Re: [PATCH bpf-next 03/10] bpf: Support stack arguments for bpf functions

bot+bpf-ci
2026-04-02T03:18:39Z

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> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 090aa26d1c98..a260610cd1c1 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h

[ ... ]

> @@ -319,6 +324,10 @@ struct bpf_func_state {
> * `stack`. allocated_stack is always a multiple of BPF_REG_SIZE.
> */
> int allocated_stack;
> +
> + u16 stack_arg_depth; /* Size of incoming + max outgoing stack args in bytes. */
> + u16 incoming_stack_arg_depth; /* Size of incoming stack args in bytes. */
> + struct bpf_stack_arg_state *stack_arg_slots;
> };

The new stack_arg_slots state is part of bpf_func_state and is
properly copied in copy_stack_state() and freed in free_func_state(),
but stacksafe() does not compare it during state pruning.

stacksafe() only iterates the regular stack:

kernel/bpf/verifier.c:stacksafe() {
for (i = 0; i < old->allocated_stack; i++) {
...
old->stack[spi].slot_type[...]
...
}
return true;
}

It never examines old->stack_arg_slots or cur->stack_arg_slots. This
means func_states_equal() -> stacksafe() treats two states as
equivalent even when they differ in their stack arg slot contents.

Could this allow the verifier to incorrectly prune a path where
outgoing stack arg slots are uninitialized? For example, if a BPF
program has two paths to a call instruction -- one that writes the
outgoing stack args and one that does not -- the verifier could
explore the initialized path first, save the state at a checkpoint,
then prune the uninitialized path as equivalent. The initialization
check in set_callee_state():

kernel/bpf/verifier.c:set_callee_state() {
for (i = 0; i < callee_incoming_slots; i++) {
int caller_spi = i + caller_incoming_slots;

if (!is_stack_arg_slot_initialized(caller, caller_spi)) {
verbose(env, "stack arg#%d not properly initialized\n",
i + 1 + MAX_BPF_FUNC_REG_ARGS);
return -EINVAL;
}
callee->stack_arg_slots[i] = caller->stack_arg_slots[caller_spi];
}
}

would never run on the pruned path, potentially allowing the callee
to receive uninitialized data.

It seems like stacksafe() needs a companion loop for stack_arg_slots,
similar to how it walks the regular stack, to maintain verifier
soundness for this new state.

---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/23879588767