Messages in this thread | | | From | Jann Horn <> | Date | Thu, 6 Feb 2020 12:28:42 +0100 | Subject | Re: [PATCH] cred: Use RCU primitives to access RCU pointers |
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On Thu, Feb 6, 2020 at 2:32 AM Joel Fernandes <joel@joelfernandes.org> wrote: > On Wed, Jan 29, 2020 at 03:14:56PM +0100, Jann Horn wrote: > > 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. > > Amol, Jann, if I understand the discussion correctly, objects ->cred > point (the subjective creds) are never (or never need to be) RCU-managed. > This makes sense in light of the commit Jann pointed out > (d7852fbd0f0423937fa287a598bfde188bb68c22). > > How about the following diff as a starting point? > > 1. Remove all ->cred accessing happening through RCU primitive.
Sounds good.
> 2. Remove __rcu from task_struct ->cred
Sounds good.
> 3. Also I removed the whole non_rcu flag, and introduced a new put_cred_non_rcu() API > which places that task-synchronously use ->cred can overwrite. Callers > doing such accesses like access() can use this API instead.
That's wrong, don't do that.
->cred is a reference without RCU semantics, ->real_cred is a reference with RCU semantics. If there have never been any references with RCU semantics to a specific instance of struct cred, then that instance can indeed be freed without an RCU grace period. But it would be possible for some filesystem code to take a reference to current->cred, and assign it to some pointer with RCU semantics somewhere, then drop that reference with put_cred() immediately before you reach put_cred_non_rcu(); with the result that despite using put_cred(), the other side doesn't get RCU semantics.
Just leave the whole ->non_rcu thing exactly as it was.
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