On Mon, Dec 01, 2025 at 10:38:12PM +0800, Shuran Liu wrote:
> Commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type
> tracking") started distinguishing read vs write accesses performed by
> helpers.
>
> The second argument of bpf_d_path() is a pointer to a buffer that the
> helper fills with the resulting path. However, its prototype currently
> uses ARG_PTR_TO_MEM without MEM_WRITE.
>
> Before 37cce22dbd51, helper accesses were conservatively treated as
> potential writes, so this mismatch did not cause issues. Since that
> commit, the verifier may incorrectly assume that the buffer contents
> are unchanged across the helper call and base its optimizations on this
> wrong assumption. This can lead to misbehaviour in BPF programs that
> read back the buffer, such as prefix comparisons on the returned path.
>
> Fix this by marking the second argument of bpf_d_path() as
> ARG_PTR_TO_MEM | MEM_WRITE so that the verifier correctly models the
> write to the caller-provided buffer.
>
> Fixes: 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking")
> Co-developed-by: Zesen Liu <[email protected]>
> Signed-off-by: Zesen Liu <[email protected]>
> Co-developed-by: Peili Gao <[email protected]>
> Signed-off-by: Peili Gao <[email protected]>
> Co-developed-by: Haoran Ni <[email protected]>
> Signed-off-by: Haoran Ni <[email protected]>
> Signed-off-by: Shuran Liu <[email protected]>
Reviewed-by: Matt Bobrowski <[email protected]>
> ---
> kernel/trace/bpf_trace.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 4f87c16d915a..49e0bdaa7a1b 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -965,7 +965,7 @@ static const struct bpf_func_proto bpf_d_path_proto = {
> .ret_type = RET_INTEGER,
> .arg1_type = ARG_PTR_TO_BTF_ID,
> .arg1_btf_id = &bpf_d_path_btf_ids[0],
> - .arg2_type = ARG_PTR_TO_MEM,
> + .arg2_type = ARG_PTR_TO_MEM | MEM_WRITE,
> .arg3_type = ARG_CONST_SIZE_OR_ZERO,
> .allowed = bpf_d_path_allowed,
> };
> --
> 2.52.0
>
