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

On Tue, Dec 12, 2023 at 5:15 AM Menglong Dong <menglong8.dong@gmail.com> 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".

On Wed, Dec 13, 2023 at 8:00 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Tue, Dec 12, 2023 at 5:15 AM Menglong Dong <menglong8.dong@gmail.com> 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