[PATCH 2/2] 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>
---
 arch/arm64/include/asm/processor.h |  2 +-
 arch/arm64/include/asm/ptrace.h    | 21 +++++++++++++++++++++
 arch/arm64/kernel/entry.S          | 10 ++++------
 arch/arm64/kernel/ptrace.c         |  2 +-
 arch/arm64/kernel/signal.c         |  8 ++++----
 arch/arm64/kernel/signal32.c       |  2 +-
 6 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 379def1..b7334f1 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h

@@ -112,7 +112,7 @@ void tls_preserve_current_state(void);
 static inline void start_thread_common(struct pt_regs *regs, unsigned long pc)
 {
 	memset(regs, 0, sizeof(*regs));
-	regs->syscallno = ~0;
+	forget_syscall(regs);
 	regs->pc = pc;
 }
 

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index 21c87dc..3a2d6cc 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h

@@ -72,8 +72,19 @@
 #define COMPAT_PT_TEXT_ADDR		0x10000
 #define COMPAT_PT_DATA_ADDR		0x10004
 #define COMPAT_PT_TEXT_END_ADDR		0x10008
+
+/*
+ * If pt_regs.syscallno == NO_SYSCALL, then the thread is not executing
+ * a syscall -- i.e., its most recent entry into the kernel from
+ * userspace was not via SVC, or otherwise a tracer cancelled the syscall.
+ *
+ * This must have the value -1, for ABI compatibility with ptrace etc.
+ */
+#define NO_SYSCALL (-1)
+
 #ifndef __ASSEMBLY__
 #include <linux/bug.h>
+#include <linux/types.h>
 
 /* sizeof(struct user) for AArch32 */
 #define COMPAT_USER_SZ	296

@@ -128,6 +139,16 @@ struct pt_regs {
 	u64 unused;	// maintain 16 byte alignment
 };
 
+static inline bool in_syscall(int syscallno)
+{
+	return syscallno != NO_SYSCALL;
+}

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 4d04b89..3a59dae 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c

@@ -387,7 +387,7 @@ static int restore_sigframe(struct pt_regs *regs,
 	/*
 	 * Avoid sys_rt_sigreturn() restarting.
 	 */
-	regs->syscallno = ~0;
+	forget_syscall(regs);
 
 	err |= !valid_user_regs(&regs->user_regs, current);
 	if (err == 0)

@@ -679,7 +679,7 @@ static void do_signal(struct pt_regs *regs)
 	/*
 	 * If we were from a system call, check for system call restarting...
 	 */
-	if (syscall >= 0) {
+	if (in_syscall(syscall)) {