arm64: Abstract syscallno manipulation

The -1 "no syscall" value is written in various ways, shared with
the user ABI in some places, and generally obscure.

This patch attempts to make things a little more consistent and
readable by replacing all these uses with a single #define.  A
couple of symbolic helpers are provided to clarify the intent
further.

Because the in-syscall check in do_signal() is changed from >= 0 to
!= NO_SYSCALL by this patch, different behaviour may be observable
if syscallno is set to values less than -1 by a tracer.  However,
this is not different from the behaviour that is already observable
if a tracer sets syscallno to a value >= __NR_(compat_)syscalls.

It appears that this can cause spurious syscall restarting, but
that is not a new behaviour either, and does not appear harmful.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
index d98ca76cbd39..4e5a664be04b 100644
--- a/arch/arm64/kernel/signal32.c
+++ b/arch/arm64/kernel/signal32.c
@@ -354,7 +354,7 @@ static int compat_restore_sigframe(struct pt_regs *regs,
 	/*
 	 * Avoid compat_sys_sigreturn() restarting.
 	 */
-	regs->syscallno = ~0;
+	forget_syscall(regs);
 
 	err |= !valid_user_regs(&regs->user_regs, current);