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

[Andrii Nakryiko]

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!

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.

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