[S390] signal race with restarting system calls

For a ERESTARTNOHAND/ERESTARTSYS/ERESTARTNOINTR restarting system call
do_signal will prepare the restart of the system call with a rewind of
the PSW before calling get_signal_to_deliver (where the debugger might
take control). For A ERESTART_RESTARTBLOCK restarting system call
do_signal will set -EINTR as return code.
There are two issues with this approach:
1) strace never sees ERESTARTNOHAND, ERESTARTSYS, ERESTARTNOINTR or
   ERESTART_RESTARTBLOCK as the rewinding already took place or the
   return code has been changed to -EINTR
2) if get_signal_to_deliver does not return with a signal to deliver
   the restart via the repeat of the svc instruction is left in place.
   This opens a race if another signal is made pending before the
   system call instruction can be reexecuted. The original system call
   will be restarted even if the second signal would have ended the
   system call with -EINTR.

These two issues can be solved by dropping the early rewind of the
system call before get_signal_to_deliver has been called and by using
the TIF_RESTART_SVC magic to do the restart if no signal has to be
delivered. The only situation where the system call restart via the
repeat of the svc instruction is appropriate is when a SA_RESTART
signal is delivered to user space.

Unfortunately this breaks inferior calls by the debugger again. The
system call number and the length of the system call instruction is
lost over the inferior call and user space will see ERESTARTNOHAND/
ERESTARTSYS/ERESTARTNOINTR/ERESTART_RESTARTBLOCK. To correct this a
new ptrace interface is added to save/restore the system call number
and system call instruction length.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/ptrace.h

@@ -328,8 +328,7 @@ struct pt_regs
 	psw_t psw;
 	unsigned long gprs[NUM_GPRS];
 	unsigned long orig_gpr2;
-	unsigned short ilc;
-	unsigned short svcnr;
+	unsigned int svc_code;
 };
 
 /*
@@ -487,6 +486,8 @@ typedef struct
 #define PTRACE_POKETEXT_AREA	      0x5004
 #define PTRACE_POKEDATA_AREA 	      0x5005
 #define PTRACE_GET_LAST_BREAK	      0x5006
+#define PTRACE_PEEK_SYSTEM_CALL       0x5007
+#define PTRACE_POKE_SYSTEM_CALL	      0x5008
 
 /*
  * PT_PROT definition is loosely based on hppa bsd definition in

arch/s390/include/asm/syscall.h

@@ -13,6 +13,7 @@
 #define _ASM_SYSCALL_H	1
 
 #include <linux/sched.h>
+#include <linux/err.h>
 #include <asm/ptrace.h>
 
 /*
@@ -25,7 +26,7 @@ extern const unsigned int sys_call_table[];
 static inline long syscall_get_nr(struct task_struct *task,
 				  struct pt_regs *regs)
 {
-	return regs->svcnr ? regs->svcnr : -1;
+	return regs->svc_code ? (regs->svc_code & 0xffff) : -1;
 }
 
 static inline void syscall_rollback(struct task_struct *task,
@@ -37,7 +38,7 @@ static inline void syscall_rollback(struct task_struct *task,
 static inline long syscall_get_error(struct task_struct *task,
 				     struct pt_regs *regs)
 {
-	return (regs->gprs[2] >= -4096UL) ? -regs->gprs[2] : 0;
+	return IS_ERR_VALUE(regs->gprs[2]) ? regs->gprs[2] : 0;
 }
 
 static inline long syscall_get_return_value(struct task_struct *task,

arch/s390/include/asm/thread_info.h

@@ -48,6 +48,7 @@ struct thread_info {
 	unsigned int		cpu;		/* current CPU */
 	int			preempt_count;	/* 0 => preemptable, <0 => BUG */
 	struct restart_block	restart_block;
+	unsigned int		system_call;
 	__u64			user_timer;
 	__u64			system_timer;
 	unsigned long		last_break;	/* last breaking-event-address. */

arch/s390/kernel/asm-offsets.c

@@ -45,8 +45,7 @@ int main(void)
 	DEFINE(__PT_PSW, offsetof(struct pt_regs, psw));
 	DEFINE(__PT_GPRS, offsetof(struct pt_regs, gprs));
 	DEFINE(__PT_ORIG_GPR2, offsetof(struct pt_regs, orig_gpr2));
-	DEFINE(__PT_ILC, offsetof(struct pt_regs, ilc));
-	DEFINE(__PT_SVCNR, offsetof(struct pt_regs, svcnr));
+	DEFINE(__PT_SVC_CODE, offsetof(struct pt_regs, svc_code));
 	DEFINE(__PT_SIZE, sizeof(struct pt_regs));
 	BLANK();
 	DEFINE(__SF_BACKCHAIN, offsetof(struct stack_frame, back_chain));

