linux-kernel - Re: [PATCH bpf-next] bpf: clear user buf when bpf_d

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Message-ID: <CAEf4BzayBd9e5c9fiEPgDKPoRm-E4uB_u__xKcRpXDz18kNnkA@mail.gmail.com>
Date: Thu, 12 Jun 2025 16:27:37 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Tao Chen <chen.dylane@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Matt Bobrowski <mattbobrowski@...gle.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: clear user buf when bpf_d_path failed

On Thu, Jun 12, 2025 at 2:40 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Jun 12, 2025 at 2:29 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Wed, Jun 11, 2025 at 8:49 AM Tao Chen <chen.dylane@...ux.dev> 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.