On Fri, Sep 25, 2015 at 11:01:13AM -0400, Tejun Heo wrote:
>
> I'm not even sure we guarantee memory barrier on kernel/user
> crossings. In practice, we probably have enough barriers (e.g. some
> syscall traps imply barrier) but I can't think of a reason why we'd
> guarantee the existence of barrier there. As an extreme example,
> imagine UML on an architecture with relaxed memory model.
You misunderstood what I wrote. I was not basing this on whether
user-space transitions contained a barrier, but on the fact that
the next syscall must recheck nlk->bound before using nlk->portid.
In fact thanks to your email I now realise that my fix to the
getsockname problem is wrong. Instead of adding a barrier to
netlink_connect I should be adding a nlk->bound check to getname.
---8<---
netlink_getname must check nlk->bound before using nlk->portid
as otherwise nlk->portid may contain garbage.
This patch also adds a netlink_bound helper that encapsulate the
barrier, as was suggested by Tejun. Also as suggested by Linus,
the lockless read of nlk->bound in netlink_bound is protected with
READ_ONCE to ensure that the compiler doesn't do double reads that
may screw up our use of the barrier.
Fixes: da314c9923fe ("netlink: Replace rhash_portid with bound")
Reported-by: Tejun Heo <t...@kernel.org>
Signed-off-by: Herbert Xu <herb...@gondor.apana.org.au>
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 2c15fae..c860464 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -125,6 +125,17 @@ static inline u32 netlink_group_mask(u32 group)
return group ? 1 << (group - 1) : 0;
}
+static inline bool netlink_bound(struct netlink_sock *nlk)
+{
+ bool bound = READ_ONCE(nlk->bound);
+
+ /* Ensure nlk is hashed and visible. */
+ if (bound)
+ smp_rmb();
+
+ return bound;
+}
+
int netlink_add_tap(struct netlink_tap *nt)
{
if (unlikely(nt->dev->type != ARPHRD_NETLINK))
@@ -1524,14 +1535,10 @@ static int netlink_bind(struct socket *sock, struct
sockaddr *addr,
return err;
}
- bound = nlk->bound;
- if (bound) {
- /* Ensure nlk->portid is up-to-date. */
- smp_rmb();
-
+ bound = netlink_bound(nlk);
+ if (bound)
if (nladdr->nl_pid != nlk->portid)
return -EINVAL;
- }
if (nlk->netlink_bind && groups) {
int group;
@@ -1547,9 +1554,6 @@ static int netlink_bind(struct socket *sock, struct
sockaddr *addr,
}
}
- /* No need for barriers here as we return to user-space without
- * using any of the bound attributes.
- */
if (!bound) {
err = nladdr->nl_pid ?
netlink_insert(sk, nladdr->nl_pid) :
@@ -1628,7 +1632,7 @@ static int netlink_getname(struct socket *sock, struct
sockaddr *addr,
nladdr->nl_pid = nlk->dst_portid;
nladdr->nl_groups = netlink_group_mask(nlk->dst_group);
} else {
- nladdr->nl_pid = nlk->portid;
+ nladdr->nl_pid = netlink_bound(nlk) ? nlk->portid : 0;
nladdr->nl_groups = nlk->groups ? nlk->groups[0] : 0;
}
return 0;
@@ -2442,13 +2446,10 @@ static int netlink_sendmsg(struct socket *sock, struct
msghdr *msg, size_t len)
dst_group = nlk->dst_group;
}
- if (!nlk->bound) {
+ if (!netlink_bound(nlk)) {
err = netlink_autobind(sock);
if (err)
goto out;
- } else {
- /* Ensure nlk is hashed and visible. */
- smp_rmb();
}
/* It's a really convoluted way for userland to ask for mmaped
--
Email: Herbert Xu <herb...@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
