Re: [RFC PATCH 02/15] rseq: Remove broken uapi field layout on 32-bit little endian

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jan 25, 2022, at 9:41 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Jan 25, 2022, at 7:21 AM, Christian Brauner brauner@kernel.org wrote:
[...]
>>>  include/uapi/linux/rseq.h | 17 ++++-------------
[...]
>>>  	union {
>> 
>> A bit unfortunate we seem to have to keep the union around even though
>> it's just one field now.
> 
> Well, as far as the user-space projects that I know of which use rseq
> are concerned (glibc, librseq, tcmalloc), those end up with their own
> copy of the uapi header anyway to deal with the big/little endian field
> on 32-bit. So I'm very much open to remove the union if we accept that
> this uapi header is really just meant to express the ABI and is not
> expected to be used as an API by user-space.
> 
> That would mean we also bring a uapi header copy into the kernel
> rseq selftests as well to minimize the gap between librseq and
> the kernel sefltests (the kernel sefltests pretty much include a
> copy of librseq for convenience. librseq is maintained out of tree).
> 
> Thoughts ?

Actually, if we go ahead and remove the union, and replace:

struct rseq {
  union {
    __u64 ptr64;
  } rseq_cs;
[...]
} v;

by:

struct rseq {
  __u64 rseq_cs;
} v;

expressions such as these are unchanged:

- sizeof(v.rseq_cs),
- &v.rseq_cs,
- __alignof__(v.rseq_cs),
- offsetof(struct rseq, rseq_cs).

So users of the uapi rseq.h (as an API) can still use rseq_abi->rseq_cs before
and after the change.

Based on this, I am inclined to remove the union, and just make the rseq_cs field
a __u64.

Any objections ?

Thanks,

Mathieu


> 
> Thanks,
> 
> Mathieu
> 
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com