[PATCH] MIPS: scall64-o32: Fix indirect syscall number load

[Aurelien Jarno <aurelien@aurel32.net>]

Commit 4c21b8fd8f14 (MIPS: seccomp: Handle indirect system calls (o32))
added indirect syscall detection for O32 processes running on MIPS64,
but it did not work correctly for big endian kernel/processes. The
reason is that the syscall number is loaded from ARG1 using the lw
instruction while this is a 64-bit value, so zero is loaded instead of
the syscall number.

Fix the code by using the ld instruction instead. When running a 32-bit
processes on a 64 bit CPU, the values are properly sign-extended, so it
ensures the value passed to syscall_trace_enter is correct.

Recent systemd versions with seccomp enabled whitelist the getpid
syscall for their internal  processes (e.g. systemd-journald), but call
it through syscall(SYS_getpid). This fix therefore allows O32 big endian
systems with a 64-bit kernel to run recent systemd versions.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Cc: <stable@vger.kernel.org> # v3.15+
---
 arch/mips/kernel/scall64-o32.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/mips/kernel/scall64-o32.S b/arch/mips/kernel/scall64-o32.S
index f158c5894a9a..feb2653490df 100644
--- a/arch/mips/kernel/scall64-o32.S
+++ b/arch/mips/kernel/scall64-o32.S
@@ -125,7 +125,7 @@ trace_a_syscall:
 	subu	t1, v0,  __NR_O32_Linux
 	move	a1, v0
 	bnez	t1, 1f /* __NR_syscall at offset 0 */
-	lw	a1, PT_R4(sp) /* Arg1 for __NR_syscall case */
+	ld	a1, PT_R4(sp) /* Arg1 for __NR_syscall case */
 	.set	pop
 
 1:	jal	syscall_trace_enter
-- 
2.20.1