sparc: Fix handling of orig_i0 wrt. debugging when restarting syscalls.

[ A combination of upstream commits 1d299bc7732c34d85bd43ac1a8745f5a2fed2078 and
  e88d2468718b0789b4c33da2f7e1cef2a1eee279 ]

Although we provide a proper way for a debugger to control whether
syscall restart occurs, we run into problems because orig_i0 is not
saved and restored properly.

Luckily we can solve this problem without having to make debuggers
aware of the issue.  Across system calls, several registers are
considered volatile and can be safely clobbered.

Therefore we use the pt_regs save area of one of those registers, %g6,
as a place to save and restore orig_i0.

Debuggers transparently will do the right thing because they save and
restore this register already.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Change-Id: I904bc58192c45c132938a97629578a954f26478d
Reviewed-on: http://git-master/r/74212
Reviewed-by: Varun Wadekar <vwadekar@nvidia.com>
Tested-by: Varun Wadekar <vwadekar@nvidia.com>

3 files changed, 63 insertions, 27 deletions

diff --git a/arch/sparc/kernel/signal32.c b/arch/sparc/kernel/signal32.c
index 2caa556db86d..023b8860dc97 100644
--- a/arch/sparc/kernel/signal32.c
+++ b/arch/sparc/kernel/signal32.c
@@ -822,21 +822,23 @@ static inline void syscall_restart32(unsigned long orig_i0, struct pt_regs *regs
  * want to handle. Thus you cannot kill init even with a SIGKILL even by
  * mistake.
  */
-void do_signal32(sigset_t *oldset, struct pt_regs * regs,
-		 int restart_syscall, unsigned long orig_i0)
+void do_signal32(sigset_t *oldset, struct pt_regs * regs)
 {
 	struct k_sigaction ka;
+	unsigned long orig_i0;
+	int restart_syscall;
 	siginfo_t info;
 	int signr;
 	
 	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
 
-	/* If the debugger messes with the program counter, it clears
-	 * the "in syscall" bit, directing us to not perform a syscall
-	 * restart.
-	 */
-	if (restart_syscall && !pt_regs_is_syscall(regs))
-		restart_syscall = 0;
+	restart_syscall = 0;
+	orig_i0 = 0;
+	if (pt_regs_is_syscall(regs) &&
+	    (regs->tstate & (TSTATE_XCARRY | TSTATE_ICARRY))) {
+		restart_syscall = 1;
+		orig_i0 = regs->u_regs[UREG_G6];
+	}
 
 	if (signr > 0) {
 		if (restart_syscall)
diff --git a/arch/sparc/kernel/signal_32.c b/arch/sparc/kernel/signal_32.c
index 8ce247ac04cc..d54c6e53aba0 100644
--- a/arch/sparc/kernel/signal_32.c
+++ b/arch/sparc/kernel/signal_32.c
@@ -519,10 +519,26 @@ static void do_signal(struct pt_regs *regs, unsigned long orig_i0)
 	siginfo_t info;
 	int signr;
 
+	/* It's a lot of work and synchronization to add a new ptrace
+	 * register for GDB to save and restore in order to get
+	 * orig_i0 correct for syscall restarts when debugging.
+	 *
+	 * Although it should be the case that most of the global
+	 * registers are volatile across a system call, glibc already
+	 * depends upon that fact that we preserve them.  So we can't
+	 * just use any global register to save away the orig_i0 value.
+	 *
+	 * In particular %g2, %g3, %g4, and %g5 are all assumed to be
+	 * preserved across a system call trap by various pieces of
+	 * code in glibc.
+	 *
+	 * %g7 is used as the "thread register".   %g6 is not used in
+	 * any fixed manner.  %g6 is used as a scratch register and
+	 * a compiler temporary, but it's value is never used across
+	 * a system call.  Therefore %g6 is usable for orig_i0 storage.
+	 */
 	if (pt_regs_is_syscall(regs) && (regs->psr & PSR_C))
-		restart_syscall = 1;
-	else
-		restart_syscall = 0;
+		regs->u_regs[UREG_G6] = orig_i0;
 
