s390/ptrace: fix setting syscall number

When strace wants to update the syscall number, it sets GPR2
to the desired number and updates the GPR via PTRACE_SETREGSET.
It doesn't update regs->int_code which would cause the old syscall
executed on syscall restart. As we cannot change the ptrace ABI and
don't have a field for the interruption code, check whether the tracee
is in a syscall and the last instruction was svc. In that case assume
that the tracer wants to update the syscall number and copy the GPR2
value to regs->int_code.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/ptrace.c

@@ -323,6 +323,25 @@ static inline void __poke_user_per(struct task_struct *child,
 		child->thread.per_user.end = data;
 }
 
+static void fixup_int_code(struct task_struct *child, addr_t data)
+{
+	struct pt_regs *regs = task_pt_regs(child);
+	int ilc = regs->int_code >> 16;
+	u16 insn;
+
+	if (ilc > 6)
+		return;
+
+	if (ptrace_access_vm(child, regs->psw.addr - (regs->int_code >> 16),
+			&insn, sizeof(insn), FOLL_FORCE) != sizeof(insn))
+		return;
+
+	/* double check that tracee stopped on svc instruction */
+	if ((insn >> 8) != 0xa)
+		return;
+
+	regs->int_code = 0x20000 | (data & 0xffff);
+}
 /*
  * Write a word to the user area of a process at location addr. This
  * operation does have an additional problem compared to peek_user.
@@ -334,7 +353,9 @@ static int __poke_user(struct task_struct *child, addr_t addr, addr_t data)
 	struct user *dummy = NULL;
 	addr_t offset;
 
+
 	if (addr < (addr_t) &dummy->regs.acrs) {
+		struct pt_regs *regs = task_pt_regs(child);
 		/*
 		 * psw and gprs are stored on the stack
 		 */
@@ -352,7 +373,11 @@ static int __poke_user(struct task_struct *child, addr_t addr, addr_t data)
 				/* Invalid addressing mode bits */
 				return -EINVAL;
 		}
-		*(addr_t *)((addr_t) &task_pt_regs(child)->psw + addr) = data;
+
+		if (test_pt_regs_flag(regs, PIF_SYSCALL) &&
+			addr == offsetof(struct user, regs.gprs[2]))
+			fixup_int_code(child, data);
+		*(addr_t *)((addr_t) &regs->psw + addr) = data;
 
 	} else if (addr < (addr_t) (&dummy->regs.orig_gpr2)) {
 		/*
@@ -718,6 +743,10 @@ static int __poke_user_compat(struct task_struct *child,
 			regs->psw.mask = (regs->psw.mask & ~PSW_MASK_BA) |
 				(__u64)(tmp & PSW32_ADDR_AMODE);
 		} else {
+
+			if (test_pt_regs_flag(regs, PIF_SYSCALL) &&
+				addr == offsetof(struct compat_user, regs.gprs[2]))
+				fixup_int_code(child, data);
 			/* gpr 0-15 */
 			*(__u32*)((addr_t) &regs->psw + addr*2 + 4) = tmp;
 		}