Re: [PATCH 1/5] tap: Better report errors receiving from QEMU socket

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 26 Jul 2024 13:25:08 +0200
Stefano Brivio <sbrivio@redhat.com> wrote:

> On Fri, 26 Jul 2024 17:20:27 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > If we get an error on recv() from the QEMU socket, we currently don't
> > print any kind of error.  Although this can happen in a non-fatal situation
> > such as a guest restarting, it's unusual enough that we realy should report
> > something for debugability.
> > 
> > Add an error message in this case.  Also always report when the qemu
> > connection closes for any reason, not just when it will cause us to exit
> > (--one-off).
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  tap.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/tap.c b/tap.c
> > index 44bd4445..a2ae7c2a 100644
> > --- a/tap.c
> > +++ b/tap.c
> > @@ -969,10 +969,10 @@ void tap_add_packet(struct ctx *c, ssize_t l2len, char *p)
> >   */
> >  static void tap_sock_reset(struct ctx *c)
> >  {
> > -	if (c->one_off) {
> > -		info("Client closed connection, exiting");
> > +	info("Client connection closed%s", c->one_off ? ", exiting" : "");
> > +
> > +	if (c->one_off)
> >  		exit(EXIT_SUCCESS);
> > -	}
> >  
> >  	/* Close the connected socket, wait for a new connection */
> >  	epoll_ctl(c->epollfd, EPOLL_CTL_DEL, c->fd_tap, NULL);
> > @@ -1005,8 +1005,10 @@ redo:
> >  
> >  	n = recv(c->fd_tap, p, TAP_BUF_FILL, MSG_DONTWAIT);
> >  	if (n < 0) {
> > -		if (errno != EINTR && errno != EAGAIN && errno != EWOULDBLOCK)
> > +		if (errno != EINTR && errno != EAGAIN && errno != EWOULDBLOCK) {
> > +			err_perror("Error receiving from QEMU socket");  
> 
> Nit I'm fixing up on merge: it's not necessarily QEMU, because libkrun
> and perhaps others also use this path (in fact, the whole problem was
> reported as part of the libkrun integration).
> 
> Maybe it's obvious to users anyway, but this might seriously confuse
> somebody who's using e.g. krun on Asahi Linux (is QEMU running, one
> might wonder):
>   https://github.com/slp/krun#motivation
>   https://github.com/slp/krun/blob/main/crates/krun/src/net.rs
> 
> On top of that, now that we have an error message, I guess it would be
> nice to state we're resetting the connection, because it's not really
> obvious: the subsequent message from tap_sock_reset() makes it look like
> the client decided to close the connection on its own.
> 
> So I'm changing this to:
> 
> 			err("Error receiving from guest, resetting connection");

...or rather:

			err_perror("Receive error on guest connection, reset");

-- 
Stefano