MIPS: Save static registers before sysmips

The MIPS sysmips system call handler may return directly from the
MIPS_ATOMIC_SET case (mips_atomic_set()) to syscall_exit. This path
restores the static (callee saved) registers, however they won't have
been saved on entry to the system call.

Use the save_static_function() macro to create a __sys_sysmips wrapper
function which saves the static registers before calling sys_sysmips, so
that the correct static register state is restored by syscall_exit.

Fixes: f1e39a4a616c ("MIPS: Rewrite sysmips(MIPS_ATOMIC_SET, ...) in C with inline assembler")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: stable@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16149/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/scall32-o32.S

@@ -371,7 +371,7 @@ EXPORT(sys_call_table)
 	PTR	sys_writev
 	PTR	sys_cacheflush
 	PTR	sys_cachectl
-	PTR	sys_sysmips
+	PTR	__sys_sysmips
 	PTR	sys_ni_syscall			/* 4150 */
 	PTR	sys_getsid
 	PTR	sys_fdatasync

arch/mips/kernel/scall64-64.S

@@ -311,7 +311,7 @@ EXPORT(sys_call_table)
 	PTR	sys_sched_getaffinity
 	PTR	sys_cacheflush
 	PTR	sys_cachectl
-	PTR	sys_sysmips
+	PTR	__sys_sysmips
 	PTR	sys_io_setup			/* 5200 */
 	PTR	sys_io_destroy
 	PTR	sys_io_getevents

arch/mips/kernel/scall64-n32.S

@@ -302,7 +302,7 @@ EXPORT(sysn32_call_table)
 	PTR	compat_sys_sched_getaffinity
 	PTR	sys_cacheflush
 	PTR	sys_cachectl
-	PTR	sys_sysmips
+	PTR	__sys_sysmips
 	PTR	compat_sys_io_setup			/* 6200 */
 	PTR	sys_io_destroy
 	PTR	compat_sys_io_getevents

arch/mips/kernel/scall64-o32.S

@@ -371,7 +371,7 @@ EXPORT(sys32_call_table)
 	PTR	compat_sys_writev
 	PTR	sys_cacheflush
 	PTR	sys_cachectl
-	PTR	sys_sysmips
+	PTR	__sys_sysmips
 	PTR	sys_ni_syscall			/* 4150 */
 	PTR	sys_getsid
 	PTR	sys_fdatasync

arch/mips/kernel/syscall.c

@@ -192,6 +192,12 @@ static inline int mips_atomic_set(unsigned long addr, unsigned long new)
 	unreachable();
 }
 
+/*
+ * mips_atomic_set() normally returns directly via syscall_exit potentially
+ * clobbering static registers, so be sure to preserve them.
+ */
+save_static_function(sys_sysmips);
+
 SYSCALL_DEFINE3(sysmips, long, cmd, long, arg1, long, arg2)
 {
 	switch (cmd) {