linux-kernel - Re: [PATCH 2/2] fs: ext4: Fix the inconsistent name exposed by /proc/self/cwd

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Date:   Fri, 1 Oct 2021 14:41:31 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Shreeya Patel <shreeya.patel@...labora.com>
Cc:     viro@...iv.linux.org.uk, adilger.kernel@...ger.ca,
        krisman@...labora.com, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com
Subject: Re: [PATCH 2/2] fs: ext4: Fix the inconsistent name exposed by
 /proc/self/cwd

On Wed, Sep 29, 2021 at 04:23:39PM +0530, Shreeya Patel wrote:
> /proc/self/cwd is a symlink created by the kernel that uses whatever
> name the dentry has in the dcache. Since the dcache is populated only
> on the first lookup, with the string used in that lookup, cwd will
> have an unexpected case, depending on how the data was first looked-up
> in a case-insesitive filesystem.
> 
> Steps to reproduce :-
> 
> root@...t-box:/src# mkdir insensitive/foo
> root@...t-box:/src# cd insensitive/FOO
> root@...t-box:/src/insensitive/FOO# ls -l /proc/self/cwd
> lrwxrwxrwx 1 root root /proc/self/cwd -> /src/insensitive/FOO
> 
> root@...t-box:/src/insensitive/FOO# cd ../fOo
> root@...t-box:/src/insensitive/fOo# ls -l /proc/self/cwd
> lrwxrwxrwx 1 root root /proc/self/cwd -> /src/insensitive/FOO
> 
> Above example shows that 'FOO' was the name used on first lookup here and
> it is stored in dcache instead of the original name 'foo'. This results
> in inconsistent name exposed by /proc/self/cwd since it uses the name
> stored in dcache.
> 
> To avoid the above inconsistent name issue, handle the inexact-match string
> ( a string which is not a byte to byte match, but is an equivalent
> unicode string ) case in ext4_lookup which would store the original name
> in dcache using d_add_ci instead of the inexact-match string name.

I'm not sure this is a problem.  /proc/<pid>/cwd just needs to point
at the current working directory for the process.  Why do we care
whether it matches the case that was stored on disk?  Whether we use
/src/insensitive/FOO, or /src/insensitive/Foo, or
/src/insensitive/foo, all of these will reach the cwd for that
process.

					- Ted