proc: fixup empty ARGV case for /proc/$PID/cmdline

/proc/*/cmdline code checks if it should look at ENVP area by checking last byte of ARGV area:

	rv = access_remote_vm(mm, arg_end - 1, &c, 1, 0);
	if (rv <= 0)
		goto out_free_page;

If ARGV is somehow made empty (by doing execve(..., NULL, ...) or manually setting ->arg_start and ->arg_end to equal values), the decision will be based on byte which doesn't even belong to ARGV/ENVP.

So, quickly check if ARGV area is empty and report 0 to match previous behaviour.

---

 fs/proc/base.c |    5 +++++
 1 file changed, 5 insertions(+)

@@ -243,6 +243,11 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 	len1 = arg_end - arg_start;
 	len2 = env_end - env_start;
 
+	/* Empty ARGV. */
+	if (len1 == 0) {
+		rv = 0;
+		goto out_free_page;
+	}
 	/*
 	 * Inherently racy -- command line shares address space
 	 * with code and data.