Re: [PATCH v2 3/7] migration: Fix socket listen backlog

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Oct 06, 2026 at 02:20:42PM -0400, Peter Xu wrote:
> QEMU incoming side may have set a wrong listen() backlog.  After the
> support of multifd + postcopy now we have four possibilities on setting the
> backlog, some of them are wrong:
> 
>   Vanilla:            1 (correct)
>   Multifd:            N (wrong, should be N+1)
>   Preempt:            2 (correct)
>   Multifd+Preempt:    N (wrong, should be N+2)
> 
> No real report for this, found by code observation only.  Logically the
> backlog (pending accept requests on destination on the listening sockets)
> needs to be the maximum of possible channels.  Otherwise if the accept
> queue full on destination, source QEMU may hit connection errors.

From a robustness POV, we might also consider retrying connections
if we see a transient failure. Though I'm not sure offhand what
errrno would result on the client when the accept() queue  is full,
hopefully there's something distinct.

This would mitigate a risk of DoS from an unauthorized client
making its own incoming connections concurrently. Such a client
is not necessarily malicious, as there are monitoring tools or
port scanners that randomly open connections to TCP listener
sockets.

> Cc: Daniel P. Berrangé <berrange@redhat.com>
> Fixes: 0705e56496 ("multifd: Use number of channels as listen backlog")
> Fixes: e274188612 ("migration: enable multifd and postcopy together")
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/socket.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>



With regards,
Daniel
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