[permalink] [raw]

Subject: [PATCH v2 tip/core/rcu 12/13] bonding/bond_main: Apply rcu_access_pointer() to avoid sparse false positive

From: "Paul E. McKenney" <[email protected]>

The sparse checking for rcu_assign_pointer() was recently upgraded
to reject non-__kernel address spaces. This also rejects __rcu,
which is almost always the right thing to do. However, the uses in
bond_change_active_slave(), bond_enslave(), and __bond_release_one()
are legitimate: They are assigning a pointer to an element from an
RCU-protected list (or a NULL pointer), and all elements of this list
are already visible to caller.

This commit therefore silences these false positives either by laundering
the pointers using rcu_access_pointer() as suggested by Josh Triplett,
or by using RCU_INIT_POINTER() for NULL pointer assignments.

Reported-by: kbuild test robot <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: [email protected]
Cc: [email protected]
---
drivers/net/bonding/bond_main.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 72df399c4ab3..2f276b971bc4 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -890,7 +890,8 @@ void bond_change_active_slave(struct bonding *bond, struct slave *new_active)
if (new_active)
bond_set_slave_active_flags(new_active);
} else {
- rcu_assign_pointer(bond->curr_active_slave, new_active);
+ rcu_assign_pointer(bond->curr_active_slave,
+ rcu_access_pointer(new_active));
}

if (bond->params.mode == BOND_MODE_ACTIVEBACKUP) {
@@ -1601,7 +1602,8 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev)
* so we can change it without calling change_active_interface()
*/
if (!bond->curr_active_slave && new_slave->link == BOND_LINK_UP)
- rcu_assign_pointer(bond->curr_active_slave, new_slave);
+ rcu_assign_pointer(bond->curr_active_slave,
+ rcu_access_pointer(new_slave));

break;
} /* switch(bond_mode) */
@@ -1801,7 +1803,7 @@ static int __bond_release_one(struct net_device *bond_dev,
}

if (all) {
- rcu_assign_pointer(bond->curr_active_slave, NULL);
+ RCU_INIT_POINTER(bond->curr_active_slave, NULL);
} else if (oldcurrent == slave) {
/*
* Note that we hold RTNL over this sequence, so there
--
1.8.1.5

2013-10-09 21:32:35

by Paul E. McKenney

[permalink] [raw]

Subject: [PATCH v2 tip/core/rcu 06/13] ipv4/ip_socketglue: Apply rcu_access_pointer() to avoid sparse false positive

From: "Paul E. McKenney" <[email protected]>

The sparse checking for rcu_assign_pointer() was recently upgraded
to reject non-__kernel address spaces. This also rejects __rcu,
which is almost always the right thing to do. However, the use in
ip_ra_control() is legitimate: It is assigning a pointer to an element
from an RCU-protected list, and all elements of this list are already
visible to caller.

This commit therefore silences this false positive by laundering the
pointer using rcu_access_pointer() as suggested by Josh Triplett.

Reported-by: kbuild test robot <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Alexey Kuznetsov <[email protected]>
Cc: James Morris <[email protected]>
Cc: Hideaki YOSHIFUJI <[email protected]>
Cc: Patrick McHardy <[email protected]>
Cc: [email protected]
---
net/ipv4/ip_sockglue.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index d9c4f113d709..d328f158f0e5 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -269,7 +269,7 @@ int ip_ra_control(struct sock *sk, unsigned char on,
}
/* dont let ip_call_ra_chain() use sk again */
ra->sk = NULL;
- rcu_assign_pointer(*rap, ra->next);
+ rcu_assign_pointer(*rap, rcu_access_pointer(ra->next));
spin_unlock_bh(&ip_ra_lock);

if (ra->destructor)
--
1.8.1.5

2013-10-09 21:33:17

by Paul E. McKenney

[permalink] [raw]

2013-10-09 21:35:17

by Paul E. McKenney

[permalink] [raw]

Subject: [PATCH v2 tip/core/rcu 02/13] notifiers: Apply rcu_access_pointer() to avoid sparse false positive

From: "Paul E. McKenney" <[email protected]>

The sparse checking for rcu_assign_pointer() was recently upgraded
to reject non-__kernel address spaces. This also rejects __rcu,
which is almost always the right thing to do. However, the use in
notifier_chain_unregister() is legitimate: It is deleting an element
from an RCU-protected list, and all elements of this list are already
visible to caller.