arch/s390/kernel/compat_signal.c

@@ -342,7 +342,7 @@ static int restore_sigregs32(struct pt_regs *regs,_sigregs32 __user *sregs)
 		return err;
 
 	restore_fp_regs(&current->thread.fp_regs);
-	regs->svcnr = 0;	/* disable syscall checks */
+	regs->svc_code = 0;	/* disable syscall checks */
 	return 0;
 }
 

arch/s390/kernel/entry.S

@@ -43,8 +43,7 @@ SP_R13	     =	STACK_FRAME_OVERHEAD + __PT_GPRS + 52
 SP_R14	     =	STACK_FRAME_OVERHEAD + __PT_GPRS + 56
 SP_R15	     =	STACK_FRAME_OVERHEAD + __PT_GPRS + 60
 SP_ORIG_R2   =	STACK_FRAME_OVERHEAD + __PT_ORIG_GPR2
-SP_ILC	     =	STACK_FRAME_OVERHEAD + __PT_ILC
-SP_SVCNR     =	STACK_FRAME_OVERHEAD + __PT_SVCNR
+SP_SVC_CODE  =	STACK_FRAME_OVERHEAD + __PT_SVC_CODE
 SP_SIZE      =	STACK_FRAME_OVERHEAD + __PT_SIZE
 
 _TIF_WORK_SVC = (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_NEED_RESCHED | \
@@ -229,7 +228,7 @@ sysc_saveall:
 	SAVE_ALL_SVC __LC_SVC_OLD_PSW,__LC_SAVE_AREA
 	CREATE_STACK_FRAME __LC_SAVE_AREA
 	mvc	SP_PSW(8,%r15),__LC_SVC_OLD_PSW
-	mvc	SP_ILC(4,%r15),__LC_SVC_ILC
+	mvc	SP_SVC_CODE(4,%r15),__LC_SVC_ILC
 	l	%r12,__LC_THREAD_INFO	# load pointer to thread_info struct
 sysc_vtime:
 	UPDATE_VTIME __LC_EXIT_TIMER,__LC_SYNC_ENTER_TIMER,__LC_USER_TIMER
@@ -239,12 +238,12 @@ sysc_update:
 	mvc	__LC_LAST_UPDATE_TIMER(8),__LC_SYNC_ENTER_TIMER
 sysc_do_svc:
 	xr	%r7,%r7
-	icm	%r7,3,SP_SVCNR(%r15)	# load svc number and test for svc 0
+	icm	%r7,3,SP_SVC_CODE+2(%r15)# load svc number and test for svc 0
 	bnz	BASED(sysc_nr_ok)	# svc number > 0
 	# svc 0: system call number in %r1
 	cl	%r1,BASED(.Lnr_syscalls)
 	bnl	BASED(sysc_nr_ok)
-	sth	%r1,SP_SVCNR(%r15)
+	sth	%r1,SP_SVC_CODE+2(%r15)
 	lr	%r7,%r1 	  # copy svc number to %r7
 sysc_nr_ok:
 	sll	%r7,2		  # svc number *4
@@ -335,10 +334,11 @@ sysc_notify_resume:
 #
 sysc_restart:
 	ni	__TI_flags+3(%r12),255-_TIF_RESTART_SVC # clear TIF_RESTART_SVC
-	l	%r7,SP_R2(%r15) 	# load new svc number
-	mvc	SP_R2(4,%r15),SP_ORIG_R2(%r15) # restore first argument
 	lm	%r2,%r6,SP_R2(%r15)	# load svc arguments
-	sth	%r7,SP_SVCNR(%r15)
+	xr	%r7,%r7			# svc 0 returns -ENOSYS
+	clc	SP_SVC_CODE+2(%r15),BASED(.Lnr_syscalls+2)
+	bnl	BASED(sysc_nr_ok)	# invalid svc number -> do svc 0
+	icm	%r7,3,SP_SVC_CODE+2(%r15)# load new svc number
 	b	BASED(sysc_nr_ok)	# restart svc
 
