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

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

> Am 17.07.2019 um 07:11 schrieb Y Song <ys114321@gmail.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@gmail.com> wrote:
>> 
>> On Tue, Jul 16, 2019 at 4:59 AM Ilya Leoshkevich <iii@linux.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@linux.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.

> 
>> 
>>>                        if (ctx_field_size <= 4) {
>>>                                if (shift)
>>>                                        insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
>>> --
>>> 2.21.0