Messages in this thread | | | From | Jann Horn <> | Date | Wed, 29 Jan 2020 15:14:56 +0100 | Subject | Re: [PATCH] cred: Use RCU primitives to access RCU pointers |
| |
On Wed, Jan 29, 2020 at 7:57 AM Amol Grover <frextrite@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@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@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.
| |