tun: signedness bug in tun_get_user()

The recent fix d9bf5f1309 "tun: compare with 0 instead of total_len" is
not totally correct.  Because "len" and "sizeof()" are size_t type, that
means they are never less than zero.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 2 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 5a8ee1cf090a..71af122edf2d 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1074,8 +1074,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
         u32 rxhash;
 
         if (!(tun->flags & TUN_NO_PI)) {
-                if ((len -= sizeof(pi)) < 0)
+                if (len < sizeof(pi))
                         return -EINVAL;
+                len -= sizeof(pi);
 
                 if (memcpy_fromiovecend((void *)&pi, iv, 0, sizeof(pi)))
                         return -EFAULT;
@@ -1083,8 +1084,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
         }
 
         if (tun->flags & TUN_VNET_HDR) {
-                if ((len -= tun->vnet_hdr_sz) < 0)
+                if (len < tun->vnet_hdr_sz)
                         return -EINVAL;
+                len -= tun->vnet_hdr_sz;
 
                 if (memcpy_fromiovecend((void *)&gso, iv, offset, sizeof(gso)))
                         return -EFAULT;