On Sun, Dec 13, 2020 at 10:30 PM Jason Wang [off-list ref] wrote:

On 2020/12/14 上午9:32, Willem de Bruijn wrote:

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On Sat, Dec 12, 2020 at 7:18 PM Willem de Bruijn
[off-list ref] wrote:

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afterwards, the error handling in vhost handle_tx() will try to
decrease the same refcount again. This is wrong and fix this by delay
copying ubuf_info until we're sure there's no errors.

I think the right approach is to address this in the error paths, rather than
complicate the normal datapath.

Is it sufficient to suppress the call to vhost_net_ubuf_put in the handle_tx
sendmsg error path, given that vhost_zerocopy_callback will be called on
kfree_skb?

We can not call kfree_skb() until the skb was created.

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Or alternatively clear the destructor in drop:

The uarg->callback() is called immediately after we decide do datacopy
even if caller want to do zerocopy. If another error occurs later, the vhost
handle_tx() will try to decrease it again.

Oh right, I missed the else branch in this path:

         /* copy skb_ubuf_info for callback when skb has no error */
         if (zerocopy) {
                 skb_shinfo(skb)->destructor_arg = msg_control;
                 skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
                 skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
         } else if (msg_control) {
                 struct ubuf_info *uarg = msg_control;
                 uarg->callback(uarg, false);
         }

So if handle_tx_zerocopy calls tun_sendmsg with ubuf_info (and thus a
reference to release), there are these five options:

1. tun_sendmsg succeeds, ubuf_info is associated with skb.
      reference released from kfree_skb calling vhost_zerocopy_callback later

2. tun_sendmsg succeeds, ubuf_info is released immediately, as skb is
not zerocopy.

3. tun_sendmsg fails before creating skb, handle_tx_zerocopy correctly
cleans up on receiving error from tun_sendmsg.

4. tun_sendmsg fails after creating skb, but with copying: decremented
at branch shown above + again in handle_tx_zerocopy

5. tun_sendmsg fails after creating skb, with zerocopy: decremented at
kfree_skb in drop: + again in handle_tx_zerocopy

Since handle_tx_zerocopy has no idea whether on error 3, 4 or 5
occurred,

Actually, it does. If sendmsg returns an error, it can test whether
vq->heads[nvq->upend_idx].len != VHOST_DMA_IN_PROGRESS.

Just to make sure I understand this. Any reason for it can't be
VHOST_DMA_IN_PROGRESS here?

It can be, and it will be if tun_sendmsg returns EINVAL before
assigning the skb destructor.

Only if tun_sendmsg released the zerocopy state through
kfree_skb->vhost_zerocopy_callback will it have been updated to
VHOST_DMA_DONE_LEN. And only then must the caller not try to release
the state again.