On Tue, Dec 19, 2023 at 2:03 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Sun, Dec 17, 2023 at 5:18 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
> >
> > Add testcase for the logic that the verifier tracks the BPF_JNE for regs.
> > The assembly function "reg_not_equal()" that we add is exactly converted
> > from the following case:
> >
> > u32 a = bpf_get_prandom_u32();
> > u64 b = 0;
> >
> > a %= 8;
> > /* the "a > 0" here will be optimized to "a != 0" */
> > if (a > 0) {
> > /* now the range of a should be [1, 7] */
> > bpf_skb_store_bytes(skb, 0, &b, a, 0);
> > }
> >
> > Signed-off-by: Menglong Dong <menglong8.dong@xxxxxxxxx>
> > ---
> > .../selftests/bpf/progs/verifier_bounds.c | 27 +++++++++++++++++++
> > 1 file changed, 27 insertions(+)
> >
>
> LGTM, but please add a comment that we rely on bpf_skb_store_byte's
> 4th argument being defined as ARG_CONST_SIZE, so zero is not allowed.
> And that r4 == 0 check is providing us this exclusion of zero from
> initial [0, 7] range.
>
Okay, sounds great! BTW, should I add such a comment to the
commit log or to the assembly function?
>
> > diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
> > index ec430b71730b..3fe2ce2b3f21 100644
> > --- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
> > +++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
> > @@ -1075,4 +1075,31 @@ l0_%=: r0 = 0; \
> > : __clobber_all);
> > }
> >
> > +SEC("tc")
> > +__description("bounds check with JMP_NE for reg edge")
> > +__success __retval(0)
> > +__naked void reg_not_equal(void)
>
> technically, you are testing `r4 == 0` :) so maybe call the test
> reg_equal_const or something. And then add similar test where you
> actually have `r4 != 0`, called req_no_equal_const?
>
Yeah, that makes sense. I'll add such a test in the next version.
Thanks!
Menglong Dong
> > +{
> > + asm volatile (" \
> > + r6 = r1; \
> > + r1 = 0; \
> > + *(u64*)(r10 - 8) = r1; \
> > + call %[bpf_get_prandom_u32]; \
> > + r4 = r0; \
> > + r4 &= 7; \
> > + if r4 == 0 goto l0_%=; \
> > + r1 = r6; \
> > + r2 = 0; \
> > + r3 = r10; \
> > + r3 += -8; \
> > + r5 = 0; \
> > + call %[bpf_skb_store_bytes]; \
> > +l0_%=: r0 = 0; \
> > + exit; \
> > +" :
> > + : __imm(bpf_get_prandom_u32),
> > + __imm(bpf_skb_store_bytes)
> > + : __clobber_all);
> > +}
> > +
> > char _license[] SEC("license") = "GPL";
> > --
> > 2.39.2
> >
