Re: [PATCH] sockets: update SOCKET_ADDRESS_TYPE_FD listen(2) backlog

[Eric Blake]

On 3/10/21 11:30 AM, Stefan Hajnoczi wrote:
> socket_get_fd() fails with the error "socket_get_fd: too many
> connections" if the given listen backlog value is not 1.
> 
> Not all callers set the backlog to 1. For example, commit
> 582d4210eb2f2ab5baac328fe4b479cd86da1647 ("qemu-nbd: Use SOMAXCONN for
> socket listen() backlog") uses SOMAXCONN. This will always fail with in
> socket_get_fd().
> 
> This patch calls listen(2) on the fd to update the backlog value. The
> socket may already be in the listen state. I have tested that this works
> on Linux 5.10 and macOS Catalina.
> 
> As a bonus this allows us to detect when the fd cannot listen. Now we'll
> be able to catch unbound or connected fds in socket_listen().
> 
> Drop the num argument from socket_get_fd() since this function is also
> called by socket_connect() where a listen backlog value does not make
> sense.
> 
> Fixes: e5b6353cf25c99c3f08bf51e29933352f7140e8f ("socket: Add backlog 
> parameter to socket_listen")
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Cc: Juan Quintela <quintela@redhat.com>
> Cc: Eric Blake <eblake@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  util/qemu-sockets.c | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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