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

The rseq rseq_cs.ptr.{ptr32,padding} uapi endianness handling is
entirely wrong on 32-bit little endian: a preprocessor logic mistake
wrongly uses the big endian field layout on 32-bit little endian
architectures.

Fortunately, those ptr32 accessors were never used within the kernel,
and only meant as a convenience for user-space.

Remove those and only leave the "ptr64" union field, as this is the only
thing really needed to express the ABI. Document how 32-bit
architectures are meant to interact with this "ptr64" union field.

Fixes: ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union, update includes")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Florian Weimer <redacted>
Cc: Thomas Gleixner <redacted>
Cc: linux-api@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <redacted>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Watson <redacted>
Cc: Paul Turner <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Russell King <redacted>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <redacted>
Cc: Christian Brauner <redacted>
Cc: Ben Maurer <redacted>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <redacted>
Cc: Michael Kerrisk <redacted>
Cc: Joel Fernandes <redacted>
Cc: Paul E. McKenney <paulmck@kernel.org>
---
 include/uapi/linux/rseq.h | 17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 9a402fdb60e9..31290f2424a7 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h

@@ -105,22 +105,13 @@ struct rseq {
 	 * Read and set by the kernel. Set by user-space with single-copy
 	 * atomicity semantics. This field should only be updated by the
 	 * thread which registered this data structure. Aligned on 64-bit.
+	 *
+	 * 32-bit architectures should update the low order bits of the
+	 * rseq_cs.ptr64 field, leaving the high order bits initialized
+	 * to 0.
 	 */
 	union {