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

[David Laight <David.Laight@ACULAB.COM>]

From: Mathieu Desnoyers
> Sent: 25 January 2022 19:01
> 
> ----- 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.

But:
	v.rseq_cs.ptr_64 = (uintptr_t)&foo;
is broken.

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

It really is a shame that you can't do:
	void   *rseq_cs __attribute__((size(8)));
and have the compiler just DTRT on 32bit systems.

	David

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