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Subject:	Re: WSL symbolic links
To:	cygwin AT cygwin DOT com
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	<20200326110059 DOT GG3261 AT calimero DOT vinschen DOT de>
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From:	Thomas Wolff <towo AT towo DOT net>
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Date:	Fri, 27 Mar 2020 00:52:17 +0100
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Am 26.03.2020 um 20:56 schrieb Corinna Vinschen:
> Brian and Thomas,
>
>
> Thanks to both of you for providing this info.
>
>
> On Mar 26 13:12, Brian Inglis wrote:
>> On 2020-03-26 05:00, Corinna Vinschen wrote:
>>> On Mar 26 10:00, Thomas Wolff wrote:
>>>> A symbolic link created with WSL is neither interpreted in cygwin nor can it
>>>> be deleted:
>>>>> touch file
>>>>> wsl ln -s file link
>>>>> wsl ls -l link
>>>> lrwxrwxrwx    1 towo     towo             1 Mar 26 08:56 link -> file
>>>>> ls -l link
>>>> -rw-r----- 1 Unknown+User Unknown+Group 0 Mar 26 00:00 link
>>> What kind of file are they in the real world?  Reparse points?  If so,
>>> what content do they have?  I attached a Q&D source from my vault
>>> of old test apps to check on reparse point content.  Please compile with
>>>    gcc -g ../src/rd-reparse.c -o rd-reparse -lntdll
>>> It takes a single native NT path as parameter, kind of like this:
>>>    ./rd-reparse '\??\C:\cygwin64\home\corinna\link'
>> They should be WSL or Windows mklink (soft) links, and the reason why mklink was
>> allowed unelevated in Windows 10 with Developer mode.
>>
>> In an *elevated* shell:
>>
>> $ ls -dln u
>> -rw-r----- 1 4294967295 4294967295 0 Nov  9 06:09 u
>> $ getfacl u
>> getfacl: u: Permission denied
>> $ icacls u
>> u NULL SID:(DENY)(Rc,S,REA,WEA,X,DC)
>>    $HOSTNAME\$USER:(F)
>>    $HOSTNAME\$USER:(RX,W,DC)
>>    BUILTIN\Users:(Rc,S,RA)
>>    BUILTIN\Administrators:(RX,W,DC)
>>    BUILTIN\Users:(DENY)(S,RD,REA,X)
>>    Everyone:(RX)
>>    NT AUTHORITY\SYSTEM:(I)(F)
>>    BUILTIN\Administrators:(I)(F)
>>    $HOSTNAME\$USER:(I)(F)
>>
>> Successfully processed 1 files; Failed processing 0 files
>> $ ./rd-reparse '\??\C:\...\u'
>> ReparseTag:           0xa000001d
>                          ^^^^^^^^^^
>
> 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
 >

If the link name itself contains non-ASCII, rd-reparse fails with
NtOpenFile: C0000034

> 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.
Thomas
>
>
> Thanks,
> Corinna
>
>
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