Symbolic link bug in recent Cygwin DLL build

Corinna Vinschen
Fri Apr 17 07:50:16 GMT 2020

Hi Mark,

On Apr 16 20:10, Mark Geisert wrote:
> Corinna Vinschen wrote:
> > On Apr 15 23:53, Mark Geisert wrote:
> > > After installing a recent DLL built from the git source tree I noticed:
> > > 
> > > ~ ln -s /tmp/foo .
> > > 
> > > ~ ls -l foo
> > > lrwxrwxrwx 1 Mark None 12 Apr 15 23:44 foo -> /mnt/tmp/foo
> > 
> > Huh?  That works for me, independently of /tmp/foo existing or not:
> > 
> >    $ ln -s /tmp/foo .
> >    $ ls -l foo
> >    lrwxrwxrwx 1 corinna Users 8 Apr 16 10:38 foo -> /tmp/foo
> > 
> > Since you're building the DLL yourself, can you please debug this with
> > GDB?  It would be very important to find out what on your system adds
> > the /mnt prefix!
> > 
> > It could occur in creating the symlink, that's in, function
> > symlink_wsl(), or it could occur in reading the symlink,,
> > function check_reparse_point_target(), in the else if (rp->ReparseTag ==
> > IO_REPARSE_TAG_LX_SYMLINK) branch at line 2534.
> > 
> > As for /mnt itself, it's the WSL equivalent to the cygdrive prefix.
> > When creating WSL symlinks, Cygwin converts the cygdrive prefix to
> > /mnt, and when reading WSL symlinks, a leading /mnt is converted
> > to the current cygdrive prefix on the fly.
> > 
> > We should just move further discussions to the cygwin-developers ML.
> Sorry about the ML flub.  Your mention of cygdrive prefix gave me some
> dread. From day 1 I've suppressed the prefix on any Cygwin machine I've
> administered; I use /c or /d etc to refer to drives.  That means having this
> line in /etc/fstab:
>     none / cygdrive binary,posix=0,user 0 0
> The "/mnt" is being added at  It's replacing the current
> cygdrive prefix with "/mnt".

Ok, that explains it.  It makes this function exactly this teeny little
bit more complicated :-P

>I'm unclear why we're in a function named
> symlink_wsl() since I'm not using WSL at all.  Maybe it means
> WSL-compatible?  OK if so.

git log --grep symlinks d2e0b65a7fdc..HEAD

> Does that string replacement need to be skipped in this situation of "empty"
> cygdrive prefix?  Or is it something more subtle?

It's a bit more subtil.  The code also tries to convert the cygdrive
prefix to /mnt if no drive letter follows, so you can create a symlink
to the cygdrive directory alone:

  $ ln -s /cygdrive /tmp/cygdrive_symlink

That also allowed the conversion cygdrive prefix to /mnt without having
to look for the drive letters.  Now, with the cygdrive prefix being /,
the first path component has to be checked for being... what?

- Just check for a single letter from a to z?  That would also wrongly
  catch /x if /x is a real subdir in the root dir. 

- Check for an existing drive letter from a to z?  That would break
  the symlink conversion on the WSL side if the symlink was supposed
  to be created pointing to, say, an USB thumb drive which was not
  plugged in at the time of the symlink creation.

- Just skip any /mnt prefix if the cygdrive prefix is /?  That would
  make all drive letter paths incompatible with WSL, and in contrast to
  /mnt symlinks, it does not autommagically "fix" drive letter paths
  if you ever change your cygdrive prefix, which is a neat side-effect
  of the new symlinks, IMHO.

I'm leaning towards solution 3.


Corinna Vinschen
Cygwin Maintainer
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: not available
URL: <>

More information about the Cygwin-developers mailing list