On Thu, Jul 29, 2021 at 3:29 PM Andrii Nakryiko
[off-list ref] wrote:

On Thu, Jul 29, 2021 at 2:38 PM Johan Almbladh
[off-list ref] wrote:

quoted

On Wed, Jul 28, 2021 at 9:13 PM Yonghong Song [off-list ref] wrote:

quoted

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.