Re: [PATCH] bpf: Fix off-by-one in tail call count limiting

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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.

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.