Messages in this thread | | | From | Jann Horn <> | Date | Thu, 6 Feb 2020 18:15:56 +0100 | Subject | Re: [PATCH] cred: Use RCU primitives to access RCU pointers |
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On Thu, Feb 6, 2020 at 5:49 PM Joel Fernandes <joel@joelfernandes.org> wrote: > On Thu, Feb 06, 2020 at 12:28:42PM +0100, Jann Horn wrote: > [snip] > > > > > > > > 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). [...] > > > 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. > > Can you point to an example in the kernel that actually uses ->cred this way? > I'm just curious. That is, reads task's ->cred pointer, and assigns it to an > RCU managed pointer?
I'm almost sure that there are no such cases at the moment. However, from a maintainability standpoint, I'm still very twitchy about this change; the current API encapsulates the RCU weirdness in the standard helper functions, but with your proposal, suddenly taking f_cred from somewhere and using it as a new task's subjective creds, or something like that, would be unsafe.
> I think such an example would be the point that the commit you mentioned > addresses. The commit basically says "as long as nobody does get_cred() on > the task_struct ->cred, we are good, but if somebody does do it, then we have > to deferred-free it". But I could not find such an example. > > That said, I agree the removal of non_rcu can be considered out of scope for > this patch.
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