Re: [PATCH bpf-next 4/7] s390: bpf: Fix off-by-one in tail call count limiting

[Johan Almbladh <johan.almbladh@anyfinetworks.com>]

On Mon, Aug 9, 2021 at 2:24 PM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> On Mon, 2021-08-09 at 11:34 +0200, Johan Almbladh wrote:
> > Before, the eBPF JIT allowed up to MAX_TAIL_CALL_CNT + 1 tail calls.
> > Now, precisely MAX_TAIL_CALL_CNT is allowed, which is in line with the
> > behaviour of the interpreter. Verified with the test_bpf test suite
> > on qemu-system-s390x.
> >
> > Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
> > ---
> >  arch/s390/net/bpf_jit_comp.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/s390/net/bpf_jit_comp.c
> > b/arch/s390/net/bpf_jit_comp.c
> > index 88419263a89a..f6cdf13285ed 100644
> > --- a/arch/s390/net/bpf_jit_comp.c
> > +++ b/arch/s390/net/bpf_jit_comp.c
> > @@ -1363,7 +1363,7 @@ static noinline int bpf_jit_insn(struct bpf_jit
> > *jit, struct bpf_prog *fp,
> >                                  jit->prg);
> >
> >                 /*
> > -                * if (tail_call_cnt++ > MAX_TAIL_CALL_CNT)
> > +                * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
> >                  *         goto out;
> >                  */
> >
> > @@ -1377,8 +1377,8 @@ static noinline int bpf_jit_insn(struct bpf_jit
> > *jit, struct bpf_prog *fp,
> >                 EMIT6_DISP_LH(0xeb000000, 0x00fa, REG_W1, REG_W0,
> > REG_15, off);
> >                 /* clij %w1,MAX_TAIL_CALL_CNT,0x2,out */
>
> This comment needs to be updated as well.
>
> >                 patch_2_clij = jit->prg;
> > -               EMIT6_PCREL_RIEC(0xec000000, 0x007f, REG_W1,
> > MAX_TAIL_CALL_CNT,
> > -                                2, jit->prg);
> > +               EMIT6_PCREL_RIEC(0xec000000, 0x007f, REG_W1,
> > +                                MAX_TAIL_CALL_CNT - 1, 2, jit->prg);
> >
> >                 /*
> >                  * prog = array->ptrs[index];
>
> With that:
>
> Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
> Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
>

Fixing it. Thanks!