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

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Date:   Fri, 28 Jul 2023 21:20:48 -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, 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?

- Eric