Re: [bpf-next] selftests/bpf: refactor snprintf_btf test to use bpf_strncmp

[Hoyeon Lee]

On Thu, Nov 6, 2025 at 8:43 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Wed, Nov 5, 2025 at 3:38 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
> >
> > On Wed, 2025-11-05 at 15:33 -0800, Alexei Starovoitov wrote:
> > > On Wed, Nov 5, 2025 at 2:52 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
> > > >
> > > > On Wed, 2025-11-05 at 14:45 -0800, Alexei Starovoitov wrote:
> > > > > On Wed, Nov 5, 2025 at 12:14 PM Hoyeon Lee <hoyeon.lee@xxxxxxxx> wrote:
> > > > > >
> > > > > > The netif_receive_skb BPF program used in snprintf_btf test still uses
> > > > > > a custom __strncmp. This is unnecessary as the bpf_strncmp helper is
> > > > > > available and provides the same functionality.
> > > > > >
> > > > > > This commit refactors the test to use the bpf_strncmp helper, removing
> > > > > > the redundant custom implementation.
> > > > > >
> > > > > > Signed-off-by: Hoyeon Lee <hoyeon.lee@xxxxxxxx>
> > > > > > ---
> > > > > >  .../selftests/bpf/progs/netif_receive_skb.c       | 15 +--------------
> > > > > >  1 file changed, 1 insertion(+), 14 deletions(-)
> > > > > >
> > > > > > diff --git a/tools/testing/selftests/bpf/progs/netif_receive_skb.c b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
> > > > > > index 9e067dcbf607..186b8c82b9e6 100644
> > > > > > --- a/tools/testing/selftests/bpf/progs/netif_receive_skb.c
> > > > > > +++ b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
> > > > > > @@ -31,19 +31,6 @@ struct {
> > > > > >         __type(value, char[STRSIZE]);
> > > > > >  } strdata SEC(".maps");
> > > > > >
> > > > > > -static int __strncmp(const void *m1, const void *m2, size_t len)
> > > > > > -{
> > > > > > -       const unsigned char *s1 = m1;
> > > > > > -       const unsigned char *s2 = m2;
> > > > > > -       int i, delta = 0;
> > > > > > -
> > > > > > -       for (i = 0; i < len; i++) {
> > > > > > -               delta = s1[i] - s2[i];
> > > > > > -               if (delta || s1[i] == 0 || s2[i] == 0)
> > > > > > -                       break;
> > > > > > -       }
> > > > > > -       return delta;
> > > > > > -}
> > > > > >
> > > > > >  #if __has_builtin(__builtin_btf_type_id)
> > > > > >  #define        TEST_BTF(_str, _type, _flags, _expected, ...)                   \
> > > > > > @@ -69,7 +56,7 @@ static int __strncmp(const void *m1, const void *m2, size_t len)
> > > > > >                                        &_ptr, sizeof(_ptr), _hflags);   \
> > > > > >                 if (ret)                                                \
> > > > > >                         break;                                          \
> > > > > > -               _cmp = __strncmp(_str, _expectedval, EXPECTED_STRSIZE); \
> > > > > > +               _cmp = bpf_strncmp(_str, EXPECTED_STRSIZE, _expectedval); \
> > > > >
> > > > > Though it's equivalent, the point of the test is to be heavy
> > > > > for the verifier with open coded __strncmp().
> > > > >
> > > > > pw-bot: cr
> > > >
> > > > I double checked that before acking, the test was added as a part of [1].
> > > > So it seems to be focused on bpf_snprintf_btf(), not on scalability.
> > > > And it's not that heavy in terms of instructions budget:
> > > >
> > > > File                     Program                  Verdict  Insns  States
> > > > -----------------------  -----------------------  -------  -----  ------
> > > > netif_receive_skb.bpf.o  trace_netif_receive_skb  success  18152     629
> > >
> > > Is this before or after?
> > > What is the % decrease in insn_processed?
> > > I'd like to better understand the impact of the change.
> >
> > That's before, after the change it is as follows:
> >
> > File                     Program                  Verdict  Insns  States
> > -----------------------  -----------------------  -------  -----  ------
> > netif_receive_skb.bpf.o  trace_netif_receive_skb  success   4353     235
> > -----------------------  -----------------------  -------  -----  ------
> >
> > So, the overall impact is 18K -> 4K instructions processed.
>
> It's large enough impact for the verifier.
> I agree that the test was mainly focusing on testing
> bpf_snprintf_btf(), but it has a nice side effect by testing
> bounded loops too.
> I prefer to keep it as-is.

Thanks for the clarification.

Removing the open-coded __strncmp would drop the bounded-loop
coverage that this test currently provides (as a side effect),
and that stress on the verifier is still valuable.

I'll drop this patch.
Thank you all for the discussion and review.