefile:24:obj-$(CONFIG_BPF_SYSCALL) += devmap.o\nkernel/bpf/Makefile:25:obj-$(CONFIG_BPF_SYSCALL) += cpumap.o\nkernel/bpf/Makefile:26:obj-$(CONFIG_BPF_SYSCALL) += offload.o\nkernel/bpf/Makefile:27:obj-$(CONFIG_BPF_SYSCALL) += net_namespace.o\nkernel/bpf/Makefile:28:obj-$(CONFIG_BPF_SYSCALL) += tcx.o\nkernel/bpf/Makefile-29-endif\nkernel/bpf/Makefile=30=ifeq ($(CONFIG_PERF_EVENTS),y)\nkernel/bpf/Makefile:31:obj-$(CONFIG_BPF_SYSCALL) += stackmap.o\nkernel/bpf/Makefile-32-endif\nkernel/bpf/Makefile=33=ifeq ($(CONFIG_CGROUPS),y)\nkernel/bpf/Makefile:34:obj-$(CONFIG_BPF_SYSCALL) += cgroup_iter.o bpf_cgrp_storage.o\nkernel/bpf/Makefile-35-endif\n--\nkernel/bpf/Makefile=37=ifeq ($(CONFIG_INET),y)\nkernel/bpf/Makefile:38:obj-$(CONFIG_BPF_SYSCALL) += reuseport_array.o\nkernel/bpf/Makefile-39-endif\n--\nkernel/bpf/Makefile=42=endif\nkernel/bpf/Makefile:43:ifeq ($(CONFIG_BPF_JIT),y)\nkernel/bpf/Makefile:44:obj-$(CONFIG_BPF_SYSCALL) += bpf_struct_ops.o\nkernel/bpf/Makefile:45:obj-$(CONFIG_BPF_SYSCALL) += cpumask.o\nkernel/bpf/Makefile-46-# bpf_lsm_proto.o must precede bpf_lsm.o. The current pahole logic\n--\nkernel/bpf/Makefile-55-# semantics within pahole are revisited accordingly.\nkernel/bpf/Makefile:56:obj-${CONFIG_BPF_LSM} += bpf_lsm_proto.o bpf_lsm.o\nkernel/bpf/Makefile-57-endif\nkernel/bpf/Makefile=58=ifneq ($(CONFIG_CRYPTO),)\nkernel/bpf/Makefile:59:obj-$(CONFIG_BPF_SYSCALL) += crypto.o\nkernel/bpf/Makefile-60-endif\nkernel/bpf/Makefile:61:obj-$(CONFIG_BPF_PRELOAD) += preload/\nkernel/bpf/Makefile-62-\nkernel/bpf/Makefile:63:obj-$(CONFIG_BPF_SYSCALL) += relo_core.o\nkernel/bpf/Makefile:64:obj-$(CONFIG_BPF_SYSCALL) += btf_iter.o\nkernel/bpf/Makefile:65:obj-$(CONFIG_BPF_SYSCALL) += btf_relocate.o\nkernel/bpf/Makefile:66:obj-$(CONFIG_BPF_SYSCALL) += kmem_cache_iter.o\nkernel/bpf/Makefile-67-ifeq ($(CONFIG_DMA_SHARED_BUFFER),y)\nkernel/bpf/Makefile:68:obj-$(CONFIG_BPF_SYSCALL) += dmabuf_iter.o\nkernel/bpf/Makefile-69-endif\n"
}

gemini-3.1-pro-preview

null

null

							input: 5764
							output: 34
							thoughts: 116
						

**Analyzing BPF Kfunc Verification**

Okay, so the configuration I'm looking at involves `CONFIG_BPF_SYSCALL`. My initial thought was to dig into `check_special_kfunc`, as it seems relevant. Hang on, that's a static function. No problem, Syzkaller *can* handle those as long as they're in the symbol table. It's good to know that I'm able to target them, but maybe there's a better starting point?

