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

[Andrii Nakryiko]

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.