patch title has a typo : there is no skb lock. On 7/9/20 12:49 PM, Thomas Habets wrote: > Here's a test program that illustrates the problem: > https://github.com/ThomasHabets/radiostuff/blob/master/ax25/axftp/examples/client_lockcheck.cc > > Before this patch, this hangs, because the read(2) blocks the > write(2). > > I see that calling skb_recv_datagram without this lock is done in > pep_sock_accept() and atalk_recvmsg() and others, which is what makes > me think it's safe to do here too. But I'm far from an expert on skb > lock semantics.
Again, do not mix skb and socket ;) > > I see some other socket types are also locking during > read. E.g. qrtr_recvmsg. Maybe they need to be fixed too. > > Before: > strace -f -eread,write ./examples/client_lockcheck M0THC-9 M0THC-0 M0THC-2 > strace: Process 3888 attached > [pid 3888] read(3, <unfinished ...> > [pid 3887] write(3, "hello world", 11 > [hang] > > After: > strace -f -eread,write ./examples/client_lockcheck M0THC-9 M0THC-0 M0THC-2 > strace: Process 2433 attached > [pid 2433] read(3, <unfinished ...> > [pid 2432] write(3, "hello world", 11) = 11 > [pid 2433] <... read resumed> "yo", 1000) = 2 > [pid 2433] write(1, "yo\n", 3yo > ) = 3 > [successful exit] > > > diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c > index fd91cd34f25e..378ee132e4d0 100644 > --- a/net/ax25/af_ax25.c > +++ b/net/ax25/af_ax25.c > @@ -1617,22 +1617,22 @@ static int ax25_recvmsg(struct socket *sock, > struct msghdr *msg, size_t size, > int copied; > int err = 0; > > - lock_sock(sk); > /* > * This works for seqpacket too. The receiver has ordered the > * queue for us! We do one quick check first though > */ > if (sk->sk_type == SOCK_SEQPACKET && sk->sk_state != TCP_ESTABLISHED) > { This would be racy, but maybe we do not care. > err = -ENOTCONN; > - goto out; > + goto out_nolock; > } > > /* Now we can treat all alike */ > skb = skb_recv_datagram(sk, flags & ~MSG_DONTWAIT, > flags & MSG_DONTWAIT, &err); Or you could use a smaller change and make this look like net/x25/af_x25.c ? In any case, this will open gates for syzbot fun I am sure ;)