netdev - Re: [PATCH bpf] bpf: fix narrower loads on s390

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Message-Id: <4311B5C3-8D1B-4958-9CDE-450662A7851D@linux.ibm.com>
Date:   Wed, 17 Jul 2019 12:36:34 +0200
From:   Ilya Leoshkevich <iii@...ux.ibm.com>
To:     Y Song <ys114321@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        gor@...ux.ibm.com, heiko.carstens@...ibm.com
Subject: Re: [PATCH bpf] bpf: fix narrower loads on s390

> Am 17.07.2019 um 11:21 schrieb Ilya Leoshkevich <iii@...ux.ibm.com>:
> 
>> Am 17.07.2019 um 07:11 schrieb Y Song <ys114321@...il.com>:
>> 
>> [sorry, resend again as previous one has come text messed out due to
>> networking issues]
>> 
>> On Tue, Jul 16, 2019 at 10:08 PM Y Song <ys114321@...il.com> wrote:
>>> 
>>> On Tue, Jul 16, 2019 at 4:59 AM Ilya Leoshkevich <iii@...ux.ibm.com> wrote:
>>>> 
>>>> test_pkt_md_access is failing on s390, since the associated eBPF prog
>>>> returns TC_ACT_SHOT, which in turn happens because loading a part of a
>>>> struct __sk_buff field produces an incorrect result.
>>>> 
>>>> The problem is that when verifier emits the code to replace partial load
>>>> of a field with a full load, a shift and a bitwise AND, it assumes that
>>>> the machine is little endian.
>>>> 
>>>> Adjust shift count calculation to account for endianness.
>>>> 
>>>> Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
>>>> Signed-off-by: Ilya Leoshkevich <iii@...ux.ibm.com>
>>>> ---
>>>> kernel/bpf/verifier.c | 8 ++++++--
>>>> 1 file changed, 6 insertions(+), 2 deletions(-)
>>>> 
>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>>> index 5900cbb966b1..3f9353653558 100644
>>>> --- a/kernel/bpf/verifier.c
>>>> +++ b/kernel/bpf/verifier.c
>>>> @@ -8616,8 +8616,12 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
>>>>               }
>>>> 
>>>>               if (is_narrower_load && size < target_size) {
>>>> -                       u8 shift = (off & (size_default - 1)) * 8;
>>>> -
>>>> +                       u8 load_off = off & (size_default - 1);
>>>> +#ifdef __LITTLE_ENDIAN
>>>> +                       u8 shift = load_off * 8;
>>>> +#else
>>>> +                       u8 shift = (size_default - (load_off + size)) * 8;
>>>> +#endif
>>> 
>> All the values are in register. The shifting operations should be the
>> same for big endian and little endian, e.g., value 64 >> 2 = 16 when
>> value "64" is in register. So I did not see a problem here.
>> 
>> Could you elaborate which field access in test_pkt_md_access
>> caused problem?
> 
> The very first one: TEST_FIELD(__u8,  len, 0xFF);
> 
>> It would be good if you can give detailed memory layout and register values
>> to illustrate the problem.
> 
> Suppose len = 0x11223344. On a big endian system, this would be
> 
> 11 22 33 44
> 
> Now, we would like to do *(u8 *)&len, the desired result is 0x11.
> Verifier should emit the following: ((*(u32 *)&len) >> 24) & 0xff, but as
> of today it misses the shift.
> 
> On a little endian system the layout is:
> 
> 44 33 22 11
> 
> and the desired result is different - 0x44. Verifier correctly emits
> (*(u32 *)&len) & 0xff.

I’ve just realized, that this example does not reflect what the test is
doing on big-endian systems (there is an #ifdef for those).

Here is a better one: len=0x11223344 and we would like to do
((u8 *)&len)[3].

len is represented as `11 22 33 44` in memory, so the desired result is
0x44. It can be obtained by doing (*(u32 *)&len) & 0xff, but today the
verifier does ((*(u32 *)&len) >> 24) & 0xff instead.