linux-user: Use safe_syscall for execve syscall

Wrap execve() in the safe-syscall handling. Although execve() is not
an interruptible syscall, it is a special case: if we allow a signal
to happen before we make the host$ syscall then we will 'lose' it,
because at the point of execve the process leaves QEMU's control.  So
we use the safe syscall wrapper to ensure that we either take the
signal as a guest signal, or else it does not happen before the
execve completes and makes it the other program's problem.

The practical upshot is that without this SIGTERM could fail to
terminate the process.
Signed-off-by: NTimothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
Message-id: 1441497448-32489-25-git-send-email-T.E.Baldwin99@members.leeds.ac.uk
[PMM: expanded commit message to explain in more detail why this is
 needed, and add comment about it too]
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>

linux-user/syscall.c

@@ -703,6 +703,7 @@ safe_syscall4(pid_t, wait4, pid_t, pid, int *, status, int, options, \
               struct rusage *, rusage)
 safe_syscall5(int, waitid, idtype_t, idtype, id_t, id, siginfo_t *, infop, \
               int, options, struct rusage *, rusage)
+safe_syscall3(int, execve, const char *, filename, char **, argv, char **, envp)
 
 static inline int host_to_target_sock_type(int host_type)
 {
@@ -6179,7 +6180,17 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
 
             if (!(p = lock_user_string(arg1)))
                 goto execve_efault;
-            ret = get_errno(execve(p, argp, envp));
+            /* Although execve() is not an interruptible syscall it is
+             * a special case where we must use the safe_syscall wrapper:
+             * if we allow a signal to happen before we make the host
+             * syscall then we will 'lose' it, because at the point of
+             * execve the process leaves QEMU's control. So we use the
+             * safe syscall wrapper to ensure that we either take the
+             * signal as a guest signal, or else it does not happen
+             * before the execve completes and makes it the other
+             * program's problem.
+             */
+            ret = get_errno(safe_execve(p, argp, envp));
             unlock_user(p, arg1, 0);
 
             goto execve_end;