 #
@@ -346,7 +346,7 @@ sysc_restart:
 #
 sysc_singlestep:
 	ni	__TI_flags+3(%r12),255-_TIF_PER_TRAP # clear TIF_PER_TRAP
-	xc	SP_SVCNR(2,%r15),SP_SVCNR(%r15)		# clear svc number
+	xc	SP_SVC_CODE(4,%r15),SP_SVC_CODE(%r15)	# clear svc code
 	la	%r2,SP_PTREGS(%r15)	# address of register-save area
 	l	%r1,BASED(.Lhandle_per)	# load adr. of per handler
 	la	%r14,BASED(sysc_return)	# load adr. of system return
@@ -361,7 +361,7 @@ sysc_tracesys:
 	la	%r2,SP_PTREGS(%r15)	# load pt_regs
 	la	%r3,0
 	xr	%r0,%r0
-	icm	%r0,3,SP_SVCNR(%r15)
+	icm	%r0,3,SP_SVC_CODE(%r15)
 	st	%r0,SP_R2(%r15)
 	basr	%r14,%r1
 	cl	%r2,BASED(.Lnr_syscalls)
@@ -454,7 +454,7 @@ ENTRY(pgm_check_handler)
 	bnz	BASED(pgm_per)		# got per exception -> special case
 	SAVE_ALL_PGM __LC_PGM_OLD_PSW,__LC_SAVE_AREA
 	CREATE_STACK_FRAME __LC_SAVE_AREA
-	xc	SP_ILC(4,%r15),SP_ILC(%r15)
+	xc	SP_SVC_CODE(4,%r15),SP_SVC_CODE(%r15)
 	mvc	SP_PSW(8,%r15),__LC_PGM_OLD_PSW
 	l	%r12,__LC_THREAD_INFO	# load pointer to thread_info struct
 	tm	SP_PSW+1(%r15),0x01	# interrupting from user ?
@@ -531,7 +531,7 @@ pgm_svcper:
 	SAVE_ALL_PGM __LC_SVC_OLD_PSW,__LC_SAVE_AREA
 	CREATE_STACK_FRAME __LC_SAVE_AREA
 	mvc	SP_PSW(8,%r15),__LC_SVC_OLD_PSW
-	mvc	SP_ILC(4,%r15),__LC_SVC_ILC
+	mvc	SP_SVC_CODE(4,%r15),__LC_SVC_ILC
 	l	%r12,__LC_THREAD_INFO	# load pointer to thread_info struct
 	UPDATE_VTIME __LC_EXIT_TIMER,__LC_SYNC_ENTER_TIMER,__LC_USER_TIMER
 	UPDATE_VTIME __LC_LAST_UPDATE_TIMER,__LC_EXIT_TIMER,__LC_SYSTEM_TIMER
@@ -550,7 +550,7 @@ pgm_svcper:
 #
 kernel_per:
 	REENABLE_IRQS
-	xc	SP_SVCNR(2,%r15),SP_SVCNR(%r15)
+	xc	SP_SVC_CODE(4,%r15),SP_SVC_CODE(%r15)
 	la	%r2,SP_PTREGS(%r15)	# address of register-save area
 	l	%r1,BASED(.Lhandle_per)	# load adr. of per handler
 	basr	%r14,%r1		# branch to do_single_step
@@ -966,7 +966,7 @@ cleanup_system_call:
 	st	%r15,12(%r12)
 	CREATE_STACK_FRAME __LC_SAVE_AREA
 	mvc	SP_PSW(8,%r15),__LC_SVC_OLD_PSW
-	mvc	SP_ILC(4,%r15),__LC_SVC_ILC
+	mvc	SP_SVC_CODE(4,%r15),__LC_SVC_ILC
 	mvc	0(4,%r12),__LC_THREAD_INFO
 cleanup_vtime:
 	clc	__LC_RETURN_PSW+4(4),BASED(cleanup_system_call_insn+12)

arch/s390/kernel/entry64.S

@@ -43,8 +43,7 @@ SP_R13	     =	STACK_FRAME_OVERHEAD + __PT_GPRS + 104
 SP_R14	     =	STACK_FRAME_OVERHEAD + __PT_GPRS + 112
 SP_R15	     =	STACK_FRAME_OVERHEAD + __PT_GPRS + 120
 SP_ORIG_R2   =	STACK_FRAME_OVERHEAD + __PT_ORIG_GPR2
-SP_ILC	     =	STACK_FRAME_OVERHEAD + __PT_ILC
-SP_SVCNR      =	STACK_FRAME_OVERHEAD + __PT_SVCNR
+SP_SVC_CODE  =	STACK_FRAME_OVERHEAD + __PT_SVC_CODE
 SP_SIZE      =	STACK_FRAME_OVERHEAD + __PT_SIZE
 