Now that I think about it, wouldn't `check_kfunc_call` be a more strategic place to start? Hmm, it's *also* static. Alright, no matter, it's still accessible. Let's head over to `kernel/bpf/verifier.c` to examine `check_kfunc_call` and see what insights it reveals.

{
  "Expression": "check_kfunc_call",
  "PathPrefix": "kernel/bpf/verifier.c"
}

{
  "Output": "kernel/bpf/verifier.c=7224=static int process_dynptr_func(struct bpf_verifier_env *env, struct bpf_reg_state *reg,\n--\nkernel/bpf/verifier.c-7298-\t\t\t * For CONST_PTR_TO_DYNPTR, reg is already scratched by check_reg_arg\nkernel/bpf/verifier.c:7299:\t\t\t * in check_helper_call and mark_btf_func_reg_size in check_kfunc_call.\nkernel/bpf/verifier.c-7300-\t\t\t */\n--\nkernel/bpf/verifier.c=12912=static int check_return_code(struct bpf_verifier_env *env, int regno, const char *reg_name);\nkernel/bpf/verifier.c-12913-\nkernel/bpf/verifier.c:12914:static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,\nkernel/bpf/verifier.c-12915-\t\t\t    int *insn_idx_p)\n--\nkernel/bpf/verifier.c=17236=static int do_check_insn(struct bpf_verifier_env *env, bool *do_print_state)\n--\nkernel/bpf/verifier.c-17304-\t\t\tif (insn-\u003esrc_reg == BPF_PSEUDO_KFUNC_CALL)\nkernel/bpf/verifier.c:17305:\t\t\t\treturn check_kfunc_call(env, insn, \u0026env-\u003einsn_idx);\nkernel/bpf/verifier.c-17306-\t\t\treturn check_helper_call(env, insn, \u0026env-\u003einsn_idx);\n"
}

gemini-3.1-pro-preview

null

null

							input: 6552
							output: 40
							thoughts: 129
						

**Analyzing Kernel Function Calls**

Okay, so I'm tracing this logic and it looks like `check_kfunc_call` is triggered by `do_check_insn`. From there, it dives into `check_special_kfunc`. Hmm, interesting. Let's make `check_special_kfunc` the primary focus for now, see where that leads.

Wait, are there any other similar functions? Ah, yes, `btf_type_has_trailing_flex_array`. That's a static one, and relatively recent, so it's probably worth adding to the set of focus symbols as well.

Right, time to go spelunking in the code. I'll need to check the definition of `btf_type_has_trailing_flex_array`, specifically within `kernel/bpf/verifier.c`. Let's see what's going on there.

{
  "Expression": "btf_type_has_trailing_flex_array",
  "PathPrefix": "kernel/bpf/verifier.c"
}

{
  "Output": "kernel/bpf/verifier.c=10919=static bool __btf_type_is_scalar_struct(struct bpf_verifier_env *env,\n--\nkernel/bpf/verifier.c-10957-\nkernel/bpf/verifier.c:10958:static bool btf_type_has_trailing_flex_array(const struct btf *btf,\nkernel/bpf/verifier.c-10959-\t\t\t\t\t     const struct btf_type *t)\n--\nkernel/bpf/verifier.c=12739=static int check_special_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_call_arg_meta *meta,\n--\nkernel/bpf/verifier.c-12775-\t\t}\nkernel/bpf/verifier.c:12776:\t\tif (btf_type_has_trailing_flex_array(ret_btf, ret_t)) {\nkernel/bpf/verifier.c-12777-\t\t\tverbose(env, \"bpf_obj_new type must not contain a flexible array\\n\");\n"
}

gemini-3.1-pro-preview

null

null

							input: 7204
							output: 31
							thoughts: 194
						

**Focusing on BPF Verification for Trailing Flexible Arrays**

Okay, so `btf_type_has_trailing_flex_array` and `check_special_kfunc` are solid starting points for fuzzing. They directly relate to the core functionality being modified in this patch: the BPF verifier's rejection of `bpf_obj_new` calls that try to allocate structs with those trailing arrays. This is a functional change, a pretty significant one, really, to the BPF core logic. The goal is to make sure this new behavior is robust. I need to ensure it handles all the different BTF types correctly, and importantly, that it doesn't introduce any regressions or allow for any bypasses. Fuzzing is definitely the right approach here.

