Re: [PATCH] bpf: fix uapi bpf_prog_info fields alignment

[Baruch Siach <baruch@tkos.co.il>]

Hi Geert,

On Tue, Jun 04 2019, Geert Uytterhoeven wrote:
> On Tue, Jun 4, 2019 at 1:40 PM Baruch Siach <baruch@tkos.co.il> wrote:
>> Merge commit 1c8c5a9d38f60 ("Merge
>> git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next") undid the
>> fix from commit 36f9814a494 ("bpf: fix uapi hole for 32 bit compat
>> applications") by taking the gpl_compatible 1-bit field definition from
>> commit b85fab0e67b162 ("bpf: Add gpl_compatible flag to struct
>> bpf_prog_info") as is. That breaks architectures with 16-bit alignment
>> like m68k. Widen gpl_compatible to 32-bit to restore alignment of the
>> following fields.
>>
>> Thanks to Dmitry V. Levin his analysis of this bug history.
>>
>> Cc: Jiri Olsa <jolsa@kernel.org>
>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Signed-off-by: Baruch Siach <baruch@tkos.co.il>
>
> Thanks for your patch!
>
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -3140,7 +3140,7 @@ struct bpf_prog_info {
>>         __aligned_u64 map_ids;
>>         char name[BPF_OBJ_NAME_LEN];
>>         __u32 ifindex;
>> -       __u32 gpl_compatible:1;
>> +       __u32 gpl_compatible;
>>         __u64 netns_dev;
>>         __u64 netns_ino;
>
> Wouldn't it be better to change the types of the fields that require
> 8-byte alignment from __u64 to __aligned_u64, like is already used
> for the map_ids fields?
>
> Without that, some day people will need to add a new flag, and will
> convert the 32-bit flag to a bitfield again to make space, reintroducing
> the issue.

This is a minimal fix that restores the original fix of commit
36f9814a494. Would __aligned_u64 cause any negative side effect on
current ABI?

baruch

>>         __u32 nr_jited_ksyms;
>> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
>> index 63e0cf66f01a..fe73829b5b1c 100644
>> --- a/tools/include/uapi/linux/bpf.h
>> +++ b/tools/include/uapi/linux/bpf.h
>> @@ -3140,7 +3140,7 @@ struct bpf_prog_info {
>>         __aligned_u64 map_ids;
>>         char name[BPF_OBJ_NAME_LEN];
>>         __u32 ifindex;
>> -       __u32 gpl_compatible:1;
>> +       __u32 gpl_compatible;
>>         __u64 netns_dev;
>>         __u64 netns_ino;
>
> Same here.
>
>>         __u32 nr_jited_ksyms;

--
     http://baruch.siach.name/blog/                  ~. .~   Tk Open Systems
=}------------------------------------------------ooO--U--Ooo------------{=
   - baruch@tkos.co.il - tel: +972.52.368.4656, http://www.tkos.co.il -