[Linux-kernel-mentees] [PATCH] cred: Use RCU primitives to access RCU pointers

[Jann Horn]

On Wed, Jan 29, 2020 at 7:57 AM Amol Grover <frextrite at gmail.com> wrote:
> On Tue, Jan 28, 2020 at 08:09:17PM +0100, Jann Horn wrote:
> > On Tue, Jan 28, 2020 at 6:04 PM Amol Grover <frextrite at gmail.com> wrote:
> > > On Tue, Jan 28, 2020 at 10:30:19AM +0100, Jann Horn wrote:
> > > > On Tue, Jan 28, 2020 at 8:28 AM Amol Grover <frextrite at gmail.com> wrote:
> > > > > task_struct.cred and task_struct.real_cred are annotated by __rcu,
> > > >
> > > > task_struct.cred doesn't actually have RCU semantics though, see
> > > > commit d7852fbd0f0423937fa287a598bfde188bb68c22. For task_struct.cred,
> > > > it would probably be more correct to remove the __rcu annotation?
> > >
> > > Hi Jann,
> > >
> > > I went through the commit you mentioned. If I understand it correctly,
> > > ->cred was not being accessed concurrently (via RCU), hence, a non_rcu
> > > flag was introduced, which determined if the clean-up should wait for
> > > RCU grace-periods or not. And since, the changes were 'thread local'
> > > there was no need to wait for an entire RCU GP to elapse.
> >
> > Yeah.
> >
> > > The commit too, as you said, mentions the removal of __rcu annotation.
> > > However, simply removing the annotation won't work, as there are quite a
> > > few instances where RCU primitives are used. Even get_current_cred()
> > > uses RCU APIs to get a reference to ->cred.
> >
> > Luckily, there aren't too many places that directly access ->cred,
> > since luckily there are helper functions like get_current_cred() that
> > will do it for you. Grepping through the kernel, I see:
[...]
> > So actually, the number of places that already don't use RCU accessors
> > is much higher than the number of places that use them.
> >
> > > So, currently, maybe we
> > > should continue to use RCU APIs and leave the __rcu annotation in?
> > > (Until someone who takes it on himself to remove __rcu annotation and
> > > fix all the instances). Does that sound good? Or do you want me to
> > > remove __rcu annotation and get the process started?
> >
> > I don't think it's a good idea to add more uses of RCU APIs for
> > ->cred; you shouldn't "fix" warnings by making the code more wrong.
> >
> > If you want to fix this, I think it would be relatively easy to fix
> > this properly - as far as I can tell, there are only seven places that
> > you'll have to change, although you may have to split it up into three
> > patches.
>
> Thank you for the detailed analysis. I'll try my best and send you a
> patch.

While you can CC me on that, I'm not a kernel maintainer; you should
send patches to the people who maintain the areas of kernel code that
you're modifying. (kernel/cred.c has no specific maintainer; for that
file, I'd probably try sending patches to Andrew Morton, Oleg
Nesterov, David Howells and Eric Biederman, as well as the
linux-kernel@ mailinglist.)

> But before I start I want to make sure one thing. The changes
> done by the commit you mentioned (which introduced non_rcu flag), should
> be now reverted, right?

No.

> Since, prior to the commit RCU semantics were
> there and RCU was being used (which was unnecessary) and the fix merely,
> removed these (unnecessary) RCU usages (with checks to either use them
> or not, but now we actually don't use RCU for subjective credentials).
>
> So, now what's left is the unused RCU code (which needs to be removed)
> and the changes done in the temporary fix (which would be reverted since
> we don't want to use RCU).

No. Instances of `struct cred` *can* still have an RCU-protected
lifetime; but only certain references to it have RCU semantics.
{task}->cred doesn't have RCU semantics, but {task}->real_cred does
have RCU semantics, and those two can point to the same object.

__rcu annotations mark that a *reference* has RCU semantics. Lack of
__rcu annotation means that the *reference* does not have RCU
semantics, but the object it points to can still have a RCU-protected
lifetime.