linux-kernel - Re: [PATCH] bpf: relax acquire for consumer_pos in ringbuf_process

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <SJ0PR11MB82960FC0F6259C67733F8415F82DA@SJ0PR11MB8296.namprd11.prod.outlook.com>
Date: Wed, 6 Aug 2025 19:43:20 +0000
From: Soham Bagchi <soham.bagchi@...h.edu>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
CC: 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>, KP Singh <kpsingh@...nel.org>, Stanislav Fomichev
	<sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Soham Bagchi
	<sohambagchi@...look.com>
Subject: Re: [PATCH] bpf: relax acquire for consumer_pos in
 ringbuf_process_ring()

I have the following reasons:

1. ringbuf is an MPSC, there will only be a single
consumer thread. Therefore, only a single thread
is modifying the consumer_pos variable. The only
functions that modify consumer_pos are _peek(),
_sample_release() and _process() in libbpf.

2. The producer functions should continue to
load-acquire the consumer_pos variable since it
could be executing on a different thread. These
would pair with the store_releases mentioned in (1).
Even while the BUSY_BIT is set, conflicting functions
are producer functions, which don't modify the
consumer_pos variable anyway, they modify
producer_pos [1].

3. Since the single user consumer thread is modifying
consumer_pos, and the code lines in question are
lines where the same user consumer thread would
read consumer_pos, the thread-locality of the loads
and stores mitigate the need for user-side
load-acquires.

Refs:
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/bpf/ringbuf.c?h=v6.16#n456

________________________________________
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
Sent: Thursday, July 31, 2025 10:46 AM
To: Soham Bagchi
Cc: Alexei Starovoitov; Daniel Borkmann; Andrii Nakryiko; Martin KaFai Lau; Eduard; Song Liu; Yonghong Song; John Fastabend; KP Singh; Stanislav Fomichev; Hao Luo; Jiri Olsa; bpf; LKML; Soham Bagchi
Subject: Re: [PATCH] bpf: relax acquire for consumer_pos in ringbuf_process_ring()

On Wed, Jul 30, 2025 at 11:53 AM Soham Bagchi <soham.bagchi@...h.edu> wrote:
>
> Since r->consumer_pos is modified only by the user thread
> in the given ringbuf context (and as such, it is thread-local)
> it does not require a load-acquire.
>
> Signed-off-by: Soham Bagchi <soham.bagchi@...h.edu>
> ---
>  tools/lib/bpf/ringbuf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> index 9702b70da44..7753a6570cf 100644
> --- a/tools/lib/bpf/ringbuf.c
> +++ b/tools/lib/bpf/ringbuf.c
> @@ -241,7 +241,7 @@ static int64_t ringbuf_process_ring(struct ring *r, size_t n)
>         bool got_new_data;
>         void *sample;
>
> -       cons_pos = smp_load_acquire(r->consumer_pos);
> +       cons_pos = *r->consumer_pos;

I don't think it's correct.
See comment in __bpf_user_ringbuf_sample_release()

--
pw-bot: cr