Re: [PATCH net-next v2 0/2] bpf: support to trace BPF_JNE

[Menglong Dong]

On Wed, Dec 13, 2023 at 8:00 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Dec 12, 2023 at 5:15 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
> >
> > For now, the reg bounds is not handled for BPF_JNE case, which can cause
> > the failure of following case:
> >
> >   /* The type of "a" is u16 */
> >   if (a > 0 && a < 100) {
> >     /* the range of the register for a is [0, 99], not [1, 99],
> >      * and will cause the following error:
> >      *
> >      *   invalid zero-sized read
> >      *
> >      * as a can be 0.
> >      */
> >     bpf_skb_store_bytes(skb, xx, xx, a, 0);
> >   }
> >
> > In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
> > TRUE branch, the dst_reg will be marked as known to 0. However, in the
> > fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
> > the [min, max] for a is [0, 99], not [1, 99].
> >
> > In the 1st patch, we reduce the range of the dst reg if the src reg is a
> > const and is exactly the edge of the dst reg For BPF_JNE.
> >
> > In the 2nd patch, we just activate the test case for this logic in
> > range_cond(), which is committed by Andrii in the
> > commit 8863238993e2 ("selftests/bpf: BPF register range bounds tester").
> >
> > Changes since v1:
> > - simplify the code in the 1st patch
> > - introduce the 2nd patch for the testing
> >
> > Menglong Dong (2):
> >   bpf: make the verifier trace the "not qeual" for regs
> >   selftests/bpf: activate the OP_NE login in range_cond()
> >
> >  kernel/bpf/verifier.c                         | 29 ++++++++++++++++++-
> >  .../selftests/bpf/prog_tests/reg_bounds.c     |  7 +----
> >  2 files changed, 29 insertions(+), 7 deletions(-)
> >
> > --
> > 2.39.2
> >
>
> +1 to all the feedback from Eduard. Besides that, please target
> bpf-next tree (so, [PATH bpf-next] for subject prefix), thanks!
>

Opps, sorry that I offered a wrong tag......:/

> Also, instead of "verifier traces", I think "verifier tracks" is less
> confusing wording. Tracing within the BPF ecosystem is usually used
> for a completely different meaning.
>

Yeah, sounds better.

> Oh, and just to keep feedback in one place. In patch #2 you have a
> typo in the subject "not qeual" -> "not equal".

Ok, I'll fix it in the next version.

Thanks!
Menglong Dong