linux-kernel - Re: [PATCH v2 2/2] exfat currently unconditionally strips trailing periods '.' when performing path lookup, but allows them in the filenames during file creation. This is done intentionally, loosely following Windows behaviour and specifications which state:

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Message-ID: <20220310210633.095f0245@suse.de>
Date:   Thu, 10 Mar 2022 21:06:33 +0100
From:   David Disseldorp <ddiss@...e.de>
To:     Vasant Karasulli <vkarasulli@...e.de>,
        Namjae Jeon <linkinjeon@...nel.org>
Cc:     Sungjong Seo <sj1557.seo@...sung.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH v2 2/2] exfat currently unconditionally strips trailing
 periods '.' when performing path lookup, but allows them in the filenames
 during file creation. This is done intentionally, loosely following Windows
 behaviour and specifications which state:

Thanks for reworking these changes, Vasant.

Please trim the 1/2 and 2/2 patch subjects down to around 50 chars
(including a "exfat: " prefix), with the details moved into the commit
message body...

On Thu, 10 Mar 2022 15:24:55 +0100, Vasant Karasulli wrote:

>   #exFAT
>   The concatenated file name has the same set of illegal characters as
>   other FAT-based file systems (see Table 31).
> 
>   #FAT
>   ...
>   Leading and trailing spaces in a long name are ignored.
>   Leading and embedded periods are allowed in a name and are stored in
>   the long name. Trailing periods are ignored.
> 
> Note: Leading and trailing space ' ' characters are currently retained
> by Linux kernel exfat, in conflict with the above specification.

I think it makes sense to mention your findings from the Windows tests
here. E.g. "Windows 10 also retains leading and trailing space
characters".

> Some implementations, such as fuse-exfat, don't perform path trailer
> removal. When mounting images which contain trailing-dot paths, these
> paths are unreachable, e.g.:
> 
>   + mount.exfat-fuse /dev/zram0 /mnt/test/
>   FUSE exfat 1.3.0
>   + cd /mnt/test/
>   + touch fuse_created_dots... '  fuse_created_spaces  '
>   + ls -l
>   total 0
>   -rwxrwxrwx 1 root 0 0 Aug 18 09:45 '  fuse_created_spaces  '
>   -rwxrwxrwx 1 root 0 0 Aug 18 09:45  fuse_created_dots...
>   + cd /
>   + umount /mnt/test/
>   + mount -t exfat /dev/zram0 /mnt/test
>   + cd /mnt/test
>   + ls -l
>   ls: cannot access 'fuse_created_dots...': No such file or directory
>   total 0
>   -rwxr-xr-x 1 root 0 0 Aug 18 09:45 '  fuse_created_spaces  '
>   -????????? ? ?    ? ?            ?  fuse_created_dots...
>   + touch kexfat_created_dots... '  kexfat_created_spaces  '
>   + ls -l
>   ls: cannot access 'fuse_created_dots...': No such file or directory
>   total 0
>   -rwxr-xr-x 1 root 0 0 Aug 18 09:45 '  fuse_created_spaces  '
>   -rwxr-xr-x 1 root 0 0 Aug 18 09:45 '  kexfat_created_spaces  '
>   -????????? ? ?    ? ?            ?  fuse_created_dots...
>   -rwxr-xr-x 1 root 0 0 Aug 18 09:45  kexfat_created_dots
>   + cd /
>   + umount /mnt/test/
> 
> With this change, the "keep_last_dots" mount option can be used to access
> paths with trailing periods and disallow creating files with names with
> trailing periods. E.g. continuing from the previous example:
> 
>   + mount -t exfat -o keep_last_dots /dev/zram0 /mnt/test
>   + cd /mnt/test
>   + ls -l
>   total 0
>   -rwxr-xr-x 1 root 0 0 Aug 18 10:32 '  fuse_created_spaces  '
>   -rwxr-xr-x 1 root 0 0 Aug 18 10:32 '  kexfat_created_spaces  '
>   -rwxr-xr-x 1 root 0 0 Aug 18 10:32  fuse_created_dots...
>   -rwxr-xr-x 1 root 0 0 Aug 18 10:32  kexfat_created_dots

It'd be nice to demonstrate "keep_last_dots" creation here as well, e.g.

  + echo > kexfat_created_dots_again...
  sh: kexfat_created_dots_again...: Invalid argument

@Namjae: not sure whether this is what you had in mind for preventing
creation of invalid paths. What's your preference?

Cheers, David