s390/vdso64: add sigreturn,rt_sigreturn and restart_syscall

Add minimalistic trampolines to vdso64 so we can return from signal
without using the stack which requires pgm check handler hacks when
NX is enabled.

restart_syscall will be called from vdso to work around the architectural
limitation that the syscall number might be encoded in the svc instruction,
and therefore can not be changed.
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/vdso64/vdso64.lds.S

@@ -137,6 +137,9 @@ VERSION
 		__kernel_clock_gettime;
 		__kernel_clock_getres;
 		__kernel_getcpu;
+		__kernel_restart_syscall;
+		__kernel_rt_sigreturn;
+		__kernel_sigreturn;
 	local: *;
 	};
 }

arch/s390/kernel/vdso64/vdso_user_wrapper.S

@@ -37,3 +37,20 @@ vdso_func gettimeofday
 vdso_func clock_getres
 vdso_func clock_gettime
 vdso_func getcpu
+
+.macro vdso_syscall func,syscall
+	.globl __kernel_\func
+	.type  __kernel_\func,@function
+	.align 8
+__kernel_\func:
+	CFI_STARTPROC
+	svc	\syscall
+	/* Make sure we notice when a syscall returns, which shouldn't happen */
+	.word	0
+	CFI_ENDPROC
+	.size	__kernel_\func,.-__kernel_\func
+.endm
+
+vdso_syscall restart_syscall,__NR_restart_syscall
+vdso_syscall sigreturn,__NR_sigreturn
+vdso_syscall rt_sigreturn,__NR_rt_sigreturn