[RFC PATCH v2] rseq: Remove broken uapi field layout on 32-bit little endian

[Mathieu Desnoyers]

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 <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Florian Weimer <fw@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-api@xxxxxxxxxxxxxxx
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Dave Watson <davejwatson@xxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
Cc: "H . Peter Anvin" <hpa@xxxxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
Cc: Ben Maurer <bmaurer@xxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 include/uapi/linux/rseq.h | 20 ++++----------------
 kernel/rseq.c             |  8 ++++----
 2 files changed, 8 insertions(+), 20 deletions(-)

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 9a402fdb60e9..77ee207623a9 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -105,23 +105,11 @@ 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 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.
diff --git a/kernel/rseq.c b/kernel/rseq.c
index 6d45ac3dae7f..97ac20b4f738 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -128,10 +128,10 @@ static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
 	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) {
@@ -217,9 +217,9 @@ static int clear_rseq_cs(struct task_struct *t)
 	 * 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
-- 
2.17.1