MIPS: seccomp: Handle indirect system calls (o32)

When userland uses syscall() to perform an indirect system call
the actually system call that needs to be checked by the filter
is on the first argument. The kernel code needs to handle this case
by looking at the original syscall number in v0 and if it's
NR_syscall, then it needs to examine the first argument to
identify the real system call that will be executed.
Similarly, we need to 'virtually' shift the syscall() arguments
so the syscall_get_arguments() function can fetch the correct
arguments for the indirect system call.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6404/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/arch/mips/kernel/scall64-o32.S b/arch/mips/kernel/scall64-o32.S
index 67dc022f6826..70f6acecd928 100644
--- a/arch/mips/kernel/scall64-o32.S
+++ b/arch/mips/kernel/scall64-o32.S
@@ -112,7 +112,18 @@ trace_a_syscall:
 
 	move	s0, t2			# Save syscall pointer
 	move	a0, sp
-	jal	syscall_trace_enter
+	/*
+	 * syscall number is in v0 unless we called syscall(__NR_###)
+	 * where the real syscall number is in a0
+	 * note: NR_syscall is the first O32 syscall but the macro is
+	 * only defined when compiling with -mabi=32 (CONFIG_32BIT)
+	 * therefore __NR_O32_Linux is used (4000)
+	 */
+	addiu	a1, v0,  __NR_O32_Linux
+	bnez	v0, 1f /* __NR_syscall at offset 0 */
+	lw	a1, PT_R4(sp)
+
+1:	jal	syscall_trace_enter
 
 	bltz	v0, 2f			# seccomp failed? Skip syscall