 STACK_SHIFT = PAGE_SHIFT + THREAD_ORDER
@@ -250,7 +249,7 @@ sysc_saveall:
 	SAVE_ALL_SVC __LC_SVC_OLD_PSW,__LC_SAVE_AREA
 	CREATE_STACK_FRAME __LC_SAVE_AREA
 	mvc	SP_PSW(16,%r15),__LC_SVC_OLD_PSW
-	mvc	SP_ILC(4,%r15),__LC_SVC_ILC
+	mvc	SP_SVC_CODE(4,%r15),__LC_SVC_ILC
 	lg	%r12,__LC_THREAD_INFO	# load pointer to thread_info struct
 sysc_vtime:
 	UPDATE_VTIME __LC_EXIT_TIMER,__LC_SYNC_ENTER_TIMER,__LC_USER_TIMER
@@ -260,14 +259,14 @@ sysc_update:
 	mvc	__LC_LAST_UPDATE_TIMER(8),__LC_SYNC_ENTER_TIMER
 	LAST_BREAK
 sysc_do_svc:
-	llgh	%r7,SP_SVCNR(%r15)
+	llgh	%r7,SP_SVC_CODE+2(%r15)
 	slag	%r7,%r7,2	# shift and test for svc 0
 	jnz	sysc_nr_ok
 	# svc 0: system call number in %r1
 	llgfr	%r1,%r1		# clear high word in r1
 	cghi	%r1,NR_syscalls
 	jnl	sysc_nr_ok
-	sth	%r1,SP_SVCNR(%r15)
+	sth	%r1,SP_SVC_CODE+2(%r15)
 	slag	%r7,%r1,2	# shift and test for svc 0
 sysc_nr_ok:
 	larl	%r10,sys_call_table
@@ -358,11 +357,12 @@ sysc_notify_resume:
 #
 sysc_restart:
 	ni	__TI_flags+7(%r12),255-_TIF_RESTART_SVC # clear TIF_RESTART_SVC
-	lg	%r7,SP_R2(%r15)		# load new svc number
-	mvc	SP_R2(8,%r15),SP_ORIG_R2(%r15) # restore first argument
 	lmg	%r2,%r6,SP_R2(%r15)	# load svc arguments
-	sth	%r7,SP_SVCNR(%r15)
-	slag	%r7,%r7,2
+	lghi	%r7,0			# svc 0 returns -ENOSYS
+	lh	%r1,SP_SVC_CODE+2(%r15)	# load new svc number
+	cghi	%r1,NR_syscalls
+	jnl	sysc_nr_ok		# invalid svc number -> do svc 0
+	slag	%r7,%r1,2
 	j	sysc_nr_ok		# restart svc
 
