Re: [PATCH bpf] bpf: fix accessing bpf_sysctl.file_pos on s390

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

> Am 16.08.2019 um 01:01 schrieb Yonghong Song <yhs@fb.com>:
> 
> 
> 
> On 8/15/19 4:20 AM, Ilya Leoshkevich wrote:
>> "ctx:file_pos sysctl:read write ok" fails on s390 with "Read value  !=
>> nux". This is because verifier rewrites a complete 32-bit
>> bpf_sysctl.file_pos update to a partial update of the first 32 bits of
>> 64-bit *bpf_sysctl_kern.ppos, which is not correct on big-endian
>> systems.
>> 
>> Fix by using an offset on big-endian systems.
>> 
>> Ditto for bpf_sysctl.file_pos reads. Currently the test does not detect
>> a problem there, since it expects to see 0, which it gets with high
>> probability in error cases, so change it to seek to offset 3 and expect
>> 3 in bpf_sysctl.file_pos.
>> 
>> Fixes: e1550bfe0de4 ("bpf: Add file_pos field to bpf_sysctl ctx")
>> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
>> ---
>>  include/linux/filter.h                    | 10 ++++++++++
>>  kernel/bpf/cgroup.c                       |  9 +++++++--
>>  tools/testing/selftests/bpf/test_sysctl.c |  9 ++++++++-
>>  3 files changed, 25 insertions(+), 3 deletions(-)
>> 
>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>> index 92c6e31fb008..94e81c56d81c 100644
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -760,6 +760,16 @@ bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
>>  #endif
>>  }
>> 
>> +static inline s16
>> +bpf_ctx_narrow_access_offset(size_t variable_size, size_t access_size)
>> +{
>> +#ifdef __LITTLE_ENDIAN
>> +	return 0;
>> +#else
>> +	return variable_size - access_size;
>> +#endif
>> +}
> 
> The change looks correct to me.
> But now in include/linux/filter.h we have to macros:
> 
> static inline u8
> bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
> {
>         u8 load_off = off & (size_default - 1);
> 
> #ifdef __LITTLE_ENDIAN
>         return load_off * 8;
> #else
>         return (size_default - (load_off + size)) * 8;
> #endif
> }
> 
> static inline s16
> bpf_ctx_narrow_access_offset(size_t variable_size, size_t access_size)
> {
> #ifdef __LITTLE_ENDIAN
>         return 0;
> #else
>         return variable_size - access_size;
> #endif
> }
> 
> It would be good if we can have ifdef __LITTLE_ENDIAN only in one place.
> How about something like below:
> 
> static inline u8
> bpf_ctx_narrow_access_offset(u32 off, u32 size, u32 size_default)
> {
>         u8 access_off = off & (size_default - 1);
> 
> #ifdef __LITTLE_ENDIAN
>         return access_off;
> #else
>         return size_default - (access_off + size);
> #endif
> }
> 
> static inline u8
> bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
> {
>         return bpf_ctx_narrow_access_offset(off, size, size_default) * 8;
> }

This does indeed look better, thanks! In this case, we don't even need
bpf_ctx_narrow_load_shift() anymore, since doing

u8 shift = bpf_ctx_narrow_access_offset(
       off, size, size_default) * 8;

directly is quite readable. I will test and send a v2.