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

[Tao Chen]

在 2025/6/13 08:06, Andrii Nakryiko 写道:
> On Thu, Jun 12, 2025 at 4:56 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> > On Thu, Jun 12, 2025 at 4:27 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > > On Thu, Jun 12, 2025 at 2:40 PM Alexei Starovoitov
> > > <alexei.starovoitov@xxxxxxxxx> wrote:
> > > > On Thu, Jun 12, 2025 at 2:29 PM Andrii Nakryiko
> > > > <andrii.nakryiko@xxxxxxxxx> wrote:
> > > > > 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.
> > > >
> > > > All old helpers had this BPF_F_PAD_ZEROS behavior
> > > > (or rather should have had).
> > > > So it makes sense to zero in this helper too for consistency.
> > > > I don't share performance concerns. This is an error path.
> > >
> > > It's just a bizarre behavior as it stands right now.
> > >
> > > On error, you'll have a zeroed out buffer, OK, good so far.
> > >
> > > On success, though, you'll have a buffer where first N bytes are
> > > filled out with good path information, but then the last sizeof(buf) -
> > > N bytes would be, effectively, garbage.
> > >
> > > All in all, you can't use that buffer as a key for hashmap looking
> > > (because of leftover non-zeroed bytes at the end), yet on error we
> > > still zero out bytes for no apparently useful reason.
> > >
> > > And then for the bpf_path_d_path(). What do we do about that one? It
> > > doesn't have zeroing out either in the error path, nor in the success
> > > path. So just more inconsistency all around.
> >
> > Consistency with bpf_path_d_path() kfunc is indeed missing.
> >
> > Ok, since you insist, dropped this patch, and force pushed.
>
> Great, thank you!

The changes in this patch are relatively simple, but the discussion 
between the two of you is more meaningful to me. I agree with Andrii's 
point of view. Thank you both for discussing this patch.

--
Best Regards
Tao Chen