 #
@@ -370,7 +370,7 @@ sysc_restart:
 #
 sysc_singlestep:
 	ni	__TI_flags+7(%r12),255-_TIF_PER_TRAP	# clear TIF_PER_TRAP
-	xc	SP_SVCNR(2,%r15),SP_SVCNR(%r15)		# clear svc number
+	xc	SP_SVC_CODE(4,%r15),SP_SVC_CODE(%r15)	# clear svc code
 	la	%r2,SP_PTREGS(%r15)	# address of register-save area
 	larl	%r14,sysc_return	# load adr. of system return
 	jg	do_per_trap
@@ -382,7 +382,7 @@ sysc_singlestep:
 sysc_tracesys:
 	la	%r2,SP_PTREGS(%r15)	# load pt_regs
 	la	%r3,0
-	llgh	%r0,SP_SVCNR(%r15)
+	llgh	%r0,SP_SVC_CODE+2(%r15)
 	stg	%r0,SP_R2(%r15)
 	brasl	%r14,do_syscall_trace_enter
 	lghi	%r0,NR_syscalls
@@ -470,7 +470,7 @@ ENTRY(pgm_check_handler)
 	jnz	pgm_per 		 # got per exception -> special case
 	SAVE_ALL_PGM __LC_PGM_OLD_PSW,__LC_SAVE_AREA
 	CREATE_STACK_FRAME __LC_SAVE_AREA
-	xc	SP_ILC(4,%r15),SP_ILC(%r15)
+	xc	SP_SVC_CODE(4,%r15),SP_SVC_CODE(%r15)
 	mvc	SP_PSW(16,%r15),__LC_PGM_OLD_PSW
 	lg	%r12,__LC_THREAD_INFO	# load pointer to thread_info struct
 	HANDLE_SIE_INTERCEPT
@@ -551,7 +551,7 @@ pgm_svcper:
 	SAVE_ALL_PGM __LC_SVC_OLD_PSW,__LC_SAVE_AREA
 	CREATE_STACK_FRAME __LC_SAVE_AREA
 	mvc	SP_PSW(16,%r15),__LC_SVC_OLD_PSW
-	mvc	SP_ILC(4,%r15),__LC_SVC_ILC
+	mvc	SP_SVC_CODE(4,%r15),__LC_SVC_ILC
 	lg	%r12,__LC_THREAD_INFO	# load pointer to thread_info struct
 	UPDATE_VTIME __LC_EXIT_TIMER,__LC_SYNC_ENTER_TIMER,__LC_USER_TIMER
 	UPDATE_VTIME __LC_LAST_UPDATE_TIMER,__LC_EXIT_TIMER,__LC_SYSTEM_TIMER
@@ -571,7 +571,7 @@ pgm_svcper:
 #
 kernel_per:
 	REENABLE_IRQS
-	xc	SP_SVCNR(2,%r15),SP_SVCNR(%r15)	# clear svc number
+	xc	SP_SVC_CODE(4,%r15),SP_SVC_CODE(%r15)	# clear svc number
 	la	%r2,SP_PTREGS(%r15)	# address of register-save area
 	brasl	%r14,do_per_trap
 	j	pgm_exit
@@ -973,7 +973,7 @@ cleanup_system_call:
 	stg	%r11,0(%r12)
 	CREATE_STACK_FRAME __LC_SAVE_AREA
 	mvc	SP_PSW(16,%r15),__LC_SVC_OLD_PSW
-	mvc	SP_ILC(4,%r15),__LC_SVC_ILC
+	mvc	SP_SVC_CODE(4,%r15),__LC_SVC_ILC
 	mvc	8(8,%r12),__LC_THREAD_INFO
 cleanup_vtime:
 	clc	__LC_RETURN_PSW+8(8),BASED(cleanup_system_call_insn+24)

arch/s390/kernel/ptrace.c

@@ -42,6 +42,7 @@ enum s390_regset {
 	REGSET_GENERAL,
 	REGSET_FP,
 	REGSET_LAST_BREAK,
+	REGSET_SYSTEM_CALL,
 	REGSET_GENERAL_EXTENDED,
 };
 
@@ -303,6 +304,13 @@ static int __poke_user(struct task_struct *child, addr_t addr, addr_t data)
 			   high order bit but older gdb's rely on it */
 			data |= PSW_ADDR_AMODE;
 #endif
+		if (addr == (addr_t) &dummy->regs.psw.addr)
+			/*
+			 * The debugger changed the instruction address,
+			 * reset system call restart, see signal.c:do_signal
+			 */
+			task_thread_info(child)->system_call = 0;
+
 		*(addr_t *)((addr_t) &task_pt_regs(child)->psw + addr) = data;
 
 	} else if (addr < (addr_t) (&dummy->regs.orig_gpr2)) {
@@ -610,6 +618,11 @@ static int __poke_user_compat(struct task_struct *child,
 			/* Build a 64 bit psw address from 31 bit address. */
 			task_pt_regs(child)->psw.addr =
 				(__u64) tmp & PSW32_ADDR_INSN;
+			/*
+			 * The debugger changed the instruction address,
+			 * reset system call restart, see signal.c:do_signal
+			 */
+			task_thread_info(child)->system_call = 0;
 		} else {
 			/* gpr 0-15 */
 			*(__u32*)((addr_t) &task_pt_regs(child)->psw
@@ -737,7 +750,7 @@ asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
 		 * debugger stored an invalid system call number. Skip
 		 * the system call and the system call restart handling.
 		 */
-		regs->svcnr = 0;
+		regs->svc_code = 0;
 		ret = -1;
 	}
 