 	if (test_thread_flag(TIF_RESTORE_SIGMASK))
 		oldset = &current->saved_sigmask;
@@ -535,8 +551,12 @@ static void do_signal(struct pt_regs *regs, unsigned long orig_i0)
 	 * the software "in syscall" bit, directing us to not perform
 	 * a syscall restart.
 	 */
-	if (restart_syscall && !pt_regs_is_syscall(regs))
-		restart_syscall = 0;
+	restart_syscall = 0;
+	if (pt_regs_is_syscall(regs) && (regs->psr & PSR_C)) {
+		restart_syscall = 1;
+		orig_i0 = regs->u_regs[UREG_G6];
+	}
+
 
 	if (signr > 0) {
 		if (restart_syscall)
diff --git a/arch/sparc/kernel/signal_64.c b/arch/sparc/kernel/signal_64.c
index a2b81598d905..f0836cd0e2f2 100644
--- a/arch/sparc/kernel/signal_64.c
+++ b/arch/sparc/kernel/signal_64.c
@@ -529,11 +529,27 @@ static void do_signal(struct pt_regs *regs, unsigned long orig_i0)
 	siginfo_t info;
 	int signr;
 	
+	/* It's a lot of work and synchronization to add a new ptrace
+	 * register for GDB to save and restore in order to get
+	 * orig_i0 correct for syscall restarts when debugging.
+	 *
+	 * Although it should be the case that most of the global
+	 * registers are volatile across a system call, glibc already
+	 * depends upon that fact that we preserve them.  So we can't
+	 * just use any global register to save away the orig_i0 value.
+	 *
+	 * In particular %g2, %g3, %g4, and %g5 are all assumed to be
+	 * preserved across a system call trap by various pieces of
+	 * code in glibc.
+	 *
+	 * %g7 is used as the "thread register".   %g6 is not used in
+	 * any fixed manner.  %g6 is used as a scratch register and
+	 * a compiler temporary, but it's value is never used across
+	 * a system call.  Therefore %g6 is usable for orig_i0 storage.
+	 */
 	if (pt_regs_is_syscall(regs) &&
-	    (regs->tstate & (TSTATE_XCARRY | TSTATE_ICARRY))) {
-		restart_syscall = 1;
-	} else
-		restart_syscall = 0;
+	    (regs->tstate & (TSTATE_XCARRY | TSTATE_ICARRY)))
+		regs->u_regs[UREG_G6] = orig_i0;
 
 	if (current_thread_info()->status & TS_RESTORE_SIGMASK)
 		oldset = &current->saved_sigmask;
@@ -542,22 +558,20 @@ static void do_signal(struct pt_regs *regs, unsigned long orig_i0)
 
 #ifdef CONFIG_COMPAT
 	if (test_thread_flag(TIF_32BIT)) {
-		extern void do_signal32(sigset_t *, struct pt_regs *,
-					int restart_syscall,
-					unsigned long orig_i0);
-		do_signal32(oldset, regs, restart_syscall, orig_i0);
+		extern void do_signal32(sigset_t *, struct pt_regs *);
+		do_signal32(oldset, regs);
 		return;
 	}
 #endif	
 
 	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
 
-	/* If the debugger messes with the program counter, it clears
-	 * the software "in syscall" bit, directing us to not perform
-	 * a syscall restart.
-	 */
-	if (restart_syscall && !pt_regs_is_syscall(regs))
-		restart_syscall = 0;
+	restart_syscall = 0;
+	if (pt_regs_is_syscall(regs) &&
+	    (regs->tstate & (TSTATE_XCARRY | TSTATE_ICARRY))) {
+		restart_syscall = 1;
+		orig_i0 = regs->u_regs[UREG_G6];
+	}
 
 	if (signr > 0) {
 		if (restart_syscall)