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 <ftyg@live.com>
Signed-off-by: Zesen Liu <ftyg@live.com>
Co-developed-by: Peili Gao <gplhust955@gmail.com>
Signed-off-by: Peili Gao <gplhust955@gmail.com>
Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Shuran Liu <electronlsr@gmail.com>
---
 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

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 <ftyg@live.com>
> Signed-off-by: Zesen Liu <ftyg@live.com>
> Co-developed-by: Peili Gao <gplhust955@gmail.com>
> Signed-off-by: Peili Gao <gplhust955@gmail.com>
> Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Shuran Liu <electronlsr@gmail.com>

Reviewed-by: Matt Bobrowski <mattbobrowski@google.com>

> ---
>  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
>