Subject: Re: [PATCH 0/4] RCU: introduce noref debug

From: Paolo Abeni <[email-masked]>
Date: Wed, 11 Oct 2017 16:50:36 +0200

On Tue, 2017-10-10 at 21:02 -0700, Paul E. McKenney wrote:
> Linus and Ingo will ask me how users decide how they should set that
> additional build flag.  Especially given that if there is code that
> requires non-strict checking, isn't everyone required to set up non-strict
> checking to avoid false positives?  Unless you can also configure out
> all the code that requires non-strict checking, I suppose, but how
> would you keep track of what to configure out?

I'm working to a new version using a single compile flag - without
additional strict option.

I don't know of any other subsytem that stores rcu pointer in
datastructures for a longer amount of time. That having said, I wonder
if the tests should completely move to the networking subsystem for the
time being. The Kconfig option would thus be called NET_DEBUG or
something along the lines. For abstraction it would be possible to add
an atomic_notifier_chain to the rcu_read/unlock methods, where multiple
users or checkers could register for. That way we keep the users
seperate from the implementation for the cost of a bit more layering
and more indirect calls. But given that this will anyway slow down
execution a lot, it will anyway only be suitable in
testing/verification/debugging environments.

> OK.  There will probably be some discussion about the API in that case.

I'll drop noref parameter, the key will became mandatory - the exact
position of where the reference of RCU managed object is stored. In the
case of noref dst it is &skb->_skb_refdst. With this kind of API it
should suite more subsystems.

> True enough.  Except that if people were good about always clearing the
> pointer, then the pointer couldn't leak, right?  Or am I missing something
> in your use cases?

This is correct. The dst_entry checking in skb, which this patch series
implements there are strict brackets in the sense of skb_dst_set,
skb_dst_set_noref, skb_dst_force, etc., which form brackets around the
safe uses of those dst_entries. This patch series validates that the
correct skb_dst_* functions are being called before the sk_buff leaves
the rcu protected section. Thus we don't need to modify and review a
lot of code but we can just patch into those helpers already.

> Or to put it another way -- have you been able to catch any real
> pointer-leak bugs with thister-leak bugs with this?  The other RCU
> debug options have had pretty long found-bug lists before we accepted
> them.

There have been two problems found so far, one is a rather minor one
while the other one seems like a normal bug. The patches for those are
part of this series (3/4 and 4/4).

Regards,

Paolo