Re: [PATCH bpf-next] selftests/bpf: Fix implementation-defined behavior in sk_lookup test

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Mon, 2022-02-21 at 22:39 +0100, Ilya Leoshkevich wrote:
> On Mon, 2022-02-21 at 19:03 +0100, Jakub Sitnicki wrote:
> > Shifting 16-bit type by 16 bits is implementation-defined for BPF
> > programs.
> > Don't rely on it in case it is causing the test failures we are
> > seeing on
> > s390x z15 target.
> > 
> > Fixes: 2ed0dc5937d3 ("selftests/bpf: Cover 4-byte load from
> > remote_port in bpf_sk_lookup")
> > Reported-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
> > ---
> > 
> > I don't have a dev env for s390x/z15 set up yet, so can't
> > definitely
> > confirm the fix.
> > That said, it seems worth fixing either way.
> > 
> >  tools/testing/selftests/bpf/progs/test_sk_lookup.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tools/testing/selftests/bpf/progs/test_sk_lookup.c
> > b/tools/testing/selftests/bpf/progs/test_sk_lookup.c
> > index bf5b7caefdd0..7d47276a8964 100644
> > --- a/tools/testing/selftests/bpf/progs/test_sk_lookup.c
> > +++ b/tools/testing/selftests/bpf/progs/test_sk_lookup.c
> > @@ -65,6 +65,7 @@ static const __u32 KEY_SERVER_A = SERVER_A;
> >  static const __u32 KEY_SERVER_B = SERVER_B;
> >  
> >  static const __u16 SRC_PORT = bpf_htons(8008);
> > +static const __u32 SRC_PORT_U32 = bpf_htonl(8008U << 16);
> >  static const __u32 SRC_IP4 = IP4(127, 0, 0, 2);
> >  static const __u32 SRC_IP6[] = IP6(0xfd000000, 0x0, 0x0,
> > 0x00000002);
> >  
> > @@ -421,7 +422,7 @@ int ctx_narrow_access(struct bpf_sk_lookup
> > *ctx)
> >  
> >         /* Load from remote_port field with zero padding (backward
> > compatibility) */
> >         val_u32 = *(__u32 *)&ctx->remote_port;
> > -       if (val_u32 != bpf_htonl(bpf_ntohs(SRC_PORT) << 16))
> > +       if (val_u32 != SRC_PORT_U32)
> >                 return SK_DROP;
> >  
> >         /* Narrow loads from local_port field. Expect DST_PORT. */
> 
> Unfortunately this doesn't help with the s390 problem.
> I'll try to debug this.

I have to admit I have a hard time wrapping my head around the
requirements here.

Based on the pre-9a69e2b385f4 code, do I understand correctly that
for the following input

Port:     0x1f48
SRC_PORT: 0x481f

we expect the following results for different kinds of loads:

Size   Offset  LE      BE
BPF_B  0       0x1f    0
BPF_B  1       0x48    0
BPF_B  2       0       0x48
BPF_B  3       0       0x1f
BPF_H  0       0x481f  0
BPF_H  1       0       0x481f
BPF_W  0       0x481f  0x481f

and this is guaranteed by the struct bpf_sk_lookup ABI? Because then it
looks as if 9a69e2b385f4 breaks it on big-endian as follows:

Size   Offset  BE-9a69e2b385f4
BPF_B  0       0x48
BPF_B  1       0x1f
BPF_B  2       0
BPF_B  3       0
BPF_H  0       0x481f
BPF_H  1       0
BPF_W  0       0x481f0000

Or is the old behavior a bug and this new one is desirable?
9a69e2b385f4 has no Fixes: tag, so I assume that's the former :-(

In which case, would it make sense to fix it by swapping remote_port
and :16 in bpf_sk_lookup on big-endian?

Best regards,
Ilya