This commit therefore silences this false positive by laundering the
pointer using rcu_access_pointer() as suggested by Josh Triplett.

Reported-by: kbuild test robot <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
---
kernel/notifier.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/notifier.c b/kernel/notifier.c
index 2d5cc4ccff7f..1857c71bae40 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -51,7 +51,7 @@ static int notifier_chain_unregister(struct notifier_block **nl,
{
while ((*nl) != NULL) {
if ((*nl) == n) {
- rcu_assign_pointer(*nl, n->next);
+ rcu_assign_pointer(*nl, rcu_access_pointer(n->next));
return 0;
}
nl = &((*nl)->next);
--
1.8.1.5

2013-10-09 21:42:33

by Eric Dumazet

[permalink] [raw]

Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive

On Wed, 2013-10-09 at 14:29 -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <[email protected]>
>
> The sparse checking for rcu_assign_pointer() was recently upgraded
> to reject non-__kernel address spaces. This also rejects __rcu,
> which is almost always the right thing to do. However, the use in
> ip6_tnl_unlink() is legitimate: It is assigning a pointer to an element
> from an RCU-protected list, and all elements of this list are already
> visible to caller.
>
> This commit therefore silences this false positive by laundering the
> pointer using rcu_access_pointer() as suggested by Josh Triplett.
>
> Reported-by: kbuild test robot <[email protected]>
> Signed-off-by: Paul E. McKenney <[email protected]>
> Cc: "David S. Miller" <[email protected]>
> Cc: Alexey Kuznetsov <[email protected]>
> Cc: James Morris <[email protected]>
> Cc: Hideaki YOSHIFUJI <[email protected]>
> Cc: Patrick McHardy <[email protected]>
> Cc: [email protected]
> ---
> net/ipv6/ip6_tunnel.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
> index 61355f7f4da5..ecc0166e1a9c 100644
> --- a/net/ipv6/ip6_tunnel.c
> +++ b/net/ipv6/ip6_tunnel.c
> @@ -245,7 +245,7 @@ ip6_tnl_unlink(struct ip6_tnl_net *ip6n, struct ip6_tnl *t)
> (iter = rtnl_dereference(*tp)) != NULL;
> tp = &iter->next) {
> if (t == iter) {
> - rcu_assign_pointer(*tp, t->next);
> + rcu_assign_pointer(*tp, rcu_access_pointer(t->next));
> break;
> }
> }

Then it seems a mere "*tp = t->next;" would be enough ?

We do not really need a barrier.

2013-10-09 21:57:56

by Paul E. McKenney

[permalink] [raw]

Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive

On Wed, Oct 09, 2013 at 02:42:29PM -0700, Eric Dumazet wrote:
> On Wed, 2013-10-09 at 14:29 -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <[email protected]>
> >
> > The sparse checking for rcu_assign_pointer() was recently upgraded
> > to reject non-__kernel address spaces. This also rejects __rcu,
> > which is almost always the right thing to do. However, the use in
> > ip6_tnl_unlink() is legitimate: It is assigning a pointer to an element
> > from an RCU-protected list, and all elements of this list are already
> > visible to caller.
> >
> > This commit therefore silences this false positive by laundering the
> > pointer using rcu_access_pointer() as suggested by Josh Triplett.
> >
> > Reported-by: kbuild test robot <[email protected]>
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > Cc: "David S. Miller" <[email protected]>
> > Cc: Alexey Kuznetsov <[email protected]>
> > Cc: James Morris <[email protected]>
> > Cc: Hideaki YOSHIFUJI <[email protected]>
> > Cc: Patrick McHardy <[email protected]>
> > Cc: [email protected]
> > ---
> > net/ipv6/ip6_tunnel.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
> > index 61355f7f4da5..ecc0166e1a9c 100644
> > --- a/net/ipv6/ip6_tunnel.c
> > +++ b/net/ipv6/ip6_tunnel.c
> > @@ -245,7 +245,7 @@ ip6_tnl_unlink(struct ip6_tnl_net *ip6n, struct ip6_tnl *t)
> > (iter = rtnl_dereference(*tp)) != NULL;
> > tp = &iter->next) {
> > if (t == iter) {
> > - rcu_assign_pointer(*tp, t->next);
> > + rcu_assign_pointer(*tp, rcu_access_pointer(t->next));
> > break;
> > }
> > }
>
> Then it seems a mere "*tp = t->next;" would be enough ?
>
> We do not really need a barrier.