@@ -899,6 +912,26 @@ static int s390_last_break_get(struct task_struct *target,
 
 #endif
 
+static int s390_system_call_get(struct task_struct *target,
+				const struct user_regset *regset,
+				unsigned int pos, unsigned int count,
+				void *kbuf, void __user *ubuf)
+{
+	unsigned int *data = &task_thread_info(target)->system_call;
+	return user_regset_copyout(&pos, &count, &kbuf, &ubuf,
+				   data, 0, sizeof(unsigned int));
+}
+
+static int s390_system_call_set(struct task_struct *target,
+				const struct user_regset *regset,
+				unsigned int pos, unsigned int count,
+				const void *kbuf, const void __user *ubuf)
+{
+	unsigned int *data = &task_thread_info(target)->system_call;
+	return user_regset_copyin(&pos, &count, &kbuf, &ubuf,
+				  data, 0, sizeof(unsigned int));
+}
+
 static const struct user_regset s390_regsets[] = {
 	[REGSET_GENERAL] = {
 		.core_note_type = NT_PRSTATUS,
@@ -925,6 +958,14 @@ static const struct user_regset s390_regsets[] = {
 		.get = s390_last_break_get,
 	},
 #endif
+	[REGSET_SYSTEM_CALL] = {
+		.core_note_type = NT_S390_SYSTEM_CALL,
+		.n = 1,
+		.size = sizeof(unsigned int),
+		.align = sizeof(unsigned int),
+		.get = s390_system_call_get,
+		.set = s390_system_call_set,
+	},
 };
 
 static const struct user_regset_view user_s390_view = {
@@ -1104,6 +1145,14 @@ static const struct user_regset s390_compat_regsets[] = {
 		.align = sizeof(long),
 		.get = s390_compat_last_break_get,
 	},
+	[REGSET_SYSTEM_CALL] = {
+		.core_note_type = NT_S390_SYSTEM_CALL,
+		.n = 1,
+		.size = sizeof(compat_uint_t),
+		.align = sizeof(compat_uint_t),
+		.get = s390_system_call_get,
+		.set = s390_system_call_set,
+	},
 	[REGSET_GENERAL_EXTENDED] = {
 		.core_note_type = NT_S390_HIGH_GPRS,
 		.n = sizeof(s390_compat_regs_high) / sizeof(compat_long_t),

arch/s390/kernel/signal.c

@@ -30,6 +30,7 @@
 #include <asm/ucontext.h>
 #include <asm/uaccess.h>
 #include <asm/lowcore.h>
+#include <asm/compat.h>
 #include "entry.h"
 
 #define _BLOCKABLE (~(sigmask(SIGKILL) | sigmask(SIGSTOP)))
@@ -156,7 +157,7 @@ static int restore_sigregs(struct pt_regs *regs, _sigregs __user *sregs)
 	current->thread.fp_regs.fpc &= FPC_VALID_MASK;
 
 	restore_fp_regs(&current->thread.fp_regs);
-	regs->svcnr = 0;	/* disable syscall checks */
+	regs->svc_code = 0;	/* disable syscall checks */
 	return 0;
 }
 
@@ -401,7 +402,6 @@ static int handle_signal(unsigned long sig, struct k_sigaction *ka,
  */
 void do_signal(struct pt_regs *regs)
 {
-	unsigned long retval = 0, continue_addr = 0, restart_addr = 0;
 	siginfo_t info;
 	int signr;
 	struct k_sigaction ka;
@@ -421,54 +421,43 @@ void do_signal(struct pt_regs *regs)
 	else
 		oldset = &current->blocked;
 
-	/* Are we from a system call? */
-	if (regs->svcnr) {
-		continue_addr = regs->psw.addr;
-		restart_addr = continue_addr - regs->ilc;
-		retval = regs->gprs[2];
+	/*
+	 * Get signal to deliver. When running under ptrace, at this point
+	 * the debugger may change all our registers, including the system
+	 * call information.
+	 */
+	current_thread_info()->system_call = regs->svc_code;
+	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
+	regs->svc_code = current_thread_info()->system_call;
 
