Re: [PATCH net 1/2] tun: unbreak truncated packet signalling

Michael S. Tsirkin
2013-12-09T10:51:56Z

On Mon, Dec 09, 2013 at 06:25:16PM +0800, Jason Wang wrote:
> Commit 6680ec68eff47d36f67b4351bc9836fd6cba9532
> (tuntap: hardware vlan tx support) breaks the truncated packet signal by never
> return a length greater than iov length in tun_put_user(). This patch fixes this
> by always return the length of packet plus possible vlan header. Caller can
> detect the truncated packet by comparing the return value and the size of iov
> length.
>
> Reported-by: Vlad Yasevich <vyasevich@gmail.com>
> Cc: Vlad Yasevich <vyasevich@gmail.com>
> Cc: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>

So writer gets back a value greater than what was written?

> ---
> The patch is needed for stable.
> ---
> drivers/net/tun.c | 23 ++++++++++++-----------
> 1 file changed, 12 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index e26cbea..dd1bd7a 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1183,7 +1183,11 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> const struct iovec *iv, int len)
> {
> struct tun_pi pi = { 0, skb->protocol };
> - ssize_t total = 0;
> + struct {
> + __be16 h_vlan_proto;
> + __be16 h_vlan_TCI;
> + } veth;
> + ssize_t total = 0, off = 0;

Why off = 0 here?
We initialize it to total unconditionally, don't we?

> int vlan_offset = 0;
>
> if (!(tun->flags & TUN_NO_PI)) {
> @@ -1248,14 +1252,11 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> total += tun->vnet_hdr_sz;
> }
>
> + off = total;
> if (!vlan_tx_tag_present(skb)) {
> len = min_t(int, skb->len, len);
> } else {
> int copy, ret;
> - struct {
> - __be16 h_vlan_proto;
> - __be16 h_vlan_TCI;
> - } veth;
>
> veth.h_vlan_proto = skb->vlan_proto;
> veth.h_vlan_TCI = htons(vlan_tx_tag_get(skb));
> @@ -1264,22 +1265,22 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> len = min_t(int, skb->len + VLAN_HLEN, len);
>
> copy = min_t(int, vlan_offset, len);
> - ret = skb_copy_datagram_const_iovec(skb, 0, iv, total, copy);
> + ret = skb_copy_datagram_const_iovec(skb, 0, iv, off, copy);
> len -= copy;
> - total += copy;
> + off += copy;
> if (ret || !len)
> goto done;
>
> copy = min_t(int, sizeof(veth), len);
> - ret = memcpy_toiovecend(iv, (void *)&veth, total, copy);
> + ret = memcpy_toiovecend(iv, (void *)&veth, off, copy);
> len -= copy;
> - total += copy;
> + off += copy;
> if (ret || !len)
> goto done;

This seems wrong: if one of the branches above is taken, total is
never incremented.

> }
>
> - skb_copy_datagram_const_iovec(skb, vlan_offset, iv, total, len);
> - total += len;
> + skb_copy_datagram_const_iovec(skb, vlan_offset, iv, off, len);
> + total += skb->len + (vlan_offset ? sizeof(veth) : 0);
>
> done:
> tun->dev->stats.tx_packets++;

I also think it's inelegant that the veth struct is now in the
outside scope, and the extra ? is also ugly.

Here's a smaller patch to fix all these problems - what do you think?

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

---

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 782e38b..3297e41 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1183,7 +1183,7 @@ static ssize_t tun_put_user(struct tun_struct *tun,
const struct iovec *iv, int len)
{
struct tun_pi pi = { 0, skb->protocol };
- ssize_t total = 0;
+ ssize_t total = 0, offset;
int vlan_offset = 0;

if (!(tun->flags & TUN_NO_PI)) {
@@ -1248,6 +1248,8 @@ static ssize_t tun_put_user(struct tun_struct *tun,
total += tun->vnet_hdr_sz;
}

+ offset = total;
+ total += skb->len;
if (!vlan_tx_tag_present(skb)) {
len = min_t(int, skb->len, len);
} else {
@@ -1257,6 +1259,8 @@ static ssize_t tun_put_user(struct tun_struct *tun,
__be16 h_vlan_TCI;
} veth;

+ total += sizeof(veth);
+
veth.h_vlan_proto = skb->vlan_proto;
veth.h_vlan_TCI = htons(vlan_tx_tag_get(skb));

@@ -1279,7 +1283,6 @@ static ssize_t tun_put_user(struct tun_struct *tun,
}

skb_copy_datagram_const_iovec(skb, vlan_offset, iv, total, len);
- total += len;

done:
tun->dev->stats.tx_packets++;

> --
> 1.8.3.2