net: fix kernel deadlock with interface rename and netdev name retrieval.

When the kernel (compiled with CONFIG_PREEMPT=n) is performing the
rename of a network interface, it can end up waiting for a workqueue
to complete. If userland is able to invoke a SIOCGIFNAME ioctl or a
SO_BINDTODEVICE getsockopt in between, the kernel will deadlock due to
the fact that read_secklock_begin() will spin forever waiting for the
writer process (the one doing the interface rename) to update the
devnet_rename_seq sequence.

This patch fixes the problem by adding a helper (netdev_get_name())
and using it in the code handling the SIOCGIFNAME ioctl and
SO_BINDTODEVICE setsockopt.

The netdev_get_name() helper uses raw_seqcount_begin() to avoid
spinning forever, waiting for devnet_rename_seq->sequence to become
even. cond_resched() is used in the contended case, before retrying
the access to give the writer process a chance to finish.

The use of raw_seqcount_begin() will incur some unneeded work in the
reader process in the contended case, but this is better than
deadlocking the system.

Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 60584b185a0c..96e4c21e15e0 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1695,6 +1695,7 @@ extern int		init_dummy_netdev(struct net_device *dev);
 extern struct net_device        *dev_get_by_index(struct net *net, int ifindex);
 extern struct net_device        *__dev_get_by_index(struct net *net, int ifindex);
 extern struct net_device        *dev_get_by_index_rcu(struct net *net, int ifindex);
+extern int              netdev_get_name(struct net *net, char *name, int ifindex);
 extern int              dev_restart(struct net_device *dev);
 #ifdef CONFIG_NETPOLL_TRAP
 extern int              netpoll_trap(void);