-		/* Prepare for system call restart.  We do this here so that a
-		   debugger will see the already changed PSW. */
-		switch (retval) {
+	if (signr > 0) {
+		/* Whee!  Actually deliver the signal.  */
+		if (regs->svc_code > 0) {
+			/* Check for system call restarting. */
+			switch (regs->gprs[2]) {
+			case -ERESTART_RESTARTBLOCK:
 			case -ERESTARTNOHAND:
+				regs->gprs[2] = -EINTR;
+				break;
 			case -ERESTARTSYS:
+				if (!(ka.sa.sa_flags & SA_RESTART)) {
+					regs->gprs[2] = -EINTR;
+					break;
+				}
+			/* fallthrough */
 			case -ERESTARTNOINTR:
 				regs->gprs[2] = regs->orig_gpr2;
-			regs->psw.addr = restart_addr;
+				regs->psw.addr = regs->psw.addr -
+					(regs->svc_code >> 16);
 				break;
-		case -ERESTART_RESTARTBLOCK:
-			regs->gprs[2] = -EINTR;
-		}
-		regs->svcnr = 0;	/* Don't deal with this again. */
-	}
-
-	/* Get signal to deliver.  When running under ptrace, at this point
-	   the debugger may change all our registers ... */
-	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
-
-	/* Depending on the signal settings we may need to revert the
-	   decision to restart the system call. */
-	if (signr > 0 && regs->psw.addr == restart_addr) {
-		if (retval == -ERESTARTNOHAND
-		    || (retval == -ERESTARTSYS
-			 && !(current->sighand->action[signr-1].sa.sa_flags
-			      & SA_RESTART))) {
-			regs->gprs[2] = -EINTR;
-			regs->psw.addr = continue_addr;
 			}
+			/* No longer in a system call */
+			regs->svc_code = 0;
 		}
 
-	if (signr > 0) {
-		/* Whee!  Actually deliver the signal.  */
-		int ret;
-#ifdef CONFIG_COMPAT
-		if (is_compat_task()) {
-			ret = handle_signal32(signr, &ka, &info, oldset, regs);
-	        }
-		else
-#endif
-			ret = handle_signal(signr, &ka, &info, oldset, regs);
-		if (!ret) {
+		if ((is_compat_task() ?
+		     handle_signal32(signr, &ka, &info, oldset, regs) :
+		     handle_signal(signr, &ka, &info, oldset, regs)) == 0) {
 			/*
 			 * A signal was successfully delivered; the saved
 			 * sigmask will have been stored in the signal frame,
@@ -487,6 +476,23 @@ void do_signal(struct pt_regs *regs)
 		return;
 	}
 
+	/* No handlers present - check for system call restart */
+	if (regs->svc_code > 0) {
+		switch (regs->gprs[2]) {
+		case -ERESTART_RESTARTBLOCK:
+			/* Restart with sys_restart_syscall */
+			regs->svc_code = __NR_restart_syscall;
+		/* fallthrough */
+		case -ERESTARTNOHAND:
+		case -ERESTARTSYS:
+		case -ERESTARTNOINTR:
+			/* Restart system call with magic TIF bit. */
+			regs->gprs[2] = regs->orig_gpr2;
+			set_thread_flag(TIF_RESTART_SVC);
+			break;
+		}
+	}
+
 	/*
 	 * If there's no signal to deliver, we just put the saved sigmask back.
 	 */
@@ -494,13 +500,6 @@ void do_signal(struct pt_regs *regs)
 		clear_thread_flag(TIF_RESTORE_SIGMASK);
 		sigprocmask(SIG_SETMASK, &current->saved_sigmask, NULL);
 	}
-
-	/* Restart a different system call. */
-	if (retval == -ERESTART_RESTARTBLOCK
-	    && regs->psw.addr == continue_addr) {
-		regs->gprs[2] = __NR_restart_syscall;
-		set_thread_flag(TIF_RESTART_SVC);
-	}
 }
 
 void do_notify_resume(struct pt_regs *regs)

include/linux/elf.h

@@ -395,6 +395,7 @@ typedef struct elf64_shdr {
 #define NT_S390_CTRS	0x304		/* s390 control registers */
 #define NT_S390_PREFIX	0x305		/* s390 prefix register */
 #define NT_S390_LAST_BREAK	0x306	/* s390 breaking event address */
+#define NT_S390_SYSTEM_CALL	0x307	/* s390 system call restart data */
 #define NT_ARM_VFP	0x400		/* ARM VFP/NEON registers */