Now, let's just do a quick sanity check and confirm there aren’t any other relevant configs. The key one appears to be `BPF_SYSCALL`, which makes sense. I need to consider `CONFIG_DEBUG_INFO_BTF`, since `bpf_obj_new` relies on BTF. Let me just confirm that `CONFIG_DEBUG_INFO_BTF` is required in the `bpf_obj_new` context. I need to make sure I’m covering all the dependencies in my fuzzing setup.

{
  "Expression": "bpf_obj_new",
  "PathPrefix": "kernel/bpf/"
}

{
  "Output": "kernel/bpf/btf.c=3910=static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,\n--\nkernel/bpf/btf.c-3932-\t\t/* Type exists only in program BTF. Assume that it's a MEM_ALLOC\nkernel/bpf/btf.c:3933:\t\t * kptr allocated via bpf_obj_new\nkernel/bpf/btf.c-3934-\t\t */\n--\nkernel/bpf/helpers.c=2336=__bpf_kfunc_start_defs();\n--\nkernel/bpf/helpers.c-2338-/**\nkernel/bpf/helpers.c:2339: * bpf_obj_new() - allocate an object described by program BTF\nkernel/bpf/helpers.c-2340- * @local_type_id__k: type ID in program BTF\n--\nkernel/bpf/helpers.c-2349- */\nkernel/bpf/helpers.c:2350:__bpf_kfunc void *bpf_obj_new(u64 local_type_id__k, struct btf_struct_meta *meta)\nkernel/bpf/helpers.c-2351-{\n--\nkernel/bpf/helpers.c-2363-\nkernel/bpf/helpers.c:2364:__bpf_kfunc void *bpf_obj_new_impl(u64 local_type_id__k, void *meta__ign)\nkernel/bpf/helpers.c-2365-{\nkernel/bpf/helpers.c:2366:\treturn bpf_obj_new(local_type_id__k, meta__ign);\nkernel/bpf/helpers.c-2367-}\n--\nkernel/bpf/helpers.c=4804=BTF_ID_FLAGS(func, crash_kexec, KF_DESTRUCTIVE)\nkernel/bpf/helpers.c-4805-#endif\nkernel/bpf/helpers.c:4806:BTF_ID_FLAGS(func, bpf_obj_new, KF_ACQUIRE | KF_RET_NULL | KF_IMPLICIT_ARGS)\nkernel/bpf/helpers.c:4807:BTF_ID_FLAGS(func, bpf_obj_new_impl, KF_ACQUIRE | KF_RET_NULL)\nkernel/bpf/helpers.c-4808-BTF_ID_FLAGS(func, bpf_percpu_obj_new, KF_ACQUIRE | KF_RET_NULL | KF_IMPLICIT_ARGS)\n--\nkernel/bpf/verifier.c=6934=enum {\n--\nkernel/bpf/verifier.c-6941- * bpf_map_lookup returns PTR_TO_MAP_VALUE_OR_NULL.\nkernel/bpf/verifier.c:6942: * bpf_obj_new returns PTR_TO_BTF_ID | MEM_ALLOC | PTR_MAYBE_NULL.\nkernel/bpf/verifier.c-6943- * Two bpf_map_lookups (even with the same key) will have different reg-\u003eid.\nkernel/bpf/verifier.c:6944: * Two separate bpf_obj_new will also have different reg-\u003eid.\nkernel/bpf/verifier.c-6945- * For traditional PTR_TO_MAP_VALUE or PTR_TO_BTF_ID | MEM_ALLOC, the verifier\n--\nkernel/bpf/verifier.c-6952- * point to different bpf_spin_locks. Likewise for pointers to allocated objects\nkernel/bpf/verifier.c:6953: * returned from bpf_obj_new.