Re: [PATCH] Fix hang if $OLDPWD points to inaccessible directory

[Mikulas Patocka]

On Fri, 29 Sep 2017, Mikulas Patocka wrote:

> 
> 
> On Thu, 28 Sep 2017, Eduardo A. Bustamante López wrote:
> 
> > On Thu, Sep 28, 2017 at 11:44:02AM +0200, Mikulas Patocka wrote:
> > [...]
> > > +++ bash/variables.c
> > > @@ -899,7 +899,7 @@ set_pwd ()
> > >       don't find OLDPWD in the environment, or it doesn't name a 
> > > directory,
> > >       make a dummy invisible variable for OLDPWD, and mark it as 
> > > exported. */
> > >    temp_var = find_variable ("OLDPWD");
> > > -  if (temp_var == 0 || value_cell (temp_var) == 0 || file_isdir 
> > > (value_cell (temp_var)) == 0)
> > > +  if (temp_var == 0 || value_cell (temp_var) == 0)
> > >      {
> > >        temp_var = bind_variable ("OLDPWD", (char *)NULL, 0);
> > 
> > This patch would revert the change introduced in:
> >   
> > http://git.savannah.gnu.org/cgit/bash.git/commit/?h=devel&id=e6f5e0c858b7c0839d346d7d55e56894648c5a33
> 
> This patch doesn't revert the above change. It only removes a test if 
> $OLDPWD is a directory. With this patch, if $OLDPWD points to a directory, 
> there will be no change, and if $OLDPWD points to non-directory, it will 
> not be cleared.
> 
> > Which was prompted by the following bug report from John Wiersba:
> >   https://lists.gnu.org/archive/html/bug-bash/2015-11/msg00115.html
> > 
> > Correct me if I'm wrong, but a hanged sshfs mount will cause many more
> > issues, not only with OLDPWD.
> 
> Hung mount shouldn't cause problems unless someone is accessing it.
> 
> What happened to me - I use the autofs filesystem to automatically mount 
> filesystems on various machines. I was on some autofs-mounted directory, I 
> moved to the home directory (which is not on autofs) and I typed startx. 
> Some times after that I shut down the remote machine that provided the 
> autofs mount.
> 
> The whole X session started to severely mibehave, starting various 
> programs caused hangs, I couldn't even start an xterm with the shell, I 
> couldn't even start the chome browser - the reason for the hangs was, that 
> the OLDPWD variable pointing to the autofs directory ended up being 
> propagated to the Xserver, to the desktop environment and to any program 
> that was running within the X session. I caused serious trouble and I 
> think it should be fixed.
> 
> If I started Xwindow on the non-functioning autofs directory, it would be 
> expected that I get hangs. However, I started Xwindow in the home 
> directory that has nothing to do with autofs - and I got hangs.
> 
> Mikulas

Another problem is that various daemons change the working directory to 
"/", so that they don't keep the directory they were started from open - 
however they don't clear the $OLDPWD environment variable.

So the $OLDPWD variable in a daemon could contain arbitrary garbage. If 
the $OLDPWD variable points to some network directory that is currently 
inaccessible, the daemon won't be able to start any bash scripts.

Mikulas