arm64: pt_regs: assert pt_regs is a multiple of 16 bytes

To ensure that the stack is correctly aligned when branching to C code,
we require that struct pt_regs is a multiple of 16 bytes, as noted in a
comment.

Add an explicit assertion for this, so that any accidental violation of
this requirement will be caught by the compiler.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Puranjay Mohan <puranjay12@gmail.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20241017092538.1859841-2-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: mrutland-arm

arch/arm64/include/asm/ptrace.h

@@ -149,8 +149,7 @@ static inline unsigned long pstate_to_compat_psr(const unsigned long pstate)
 
 /*
  * This struct defines the way the registers are stored on the stack during an
- * exception. Note that sizeof(struct pt_regs) has to be a multiple of 16 (for
- * stack alignment). struct user_pt_regs must form a prefix of struct pt_regs.
+ * exception. struct user_pt_regs must form a prefix of struct pt_regs.
  */
 struct pt_regs {
 	union {
@@ -180,6 +179,9 @@ struct pt_regs {
 	u64 exit_rcu;
 };
 
+/* For correct stack alignment, pt_regs has to be a multiple of 16 bytes. */
+static_assert(IS_ALIGNED(sizeof(struct pt_regs), 16));
+
 static inline bool in_syscall(struct pt_regs const *regs)
 {
 	return regs->syscallno != NO_SYSCALL;