Re: [PATCH bpf-next] bpf: clear user buf when bpf_d_path failed

[Andrii Nakryiko]

On Wed, Jun 11, 2025 at 8:49 AM Tao Chen <chen.dylane@xxxxxxxxx> wrote:
>
> The bpf_d_path() function may fail. If it does,
> clear the user buf, like bpf_probe_read etc.
>

But that doesn't mean we *have to* do memset(0) for bpf_d_path(),
though. Especially given that path buffer can be pretty large (4KB).

Is there an issue you are trying to address with this, or is it more
of a consistency clean up? Note, that more or less recently we made
this zero filling behavior an option with an extra flag
(BPF_F_PAD_ZEROS) for newer APIs. And if anything, bpf_d_path() is
more akin to variable-sized string probing APIs rather than
fixed-sized bpf_probe_read* family.

In short, I feel like we should revert this and let users do
zero-filling, if they really need to. bpf_probe_read_kernel(dst, sz,
NULL) would do. But we should think about adding dynptr-based
bpf_dynptr_memset() API for cases when the size is not known
statically, IMO.


> Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> ---
>  kernel/trace/bpf_trace.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 0998cbbb963..bb1003cb271 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -916,11 +916,14 @@ BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, u32, sz)
>          * potentially broken verifier.
>          */
>         len = copy_from_kernel_nofault(&copy, path, sizeof(*path));
> -       if (len < 0)
> +       if (len < 0) {
> +               memset(buf, 0, sz);
>                 return len;
> +       }
>
>         p = d_path(&copy, buf, sz);
>         if (IS_ERR(p)) {
> +               memset(buf, 0, sz);
>                 len = PTR_ERR(p);
>         } else {
>                 len = buf + sz - p;
> --
> 2.48.1
>
>