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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Cc: Yonghong Song <yhs@fb.com>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Tony Ambardar <Tony.Ambardar@gmail.com>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH] bpf: Fix off-by-one in tail call count limiting
Date: Thu, 29 Jul 2021 15:48:40 -0700	[thread overview]
Message-ID: <CAEf4BzZ1nNv12s-NJEayct5Yih_G6vNkEvFPst6dLcbhxWV_0g@mail.gmail.com> (raw)
In-Reply-To: <CAEf4BzbYbSAqU91r8RzXWWR81mq9kwJ0=r8-1aRU1UaeDqxMeg@mail.gmail.com>

On Thu, Jul 29, 2021 at 3:29 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Thu, Jul 29, 2021 at 2:38 PM Johan Almbladh
> <johan.almbladh@anyfinetworks.com> wrote:
> >
> > On Wed, Jul 28, 2021 at 9:13 PM Yonghong Song <yhs@fb.com> wrote:
> > > I also checked arm/arm64 jit. I saw the following comments:
> > >
> > >          /* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
> > >           *      goto out;
> > >           * tail_call_cnt++;
> > >           */
> > >
> > > Maybe we have this MAX_TAIL_CALL_CNT + 1 issue
> > > for arm/arm64 jit?
> >
> > That wouldn't be unreasonable. I don't have an arm or arm64 setup
> > available right now, but I can try to test it in qemu.
>
> On a brief check, there seems to be quite a mess in terms of the code
> and comments.
>
> E.g., in arch/x86/net/bpf_jit_comp32.c:
>
>         /*
>          * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
>          *     goto out;
>          */
>
>                             ^^^^ here comment is wrong
>
>         [...]
>
>         /* cmp edx,hi */
>         EMIT3(0x83, add_1reg(0xF8, IA32_EBX), hi);
>         EMIT2(IA32_JNE, 3);
>         /* cmp ecx,lo */
>         EMIT3(0x83, add_1reg(0xF8, IA32_ECX), lo);
>
>         /* ja out */
>         EMIT2(IA32_JAE, jmp_label(jmp_label1, 2));
>
>         ^^^ JAE is >=, right? But the comment says JA.
>
>
> As for arch/x86/net/bpf_jit_comp.c, both comment and the code seem to
> do > MAX_TAIL_CALL_CNT, but you are saying JIT is correct. What am I
> missing?
>
> Can you please check all the places where MAX_TAIL_CALL_CNT is used
> throughout the code? Let's clean this up in one go.
>
> Also, given it's so easy to do this off-by-one error, can you please
> add a negative test validating that 33 tail calls are not allowed? I
> assume we have a positive test that allows exactly MAX_TAIL_CALL_CNT,
> but please double-check that as well.

Ok, I see that you've added this in your bpf tests patch set. Please
consider, additionally, implementing a similar test as part of
selftests/bpf (specifically in test_progs). We run test_progs
continuously in CI for every incoming patch/patchset, so it has much
higher chances of capturing any regressions.

I'm also thinking that this MAX_TAIL_CALL_CNT change should probably
go into the bpf-next tree. First, this off-by-one behavior was around
for a while and it doesn't cause serious issues, even if abused. But
on the other hand, it will make your tail call tests fail, when
applied into bpf-next without your change. So I think we should apply
both into bpf-next.

On a related topic, please don't forget to include the target kernel
tree for your patches: [PATCH bpf] or [PATCH bpf-next].


>
> I also wonder if it would make sense to convert these
> internal-but-sort-of-advertised constants like MAX_TAIL_CALL_CNT and
> BPF_COMPLEXITY_LIMIT_INSNS into enums so that they can be "discovered"
> from BTF. This should be discussed/attempted outside of this fix,
> though. Just bringing it up here.

  reply	other threads:[~2021-07-29 22:49 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-26  8:17 [RFC PATCH 00/14] bpf/tests: Extend the eBPF test suite Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 01/14] bpf/tests: add BPF_JMP32 test cases Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 02/14] bpf/tests: add BPF_MOV tests for zero and sign extension Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 03/14] bpf/tests: fix typos in test case descriptions Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 04/14] bpf/tests: add more tests of ALU32 and ALU64 bitwise operations Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 05/14] bpf/tests: add more ALU32 tests for BPF_LSH/RSH/ARSH Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 06/14] bpf/tests: add more BPF_LSH/RSH/ARSH tests for ALU64 Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 07/14] bpf/tests: add more ALU64 BPF_MUL tests Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 08/14] bpf/tests: add tests for ALU operations implemented with function calls Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 09/14] bpf/tests: add word-order tests for load/store of double words Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 10/14] bpf/tests: add branch conversion JIT test Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 11/14] bpf/tests: add test for 32-bit context pointer argument passing Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 12/14] bpf/tests: add tests for atomic operations Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 13/14] bpf/tests: add tests for BPF_CMPXCHG Johan Almbladh
2021-07-26  8:17 ` [RFC PATCH 14/14] bpf/tests: add tail call test suite Johan Almbladh
2021-07-26 22:53 ` [RFC PATCH 00/14] bpf/tests: Extend the eBPF " Andrii Nakryiko
2021-07-28  8:27   ` Daniel Borkmann
2021-07-28 12:15     ` Johan Almbladh
2021-07-28 16:47     ` [PATCH] bpf: Fix off-by-one in tail call count limiting Johan Almbladh
2021-07-28 19:13       ` Yonghong Song
2021-07-29 21:37         ` Johan Almbladh
2021-07-29 22:29           ` Andrii Nakryiko
2021-07-29 22:48             ` Andrii Nakryiko [this message]
2021-08-01  8:37               ` Johan Almbladh
2021-08-02 20:28                 ` Andrii Nakryiko
2021-08-05 14:37                   ` Johan Almbladh
2021-08-05 22:54                     ` Andrii Nakryiko

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