[PATCH] s390/ptrace: Preserve ILC when updating system call number

[Heiko Carstens <hca@linux.ibm.com>]

When an attached debugger changes gpr2 via PTRACE_POKEUSR while the
traced process has PIF_SYSCALL set, the int_code in pt_regs is also
updated. In case of an svc it contains the instruction length code
(ILC), as well as the system call number.

The ptrace code sets the ILC unconditionally to a value which matches the
svc instruction. However the svc instruction also may have been executed
via the execute or execute relative long instruction, which have a
different length. Therefore preserve the ILC and only change the system
call number using syscall_set_nr().

This makes sure that in case of a PSW rewind the instruction address
points to the correct instruction.

Fixes: 56e62a737028 ("s390: convert to generic entry")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---
 arch/s390/kernel/ptrace.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/s390/kernel/ptrace.c b/arch/s390/kernel/ptrace.c
index 125ca4c4e30c..d8d5e843859c 100644
--- a/arch/s390/kernel/ptrace.c
+++ b/arch/s390/kernel/ptrace.c
@@ -34,6 +34,7 @@
 #include <asm/ptrace.h>
 #include <asm/rwonce.h>
 #include <asm/fpu.h>
+#include <asm/syscall.h>
 
 #include "entry.h"
 
@@ -345,11 +346,9 @@ static int __poke_user(struct task_struct *child, addr_t addr, addr_t data)
 				return -EINVAL;
 		}
 
-		if (test_pt_regs_flag(regs, PIF_SYSCALL) &&
-			addr == offsetof(struct user, regs.gprs[2])) {
-			struct pt_regs *regs = task_pt_regs(child);
-
-			regs->int_code = 0x20000 | (data & 0xffff);
+		if (addr == offsetof(struct user, regs.gprs[2])) {
+			if (test_pt_regs_flag(regs, PIF_SYSCALL))
+				syscall_set_nr(child, regs, data);
 		}
 		*(addr_t *)((addr_t) &regs->psw + addr) = data;
 	} else if (addr < offsetof(struct user, regs.orig_gpr2)) {
-- 
2.53.0