Re: [V9fs-developer] [GIT PULL] 9p file system bug fixes for 2.6.35-rc2

Venkateswararao Jujjuri (JV)
2010-06-08T14:30:01Z

Linus Torvalds wrote:
>
> On Mon, 7 Jun 2010, Eric Van Hensbergen wrote:
>> jvrao (2):
>> Add a helper function to get fsgid for a file create.
>> 9p: Add a wstat after TCREATE to fix the gid.
>
> Quite frankly, this looks rather broken.
>
> It uses "dentry->d_parent" without locking (it so happens to likely be ok,
> since we are in "create()" and thus should be holding the parent
> semaphore). On its own, that might be excusable (if people were even
> _aware_ of the this locking rule!), but it does so just to get the inode
> pointer to that parent.
>
> And the only thing that makes it ok to access dentry->d_parent - the fact
> that we are in v9fs_create() - is also the thing that should have made
> people look at the arguments to the function and say "hmm".

Silly me. I sent out another patch using the dir inode passed through arguments.
But we still need to analyze the use of dentry->d_parent in other parts of code..

- JV

>
> We pass in the directory inode pointer as an argument to the create
> function! The code could have used that thing directly, instead of
> mucking around with dentry pointers that it had no business looking at.
>
> I see why it seems to have happened: v9fs does the exact same thing for
> the pre-existing "v9fs_fid_lookup()". So there is history to this
> behavior.
>
> Maybe people weren't aware of the fact that just dereferencing
> dentry->d_parent willy-nilly isn't actually allowed. That field changes.
> Sure, there are cases where it's ok, but this is a dangerous thing to do
> in general.
>
> In fact, the other thing that I find doing that whole "dentry->d_parent"
> thing seems to literally be broken. If you look at v9fs_fid_lookup(),
> you'll notice how it walks up the d_parent chain, and at that point you do
> NOT own the directory i_mutex, so at that point d_parent really _can_ be
> changing wildly due to concurrent renames or whatever.
>
> So 9pfs seems to have some preexisting bugs in this area. I'm not going to
> pull new bug-prone code. See the other discussions about being tight this
> release about really _only_ taking regressions after the merge window
> closed.
>
> Linus
>
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