1.7.5: Occasional failure of CreatePipe or signal handing due to thread-unsafe code in cwdstuff::set

Corinna Vinschen corinna-cygwin@cygwin.com
Wed Aug 11 14:50:00 GMT 2010


On Aug 11 15:15, Andy Koppe wrote:
> On 11 August 2010 14:55, Corinna Vinschen wrote:
> >> there's no Win32-safe way to set a new
> >> directory handle as cwd in Vista and later anymore.  Since there's no
> >> official API to set the cwd using a directory handle, there's no way to
> >> set the Win32 cwd to a directory with restricted permissions.
> >> This *is* frustrating.
> >>
> >> I'll look into another solution.  Probably we will have to call
> >> SetCurrentDirectory again and ignore any error.  I don't accept the
> >> aforementioned restriction for POSIX calls.
> >
> > I'm wondering what you say about this.  From John's description I take
> > it that the construct really is unsafe, and we can't discount the chance
> > that this race can break Cygwin itself in some scenarios.
> >
> > Since SetCurrentDirectory appears to be the only valid way to change the
> > Win32 CWD in Vista and later, I think we should do so as well.
> >
> > This means, the Win32 CWD will be wrong as soon as
> >
> > - The path is not accessible to admins w/o backup privileges.
> >
> > - The pathlen is > 259.
> 
> Why did they ever bother with long path names if fundamental stuff
> like that doesn't work?

Weeeell, don't ask me!  Actually, since the NT kernel doesn't have a
notion of a current working directory, the CWD is only used in the Win32
API anyway.  And, come on, *nobody* ever needs a CWD path longer than
MAX_PATH, right?

> > The latter is already true anyway since no tinkering with internal
> > Win32 structures will allow that.  At least the pathname in the PEB
> > is wrong, just the handle was correct so far.
> >
> > That also leads to the question what to do in these cases?
> >
> >  1) Return with error
> >
> >    --> Disallows these paths in Cygwin's POSIX API as well.  Not an
> >        option, IMHO.
> >
> >  2) Just ignore that SetCurrentDirectory failed?
> >
> >    --> Win32 CWD == previous CWD.
> >
> >  3) If SetCurrentDirectory fails, call
> >
> >       SetCurrentDirectory (GetSystemDirectory ())
> >
> >    That's basically what CMD.EXE does if it can't handle the current
> >    CWD at startup.  You can easily test that by setting the CWD to a
> >    network UNC path and start CMD.EXE.
> >
> > I think option 3) is what we should use, but I'm open to other
> > suggestions.
> 
> I agree with approach 3, but could we send it somewhere safer? Ideally
> somewhere that isn't writable. I'm concerned about the damage that a
> process that thinks it's elsewhere could do to the system32 directory.

Where is "somewhere safer"?  I mean, even CMD.EXE uses it as fallback.
What about Cygwin's root dir?


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat



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