From: Eduard Zingerman <eddyz87@gmail.com>
To: Yonghong Song <yonghong.song@linux.dev>, bpf@vger.kernel.org
Cc: Alexei Starovoitov <ast@kernel.org>,
Andrii Nakryiko <andrii@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
kernel-team@fb.com, Martin KaFai Lau <martin.lau@kernel.org>,
Emil Tsalapatis <etsal@meta.com>
Subject: Re: [PATCH bpf-next 1/3] bpf: Allow 'may_goto 0' instruction
Date: Thu, 16 Jan 2025 11:23:35 -0800 [thread overview]
Message-ID: <ba55fbfc5333a9604b482a62686b9aa8652d47bd.camel@gmail.com> (raw)
In-Reply-To: <20250116055129.604354-1-yonghong.song@linux.dev>
On Wed, 2025-01-15 at 21:51 -0800, Yonghong Song wrote:
> Commit 011832b97b31 ("bpf: Introduce may_goto instruction") added support
> for may_goto insn. The 'may_goto 0' insn is disallowed since the insn is
> equivalent to a nop as both branch will go to the next insn.
>
> But it is possible that compiler transformation may generate 'may_goto 0'
> insn. Emil Tsalapatis from Meta reported such a case which caused
> verification failure. For example, for the following code,
> int i, tmp[3];
> for (i = 0; i < 3 && can_loop; i++)
> tmp[i] = 0;
> ...
>
> clang 20 may generate code like
> may_goto 2;
> may_goto 1;
> may_goto 0;
> r1 = 0; /* tmp[0] = 0; */
> r2 = 0; /* tmp[1] = 0; */
> r3 = 0; /* tmp[2] = 0; */
>
> Let us permit 'may_goto 0' insn to avoid verification failure for codes
> like the above.
>
> Reported-by: Emil Tsalapatis <etsal@meta.com>
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
[...]
next prev parent reply other threads:[~2025-01-16 19:23 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-01-16 5:51 [PATCH bpf-next 0/3] bpf: Allow 'may_goto 0' instruction Yonghong Song
2025-01-16 5:51 ` [PATCH bpf-next 1/3] " Yonghong Song
2025-01-16 19:23 ` Eduard Zingerman [this message]
2025-01-16 5:51 ` [PATCH bpf-next 2/3] bpf: Remove " Yonghong Song
2025-01-16 19:42 ` Eduard Zingerman
2025-01-17 1:45 ` Alexei Starovoitov
2025-01-17 3:43 ` Yonghong Song
2025-01-16 5:51 ` [PATCH bpf-next 3/3] selftests/bpf: Add some tests related to 'may_goto 0' insns Yonghong Song
2025-01-16 19:49 ` Eduard Zingerman
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