WSL symbolic links

[Thomas Wolff]

Am 27.03.2020 um 12:21 schrieb Corinna Vinschen:
> On Mar 27 00:52, Thomas Wolff wrote:
>> Am 26.03.2020 um 20:56 schrieb Corinna Vinschen:
>>> This is a reparse point tag different from the normal Windows symlink
>>> reparse point tag, 0xa000000c.  Searching for this value shows this
>>> is defined in ntifs.h as IO_REPARSE_TAG_LX_SYMLINK.
>>>
>>> Unfortunately I don't see a definition of the reparse point data for
>>> that reparse point type.
>>>
>>> In your examples the data part looks like a 4 byte int value, being 2 in
>>> both of your examples, maybe a file type, followed by the path in
>>> multibyte, no trailing \0.
>>>
>>> Unfortunately, in both cases the path is relative, just the file name it
>>> points to.  To get more information, could one of you two please create
>>> a few more symlinks?
>>>
>>> - A symlink pointing to a local path, given in absolute path syntax.
>>>     I assume the path will be in POSIX syntax, contain slashes, but it
>>>     would be helpful to see it.
>>>
>>> - A symlink with a target path pointing to a remote file (what syntax
>>>     does this use?)
>>>
>>> - Last but not least, could you please create a symlink pointing to a
>>>     target with a non-ASCII char, e. g., some german umlaut?
>> Not sure what kind of remote you'd like to see. I have a 'net use'
>> (cifs/smbfs) mounted drive but couldn't mount it in WSL. Otherwise:
>>
>>> wsl -d Ubuntu ls -l link*
>> lrwxrwxrwx 1 towo towo  4 Mar 27 00:31 link -> file
>> lrwxrwxrwx 1 towo towo 15 Mar 27 00:31 link-abs -> /mnt/c/tmp/file
>> lrwxrwxrwx 1 towo towo  5 Mar 27 00:39 link-foo -> föö
>> lrwxrwxrwx 1 towo towo 16 Mar 27 00:39 link-foo-abs -> /mnt/c/tmp/föö
>>> rd-reparse '\??\C:\tmp\link' ; echo
>> ReparseTag:           0xa000001d
>> ReparseDataLength:             8
>> Reserved:                      0
>> 02 00 00 00 66 69 6c 65
>>> rd-reparse '\??\C:\tmp\link-abs' ; echo
>> ReparseTag:           0xa000001d
>> ReparseDataLength:            19
>> Reserved:                      0
>> 02 00 00 00 2f 6d 6e 74 2f 63 2f 74 6d 70 2f 66
>> 69 6c 65
>>> rd-reparse '\??\C:\tmp\link-foo' ; echo
>> ReparseTag:           0xa000001d
>> ReparseDataLength:             9
>> Reserved:                      0
>> 02 00 00 00 66 c3 b6 c3 b6
>>> rd-reparse '\??\C:\tmp\link-foo-abs' ; echo
>> ReparseTag:           0xa000001d
>> ReparseDataLength:            20
>> Reserved:                      0
>> 02 00 00 00 2f 6d 6e 74 2f 63 2f 74 6d 70 2f 66
>> c3 b6 c3 b6
> Thanks!  In the meantime I could fix my problems with the Windows Store
> and was finally able to install WSL for further testing.  See below.
>
>> If the link name itself contains non-ASCII, rd-reparse fails with
>> NtOpenFile: C0000034
> Yeah, that's expected.  The test app only handles filename with ASCII
> chars.
>
>>> It's questionable if supporting this new symlink type makes sense, but
>>> taking a closer look doesn't hurt, I guess.
>> Well, at least they should be deletable, I think.
> I debugged this now and I found that practically all problems, including
> the inability to delete the symlink, are a result of not being able to
> open the reparse point correctly as reparse point within Cygwin.  So as
> not to destroy something important, Cygwin only opens reparse points as
> reparse points if it recognizes the reparse point type.
>
> Consequentially, all immediate problems go away, as soon as Cygwin
> recognizes and handles the symlink :)
>
> So I created a patch and pushed it.  The latest developer snapshot from
> https://cygwin.com/snapshots/ contains this patch.
Works, great, thank you!
> Funny sidenote: Assuming you create symlinks pointing to files with
> non-UTF-8 chars, e. g., umlauts in ISO-8859-1, then the symlink converts
> *all* these chars to the Unicode REPLACEMENT CHARACTER 0xfffd.  I assume
> this will also happen if you try to create the file with these chars in
> the first place, so it's not much of a problem.
As Windows filenames are character strings as opposed to Linux filenames 
which are byte strings, some strange behaviour is unavoidable. I see:
$ wsl ls -l link_LW
lrwxrwxrwx    1 towo     towo            19 Mar 27 12:11 link_LW -> 
file_L_
$ ls -l link_LW
lrwxrwxrwx 1 towo Kein 11 27. Mrz 13:11 link_LW -> file_L_����
which looks OK for me.
Thomas