MIPS: Traced negative syscalls should return -ENOSYS

If a negative system call number is used when system call tracing is
enabled, syscall_trace_enter() will return that negative system call
number without having written the return value and error flag into the
pt_regs.

The caller then treats it as a cancelled system call and assumes that
the return value and error flag are already written, leaving the
negative system call number in the return register ($v0), and the 4th
system call argument in the error register ($a3).

Add a special case to detect this at the end of syscall_trace_enter(),
to set the return value to error -ENOSYS when this happens.

Fixes: d218af7 ("MIPS: scall: Always run the seccomp syscall filters")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16653/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/ptrace.c

@@ -894,6 +894,13 @@ asmlinkage long syscall_trace_enter(struct pt_regs *regs, long syscall)
 
 	audit_syscall_entry(syscall, regs->regs[4], regs->regs[5],
 			    regs->regs[6], regs->regs[7]);
+
+	/*
+	 * Negative syscall numbers are mistaken for rejected syscalls, but
+	 * won't have had the return value set appropriately, so we do so now.
+	 */
+	if (syscall < 0)
+		syscall_set_return_value(current, regs, -ENOSYS, 0);
 	return syscall;
 }