Messages in this thread | | | Date | Thu, 13 Dec 2012 14:15:40 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 3/7] uprobes: Kill the pointless inode/uc checks in register/unregister |
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On 12/13, Srikar Dronamraju wrote: > > * Oleg Nesterov <oleg@redhat.com> [2012-12-10 20:12:32]: > > > On 12/10, Srikar Dronamraju wrote: > > > > > > * Oleg Nesterov <oleg@redhat.com> [2012-11-23 21:28:06]: > > > > > > > register/unregister verifies that inode/uc != NULL. For what? > > > > This really looks like "hide the potential problem", the caller > > > > should pass the valid data. > > > > > > > > > > Agree that users should pass valid data. > > > I do understand that we expect the users to be knowledge-able. > > > Also users are routed thro in-kernel api that does this check. > > > > > > However from an api perspective, if a user passes invalid data, do we > > > want the system to crash. > > > > > > Esp if kernel can identify that users has indeed passed wrong info. I do agree > > > that users can still pass invalid data that kernel maynot be able to > > > identify in most cases. > > > > inode != NULL can't verify that it actually points to the valid inode, > > NULL is only one example of invalid data. > > > > I agree, sometimes it makes sense to protect against the stupid mistakes, > > but if we want to check against NULL we should do > > > > if (WARN_ON(!inode)) > > return; > > > > agree, that warn_on is better than a simple check
and this one
if (WARN_ON(inode < PAGE_OFFSET))
is even better ;)
> > Especially in uprobe_unregister(). The current code is really "hide > > the possible problem" and nothing more. It is better to crash imho > > than silently return. > > > > > > register() also checks uc->next == NULL, probably to prevent the > > > > double-register but the caller can do other stupid/wrong things. > > > > > > Users can surely do more stupid things. But this is again something that > > > kernel can identify. By allowing a double-register of a consumer, thats > > > already registered, we might end up allowing circular loop of consumers. > > > > I understand. But in this case we should document that uc->next must > > be cleared before uprobe_register(). Or add init_consumer(). > > > > And we should change uprobe_unregister() to clear uc->next as well. > > I think that the code like this > > > > uprobe_register(uc); > > uprobe_unregister(uc); > > > > uprobe_register(uc); > > > > should work. Currently it doesn't because of this check. > > > > yes, these should work and makes a case to nullify ->next on unregister. > > However, what if someone tries > > uprobe_register(uc1); > uprobe_register(uc2); > uprobe_register(uc1); > > i.e somebody tries to re-register uc1, while its active and has a valid > next. After the re-registration of uc1, the uprobe->consumers will no more reference uc2.
Yes. And even without uprobe_register(uc2) the result won't be good. This is like list_add(node).
> Should we leave this case as a fool shooting himself?
IMHO yes, or we should create init_consumer() or at least document that the private ->next member should be nullified.
But let me repeat,
> > So I still think these checks are pointless and (at least in unregister) > > even harmful.
Yes, but I am not going to argue if you want to keep these checks.
Oleg.
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