MIPS: Fix syscall_get_nr for the syscall exit tracing.

Register 2 is alredy overwritten by the return value when
syscall_trace_leave() is called.
Signed-off-by: Lars Persson <larper@axis.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9187/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/syscall.h

@@ -29,13 +29,7 @@
 static inline long syscall_get_nr(struct task_struct *task,
 				  struct pt_regs *regs)
 {
-	/* O32 ABI syscall() - Either 64-bit with O32 or 32-bit */
-	if ((config_enabled(CONFIG_32BIT) ||
-	    test_tsk_thread_flag(task, TIF_32BIT_REGS)) &&
-	    (regs->regs[2] == __NR_syscall))
-		return regs->regs[4];
-	else
-		return regs->regs[2];
+	return current_thread_info()->syscall;
 }
 
 static inline unsigned long mips_get_syscall_arg(unsigned long *arg,

arch/mips/include/asm/thread_info.h

@@ -36,6 +36,7 @@ struct thread_info {
 						 */
 	struct restart_block	restart_block;
 	struct pt_regs		*regs;
+	long			syscall;	/* syscall number */
 };
 
 /*

arch/mips/kernel/ptrace.c

@@ -770,6 +770,8 @@ asmlinkage long syscall_trace_enter(struct pt_regs *regs, long syscall)
 	long ret = 0;
 	user_exit();
 
+	current_thread_info()->syscall = syscall;
+
 	if (secure_computing() == -1)
 		return -1;