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 replace the whole rseq_cs union by a __u64 type, as
this is the only thing really needed to express the ABI. Document how
32-bit architectures are meant to interact with this field.

Fixes: ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union, update includes")
Signed-off-by: Mathieu Desnoyers <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

include/uapi/linux/rseq.h

···
 	 * 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 field, leaving the high order bits initialized to 0.
 	 */
-	union {
-		__u64 ptr64;
-#ifdef __LP64__
-		__u64 ptr;
-#else
-		struct {
-#if (defined(__BYTE_ORDER) && (__BYTE_ORDER == __BIG_ENDIAN)) || defined(__BIG_ENDIAN)
-			__u32 padding;		/* Initialized to zero. */
-			__u32 ptr32;
-#else /* LITTLE */
-			__u32 ptr32;
-			__u32 padding;		/* Initialized to zero. */
-#endif /* ENDIAN */
-		} ptr;
-#endif
-	} rseq_cs;
+	__u64 rseq_cs;
 
 	/*
 	 * Restartable sequences flags field.

kernel/rseq.c

···
 	int ret;
 
 #ifdef CONFIG_64BIT
-	if (get_user(ptr, &t->rseq->rseq_cs.ptr64))
+	if (get_user(ptr, &t->rseq->rseq_cs))
 		return -EFAULT;
 #else
-	if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
+	if (copy_from_user(&ptr, &t->rseq->rseq_cs, sizeof(ptr)))
 		return -EFAULT;
 #endif
 	if (!ptr) {
···
 	 * Set rseq_cs to NULL.
 	 */
 #ifdef CONFIG_64BIT
-	return put_user(0UL, &t->rseq->rseq_cs.ptr64);
+	return put_user(0UL, &t->rseq->rseq_cs);
 #else
-	if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
+	if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
 		return -EFAULT;
 	return 0;
 #endif