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Date: Tue, 22 Feb 2022 19:24:11 +0100
From: Jakub Sitnicki <jakub@...udflare.com>
To: Ilya Leoshkevich <iii@...ux.ibm.com>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
kernel-team@...udflare.com,
Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix implementation-defined
behavior in sk_lookup test
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@...il.com>
>> > > > > Signed-off-by: Jakub Sitnicki <jakub@...udflare.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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