From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Kernel Team <kernel-team@fb.com>,
John Fastabend <john.fastabend@gmail.com>,
Jesper Dangaard Brouer <brouer@redhat.com>
Subject: Re: [PATCH bpf-next 1/2] bpf: fix a verifier failure with xor
Date: Wed, 02 Sep 2020 11:33:09 +0200 [thread overview]
Message-ID: <87d034ikve.fsf@toke.dk> (raw)
In-Reply-To: <465da51a-793e-5ea0-85dc-56ab4f36ae34@fb.com>
Yonghong Song <yhs@fb.com> writes:
> On 9/1/20 1:07 PM, Andrii Nakryiko wrote:
>> On Mon, Aug 24, 2020 at 11:47 PM Yonghong Song <yhs@fb.com> wrote:
>>>
>>> bpf selftest test_progs/test_sk_assign failed with llvm 11 and llvm 12.
>>> Compared to llvm 10, llvm 11 and 12 generates xor instruction which
>>
>> Does this mean that some perfectly working BPF programs will now fail
>> to verify on older kernels, if compiled with llvm 11 or llvm 12? If
>
> Right.
>
>> yes, is there something that one can do to prevent Clang from using
>> xor in such situations?
>
> The xor is generated by the combination of llvm simplifyCFG and
> instrCombine phase.
>
> The following is a hack to prevent compiler from generating xor's.
Wait, so this means that we can no longer tell people to just use the
newest LLVM version - now we have to keep track of a minimum *and*
maximum LLVM version for each kernel version?
Could we maybe try to not *keep* making it harder for people to use BPF? :/
As for the patch, sure, make the verifier smarter, but I also feel like
LLVM should be fixed to not suddenly emit such xor instructions...
-Toke
next prev parent reply other threads:[~2020-09-02 9:33 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-25 6:46 [PATCH bpf-next 0/2] fix a verifier failure with xor Yonghong Song
2020-08-25 6:46 ` [PATCH bpf-next 1/2] bpf: " Yonghong Song
2020-08-26 1:58 ` Alexei Starovoitov
2020-08-26 3:36 ` Yonghong Song
2020-08-26 22:06 ` John Fastabend
2020-08-27 5:12 ` Alexei Starovoitov
2020-08-27 18:43 ` John Fastabend
2020-09-01 20:07 ` Andrii Nakryiko
2020-09-02 2:17 ` Yonghong Song
2020-09-02 5:27 ` John Fastabend
2020-09-02 5:43 ` Yonghong Song
2020-09-04 5:29 ` John Fastabend
2020-09-02 9:33 ` Toke Høiland-Jørgensen [this message]
2020-09-02 14:21 ` Alexei Starovoitov
2020-09-02 15:01 ` Toke Høiland-Jørgensen
2020-09-02 21:40 ` Alexei Starovoitov
2020-08-25 6:46 ` [PATCH bpf-next 2/2] selftests/bpf: add verifier tests for xor operation Yonghong Song
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