s390, s390x: fix printing of syscalls unknown to the kernel

On s390/s390x, syscalls with NR up to 255 can be implemented
directly using "svc NR", for NR >= 256 "svc 0" with %r1=NR is used.
The latter method is allowed for NR < 256, too.

When the syscall number specified directly or indirectly is recognized
by the kernel, i.e. it is less than its NR_syscalls value, it is stored
in %r2 and is available to arch_get_scno via s390_regset.gprs[2].
For syscall numbers >= NR_syscalls this register is set to 0,
but %r1 remains unchanged and could be used by arch_get_scno
via s390_regset.gprs[1] to decide what the syscall number is.

* linux/s390/get_scno.c (arch_get_scno): If s390_regset.gprs[2] is zero,
take syscall number from s390_regset.gprs[1].
* NEWS: Mention this fix.

This fixes Debian bug #485979 and Fedora bug #1298294.

Author: ldv-alt

NEWS

@@ -11,6 +11,8 @@ Noteworthy changes in release ?.?? (????-??-??)
   * Fixed build on arc, metag, nios2, or1k, and tile architectures.
   * Fixed decoding of 32-bit times syscall return value on 64-bit architectures.
   * Fixed decoding of mlock2 syscall on sparc.
+  * Fixed decoding of syscalls unknown to the kernel on s390/s390x.
+    (addresses Debian bug #485979 and Fedora bug #1298294).
 
 Noteworthy changes in release 4.11 (2015-12-21)
 ===============================================

linux/s390/get_scno.c

@@ -2,6 +2,7 @@
 static int
 arch_get_scno(struct tcb *tcp)
 {
-	tcp->scno = s390_regset.gprs[2];
+	tcp->scno = s390_regset.gprs[2] ?
+		    s390_regset.gprs[2] : s390_regset.gprs[1];
 	return 1;
 }