\nkernel/bpf/verifier.c-6954- * The verifier allows taking only one bpf_spin_lock at a time to avoid\n--\nkernel/bpf/verifier.c=11006=enum special_kfunc_type {\nkernel/bpf/verifier.c:11007:\tKF_bpf_obj_new_impl,\nkernel/bpf/verifier.c:11008:\tKF_bpf_obj_new,\nkernel/bpf/verifier.c-11009-\tKF_bpf_obj_drop_impl,\n--\nkernel/bpf/verifier.c=11079=BTF_ID_LIST(special_kfunc_list)\nkernel/bpf/verifier.c:11080:BTF_ID(func, bpf_obj_new_impl)\nkernel/bpf/verifier.c:11081:BTF_ID(func, bpf_obj_new)\nkernel/bpf/verifier.c-11082-BTF_ID(func, bpf_obj_drop_impl)\n--\nkernel/bpf/verifier.c=11173=BTF_ID(func, bpf_stream_print_stack)\nkernel/bpf/verifier.c-11174-\nkernel/bpf/verifier.c:11175:static bool is_bpf_obj_new_kfunc(u32 func_id)\nkernel/bpf/verifier.c-11176-{\nkernel/bpf/verifier.c:11177:\treturn func_id == special_kfunc_list[KF_bpf_obj_new] ||\nkernel/bpf/verifier.c:11178:\t       func_id == special_kfunc_list[KF_bpf_obj_new_impl];\nkernel/bpf/verifier.c-11179-}\n--\nkernel/bpf/verifier.c=11514=static void ref_convert_owning_non_owning(struct bpf_verifier_env *env, u32 id)\n--\nkernel/bpf/verifier.c-11572- * reg-\u003eid is a unique ID preserved after the NULL pointer check on the pointer\nkernel/bpf/verifier.c:11573: * returned from bpf_obj_new. Each allocation receives a new reg-\u003eid.\nkernel/bpf/verifier.c-11574- */\n--\nkernel/bpf/verifier.c=12739=static int check_special_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_call_arg_meta *meta,\n--\nkernel/bpf/verifier.c-12748-\nkernel/bpf/verifier.c:12749:\tif (is_bpf_obj_new_kfunc(meta-\u003efunc_id) || is_bpf_percpu_obj_new_kfunc(meta-\u003efunc_id)) {\nkernel/bpf/verifier.c-12750-\t\tstruct btf_struct_meta *struct_meta;\n--\nkernel/bpf/verifier.c-12753-\nkernel/bpf/verifier.c:12754:\t\tif (is_bpf_obj_new_kfunc(meta-\u003efunc_id) \u0026\u0026 !bpf_global_ma_set)\nkernel/bpf/verifier.c-12755-\t\t\treturn -ENOMEM;\n--\nkernel/bpf/verifier.c-12766-\t\tif (!ret_btf) {\nkernel/bpf/verifier.c:12767:\t\t\tverbose(env, \"bpf_obj_new/bpf_percpu_obj_new requires prog BTF\\n\");\nkernel/bpf/verifier.c-12768-\t\t\treturn -EINVAL;\n--\nkernel/bpf/verifier.c-12772-\t\tif (!ret_t || !__btf_type_is_struct(ret_t)) {\nkernel/bpf/verifier.c:12773:\t\t\tverbose(env, \"bpf_obj_new/bpf_percpu_obj_new type ID argument must be of a struct\\n\");\nkernel/bpf/verifier.c-12774-\t\t\treturn -EINVAL;\n--\nkernel/bpf/verifier.c-12776-\t\tif (btf_type_has_trailing_flex_array(ret_btf, ret_t)) {\nkernel/bpf/verifier.c:12777:\t\t\tverbose(env, \"bpf_obj_new type must not contain a flexible array\\n\");\nkernel/bpf/verifier.c-12778-\t\t\treturn -EINVAL;\n--\nkernel/bpf/verifier.c=12914=static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,\n--\nkernel/bpf/verifier.c-13132-\t\tif (meta.btf != btf_vmlinux ||\nkernel/bpf/verifier.c:13133:\t\t    (!is_bpf_obj_new_kfunc(meta.func_id) \u0026\u0026\nkernel/bpf/verifier.c-13134-\t\t     !is_bpf_percpu_obj_new_kfunc(meta.func_id) \u0026\u0026\n--\nkernel/bpf/verifier.c=19590=int bpf_fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,\n--\nkernel/bpf/verifier.c-19620-\nkernel/bpf/verifier.c:19621:\tif (is_bpf_obj_new_kfunc(desc-\u003efunc_id) || is_bpf_percpu_obj_new_kfunc(desc-\u003efunc_id)) {\nkernel/bpf/verifier.c-19622-\t\tstruct btf_struct_meta *kptr_struct_meta = env-\u003einsn_aux_data[insn_idx].kptr_struct_meta;\n"
}

