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

[Jakub Sitnicki <jakub@cloudflare.com>]

On Tue, Feb 22, 2022 at 06:42 PM +01, Ilya Leoshkevich wrote:
> On Tue, 2022-02-22 at 15:53 +0100, Jakub Sitnicki wrote:
>> On Tue, Feb 22, 2022 at 03:22 AM +01, Ilya Leoshkevich wrote:
>> > On Tue, 2022-02-22 at 01:43 +0100, Ilya Leoshkevich wrote:
>> > > 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
>> > 
>> > Sorry, I worded this incorrectly: 9a69e2b385f4 did not change the
>> > kernel behavior, the ABI is not broken and the old compiled code
>> > should
>> > continue to work.
>> > What the second table really shows are what the results should be
>> > according to the 9a69e2b385f4 struct bpf_sk_lookup definition,
>> > which I
>> > still think is broken on big-endian and needs to be adjusted to
>> > match
>> > the ABI.
>> > 
>> > I noticed one other strange thing in the meantime: loads from
>> > *(__u32 *)&ctx->remote_port, *(__u16 *)&ctx->remote_port and
>> > *((__u16 *)&ctx->remote_port + 1) all produce 8008 on s390, which
>> > is
>> > clearly inconsistent. It looks as if convert_ctx_accesses() needs
>> > to be
>> > adjusted to handle combinations like ctx_field_size == 4 && size ==
>> > 2
>> > && target_size == 2. I will continue with this tomorrow.
>> > 
>> > > 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?
>> 
>> Thanks for looking into it.
>> 
>> When it comes to requirements, my intention was to keep the same
>> behavior as before the split up of the remote_port field in
>> 9a69e2b385f4
>> ("bpf: Make remote_port field in struct bpf_sk_lookup 16-bit wide").
>> 
>> 9a69e2b385f4 was supposed to be a formality, after a similar change
>> in
>> 4421a582718a ("bpf: Make dst_port field in struct bpf_sock 16-bit
>> wide"), which went in earlier.
>> 
>> In 4421a582718a I've provided a bit more context. I understand that
>> the
>> remote_port value, even before the type changed from u32 to u16,
>> appeared to the BPF program as if laid out in memory like so:
>> 
>>       offsetof(struct bpf_sk_lookup, remote_port) +0  <port MSB>
>>                                                   +1  <port LSB>
>>                                                   +2  0x00
>>                                                   +3  0x00
>> 
>> Translating it to your handy table format, I expect should result in
>> loads as so if port is 8008 = 0x1f48:
>> 
>>       Size   Offset  LE      BE
>>       BPF_B  0       0x1f    0x1f
>>       BPF_B  1       0x48    0x48
>>       BPF_B  2       0       0
>>       BPF_B  3       0       0
>>       BPF_H  0       0x481f  0x1f48
>>       BPF_H  1       0       0
>>       BPF_W  0       0x481f  0x1f480000
>
> Hmm, I think for big-endian the layout is different.
> If we look at test_sk_lookup.c from 9a69e2b385f4^:
>
>         /* Narrow loads from remote_port field. Expect SRC_PORT. */
>         if (LSB(ctx->remote_port, 0) != ((SRC_PORT >> 0) & 0xff) ||
>             LSB(ctx->remote_port, 1) != ((SRC_PORT >> 8) & 0xff) ||
>             LSB(ctx->remote_port, 2) != 0 || LSB(ctx->remote_port, 3)
> != 0)
>                 return SK_DROP;
>
> LSB() on little-endian is just byte indexing, so it's indeed 
> 1f,48,00,00. However, on big-endian it's indexing from the end, so
> it's 00,00,48,1f.

I understood that LSB() is indexing from the end on BE because SRC_PORT
constant value differs on LE (= 0x481f) and BE (= 0x1f48) platforms, so

                 LE  BE
  SRC_PORT >> 0  1f  48
  SRC_PORT >> 8  48  1f

So on LE we first compare remote_port MSB, then LSB.
While on BE we start with remote_port LSB, then MSB.

But, now that you have pointed it out, I notice that sizeof(remote_port)
has changed and from 4 to 2, and I can't see how LSB(…, 3) and LSB(…, 4)
loads can keep working on big-endian.

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