Hmmm... I could use RCU_INIT_POINTER(). Something like the following?

RCU_INIT_POINTER(ACCESS_ONCE(*tp), t->next);

The ACCESS_ONCE() to prevent the compiler from doing anything stupid.
Presumably the value of t->next cannot change, so a normal load suffices.

Or did you have something else in mind?

Thanx, Paul

2013-10-09 22:10:27

by Eric Dumazet

[permalink] [raw]

Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive

On Wed, 2013-10-09 at 14:57 -0700, Paul E. McKenney wrote:

> Hmmm... I could use RCU_INIT_POINTER(). Something like the following?
>
> RCU_INIT_POINTER(ACCESS_ONCE(*tp), t->next);
>
> The ACCESS_ONCE() to prevent the compiler from doing anything stupid.
> Presumably the value of t->next cannot change, so a normal load suffices.
>
> Or did you have something else in mind?

Well, *tp and t->next are both of the same type, with __rcu attribute.

struct ip6_tnl __rcu **tp;

So I meant :

ACCESS_ONCE(*tp) = t->next;

If really we can have a really stupid compiler.

2013-10-09 22:18:26

by Josh Triplett

[permalink] [raw]

On Sun, Oct 13, 2013 at 04:14:39AM -0700, Paul E. McKenney wrote:
> On Sat, Oct 12, 2013 at 07:42:18PM +0000, Mathieu Desnoyers wrote:
> > ----- Original Message -----
> > > On Sat, Oct 12, 2013 at 06:43:45PM +0200, Hannes Frederic Sowa wrote:
> > > > Regarding the volatile access, I hope that the C11 memory model
> > > > and enhancements to the compiler will some day provide a better
> > > > way to express the semantics of what is tried to express here
> > > > (__atomic_store_n/__atomic_load_n with the accompanied memory model,
> > > > which could be even weaker to what a volatile access would enfore
> > > > now and could guarantee atomic stores/loads).
> > >
> > > I just played around a bit more. Perhaps we could try to warn of silly
> > > usages of ACCESS_ONCE():
> > >
> > > --- a/include/linux/compiler.h
> > > +++ b/include/linux/compiler.h
> > > @@ -349,7 +349,11 @@ void ftrace_likely_update(struct ftrace_branch_data *f,
> > > int val, int expect);
> > > * use is to mediate communication between process-level code and irq/NMI
> > > * handlers, all running on the same CPU.
> > > */
> > > -#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> > > +#define ACCESS_ONCE(x) (*({ \
> > > + compiletime_assert(sizeof(typeof(x)) <= sizeof(typeof(&x)), \
> > > + "ACCESS_ONCE likely not atomic"); \
> >
> > AFAIU, ACCESS_ONCE() is not meant to ensure atomicity of load/store,
> > but rather merely ensures that the compiler will not merge nor refetch
> > accesses. I don't think the assert check you propose is appropriate with
> > respect to the ACCESS_ONCE() semantic.
>
> I am with Mathieu on this one, at least unless there is some set of actual
> bugs already in the kernel that these length checks would find.

I guess my wording of "ACCESS_ONCE likely not atomic" was misplaced. Something
like volatile access to memory larger than the processor register size is
probably not what you intended. Use atomics or proper locking. ;)
And maybe that is not even correct.

> /me wonders about structs of size 3, 5, 6, and 7...

Checked a x86_64 allyesconfig build with sizes above pointer size and odd
parity and nothing broke.

Greetings,

Hannes