gemini-3.1-pro-preview

null

null

							input: 10140
							output: 136
							thoughts: 342
						

**My Assessment of the BPF Patch**

Alright, let's break this down. `bpf_obj_new` is a kfunc, got it. The primary config dependency is `CONFIG_BPF_SYSCALL`, but I always double-check these things. Let me see... I'll quickly check the `linked_list_fail.c` selftest, just to be sure there are no other gotchas. Yep, seems like the usual suspects are required: `CONFIG_BPF_SYSCALL`, `CONFIG_DEBUG_INFO_BTF`, and `CONFIG_BPF_JIT`. Okay, I will include `BPF_SYSCALL` and `DEBUG_INFO_BTF`.

Now, I need to confirm that `check_special_kfunc` is reachable. Ah, yes, it's called from `check_kfunc_call`, which in turn is part of the `do_check_insn` verification process for BPF programs. Makes sense. This patch introduces a check for flexible arrays within `bpf_obj_new` calls. Functionally, this is a significant change, right?

Key symbols to keep in mind here are `check_special_kfunc` and `btf_type_has_trailing_flex_array`.

So, the patch fundamentally alters the BPF verifier. It will now reject `bpf_obj_new` calls that try to allocate structs containing trailing flexible arrays. This is core BPF logic being tweaked, and my focus should be on fuzzing this extensively. We need to ensure it handles various BTF types gracefully and that there are no regressions or any sneaky ways to bypass this new check.

Right, let's get the set-results tool running.

{
  "EnableConfigs": [
    "BPF_SYSCALL",
    "DEBUG_INFO_BTF"
  ],
  "FocusSymbols": [
    "check_special_kfunc",
    "btf_type_has_trailing_flex_array"
  ],
  "Reasoning": "The patch modifies the BPF verifier to reject `bpf_obj_new` calls that attempt to allocate structs with trailing flexible arrays. This is a functional change in the core BPF logic that should be fuzzed to ensure it correctly handles various BTF types and doesn't introduce regressions or bypasses.",
  "WorthFuzzing": true
}

{
  "EnableConfigs": [
    "BPF_SYSCALL",
    "DEBUG_INFO_BTF"
  ],
  "FocusSymbols": [
    "check_special_kfunc",
    "btf_type_has_trailing_flex_array"
  ],
  "Reasoning": "The patch modifies the BPF verifier to reject `bpf_obj_new` calls that attempt to allocate structs with trailing flexible arrays. This is a functional change in the core BPF logic that should be fuzzed to ensure it correctly handles various BTF types and doesn't introduce regressions or bypasses.",
  "WorthFuzzing": true
}