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From: Daniel Borkmann <daniel@iogearbox.net>
To: Alexei Starovoitov <ast@kernel.org>, davem@davemloft.net
Cc: kpsingh@google.com, jannh@google.com, netdev@vger.kernel.org,
	bpf@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH bpf-next] bpf: Fix trampoline generation for fmod_ret programs
Date: Wed, 11 Mar 2020 14:23:12 +0100	[thread overview]
Message-ID: <bcf81455-6b78-1ff2-d5db-3b203696b8f0@iogearbox.net> (raw)
In-Reply-To: <20200311003906.3643037-1-ast@kernel.org>

On 3/11/20 1:39 AM, Alexei Starovoitov wrote:
> fmod_ret progs are emitted as:
> 
> start = __bpf_prog_enter();
> call fmod_ret
> *(u64 *)(rbp - 8) = rax
> __bpf_prog_exit(, start);
> test eax, eax
> jne do_fexit
> 
> That 'test eax, eax' is working by accident. The compiler is free to use rax
> inside __bpf_prog_exit() or inside functions that __bpf_prog_exit() is calling.
> Which caused "test_progs -t modify_return" to sporadically fail depending on
> compiler version and kconfig. Fix it by using 'cmp [rbp - 8], 0' instead of
> 'test eax, eax'.
> 
> Fixes: ae24082331d9 ("bpf: Introduce BPF_MODIFY_RETURN")
> Reported-by: Andrii Nakryiko <andriin@fb.com>
> Acked-by: Andrii Nakryiko <andriin@fb.com>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Applied, thanks!

      parent reply	other threads:[~2020-03-11 13:23 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-11  0:39 [PATCH bpf-next] bpf: Fix trampoline generation for fmod_ret programs Alexei Starovoitov
2020-03-11  1:15 ` KP Singh
2020-03-11 13:23 ` Daniel Borkmann [this message]

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