Re: [bpf PATCH 2/3] bpf: sockmap, zero sg_size on error when buffer is released

[David Miller <davem@davemloft.net>]

From: John Fastabend <john.fastabend@gmail.com>
Date: Wed, 02 May 2018 10:47:32 -0700

> When an error occurs during a redirect we have two cases that need
> to be handled (i) we have a cork'ed buffer (ii) we have a normal
> sendmsg buffer.
> 
> In the cork'ed buffer case we don't currently support recovering from
> errors in a redirect action. So the buffer is released and the error
> should _not_ be pushed back to the caller of sendmsg/sendpage. The
> rationale here is the user will get an error that relates to old
> data that may have been sent by some arbitrary thread on that sock.
> Instead we simple consume the data and tell the user that the data
> has been consumed. We may add proper error recovery in the future.
> However, this patch fixes a bug where the bytes outstanding counter
> sg_size was not zeroed. This could result in a case where if the user
> has both a cork'ed action and apply action in progress we may
> incorrectly call into the BPF program when the user expected an
> old verdict to be applied via the apply action. I don't have a use
> case where using apply and cork at the same time is valid but we
> never explicitly reject it because it should work fine. This patch
> ensures the sg_size is zeroed so we don't have this case.
> 
> In the normal sendmsg buffer case (no cork data) we also do not
> zero sg_size. Again this can confuse the apply logic when the logic
> calls into the BPF program when the BPF programmer expected the old
> verdict to remain. So ensure we set sg_size to zero here as well. And
> additionally to keep the psock state in-sync with the sk_msg_buff
> release all the memory as well. Previously we did this before
> returning to the user but this left a gap where psock and sk_msg_buff
> states were out of sync which seems fragile. No additional overhead
> is taken here except for a call to check the length and realize its
> already been freed. This is in the error path as well so in my
> opinion lets have robust code over optimized error paths.
> 
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>

Acked-by: David S. Miller <davem@davemloft.net>