linux-user: allow NULL msg in recvfrom

The kernel allows a NULL msg in recvfrom so that he size of the next
message may be queried before allocating a correctly sized buffer. This
change allows the syscall translator to pass along the NULL msg pointer
instead of returning early with EFAULT.

Signed-off-by: Zach Reizner <zachr@google.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <CAFNex=DvFCq=AQf+=19fTfw-T8eZZT=3NnFFm2JMFvVr5QgQyA@mail.gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

Author: zachreizner

linux-user/syscall.c

@@ -3679,9 +3679,14 @@ static abi_long do_recvfrom(int fd, abi_ulong msg, size_t len, int flags,
     void *host_msg;
     abi_long ret;
 
-    host_msg = lock_user(VERIFY_WRITE, msg, len, 0);
-    if (!host_msg)
-        return -TARGET_EFAULT;
+    if (!msg) {
+        host_msg = NULL;
+    } else {
+        host_msg = lock_user(VERIFY_WRITE, msg, len, 0);
+        if (!host_msg) {
+            return -TARGET_EFAULT;
+        }
+    }
     if (target_addr) {
         if (get_user_u32(addrlen, target_addrlen)) {
             ret = -TARGET_EFAULT;