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

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Date:   Thu, 03 Aug 2023 13:37:45 -0400
From:   Gabriel Krisman Bertazi <krisman@...e.de>
To:     Eric Biggers <ebiggers@...nel.org>
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

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?

-- 
Gabriel Krisman Bertazi