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

Michael S. Tsirkin
2013-12-09T10:53:09Z

On Mon, Dec 09, 2013 at 12:55:29PM +0200, Michael S. Tsirkin wrote:
> 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?

Pls ignore this question - wrote it before I understood the
patch, and forgot to remove.
The rest of the comments and the proposed alternative patch
still stand.

> > ---
> > 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