[S390] ptrace: fix return value of do_syscall_trace_enter()

strace may change the system call number, so regs->gprs[2] must not
be read before tracehook_report_syscall_entry(). This fixes a bug
where "strace -f" will hang after a vfork().

Cc: <stable@kernel.org>
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 2 insertions, 3 deletions

diff --git a/arch/s390/kernel/ptrace.c b/arch/s390/kernel/ptrace.c
index 33fdc5a7976..9f654da4cec 100644
--- a/arch/s390/kernel/ptrace.c
+++ b/arch/s390/kernel/ptrace.c
@@ -640,7 +640,7 @@ long compat_arch_ptrace(struct task_struct *child, compat_long_t request,
 
 asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
 {
-        long ret;
+        long ret = 0;
 
         /* Do the secure computing check first. */
         secure_computing(regs->gprs[2]);
@@ -649,7 +649,6 @@ asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
          * The sysc_tracesys code in entry.S stored the system
          * call number to gprs[2].
          */
-        ret = regs->gprs[2];
         if (test_thread_flag(TIF_SYSCALL_TRACE) &&
             (tracehook_report_syscall_entry(regs) ||
              regs->gprs[2] >= NR_syscalls)) {
@@ -671,7 +670,7 @@ asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
                                     regs->gprs[2], regs->orig_gpr2,
                                     regs->gprs[3], regs->gprs[4],
                                     regs->gprs[5]);
-        return ret;
+        return ret ?: regs->gprs[2];
 }
 
 asmlinkage void do_syscall_trace_exit(struct pt_regs *regs)