linux-ext4 - Re: [PATCH v4 3/7] libfs: Validate negative dentries in case-insensitive directories

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Date:   Thu, 3 Aug 2023 21:41:56 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Gabriel Krisman Bertazi <krisman@...e.de>
Cc:     viro@...iv.linux.org.uk, brauner@...nel.org, tytso@....edu,
        jaegeuk@...nel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Subject: Re: [PATCH v4 3/7] libfs: Validate negative dentries in
 case-insensitive directories

On Thu, Aug 03, 2023 at 01:37:45PM -0400, Gabriel Krisman Bertazi wrote:
> Eric Biggers <ebiggers@...nel.org> writes:
> 
> > On Thu, Jul 27, 2023 at 01:28:39PM -0400, Gabriel Krisman Bertazi wrote:
> >>   - In __lookup_slow, either the parent inode is read locked by the
> >>     caller (lookup_slow), or it is called with no flags (lookup_one*).
> >>     The read lock suffices to prevent ->d_name modifications, with the
> >>     exception of one case: __d_unalias, will call __d_move to fix a
> >>     directory accessible from multiple dentries, which effectively swaps
> >>     ->d_name while holding only the shared read lock.  This happens
> >>     through this flow:
> >> 
> >>     lookup_slow()  //LOOKUP_CREATE
> >>       d_lookup()
> >>         ->d_lookup()
> >>           d_splice_alias()
> >>             __d_unalias()
> >>               __d_move()
> >> 
> >>     Nevertheless, this case is not a problem because negative dentries
> >>     are not allowed to be moved with __d_move.
> >
> > Isn't it possible for a negative dentry to become a positive one concurrently?
> 
> Do you mean d_splice_alias racing with a dentry instantiation and
> __d_move being called on a negative dentry that is turning positive?
> 
> It is not possible for __d_move to be called with a negative dentry for
> d_splice_alias, since the inode->i_lock is locked during __d_find_alias,
> so it can't race with __d_instantiate or d_add. Then, __d_find_alias
> can't find negative dentries in the first place, so we either have a
> positive dentry, in which case __d_move is fine with regard to
> d_revalidate_name, or we don't have any aliases and don't call
> __d_move.
> 
> Can you clarify what problem you see here?
> 

I agree that negative dentries can't be moved --- I pointed this out earlier
(https://lore.kernel.org/linux-fsdevel/20230720060657.GB2607@sol.localdomain).
The question is whether if ->d_revalidate sees a negative dentry, when can it
assume that it remains a negative dentry for the remainder of ->d_revalidate.
I'm not sure there is a problem, I just don't understand your explanation.

- Eric