[lxc-devel] procfs and unnamed pipes

[Serge Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> "Timofey.Kirillov" <distorhead at gmail.com> writes:
> 
> > Hi,
> >
> > I have a question about using unnamed pipes with procfs.
> >
> > Suppose a chrooted environment with proc mounted as procfs. I am trying 
> > to use bash process substitution feature and get this:
> >
> > $ cat <(echo hello)
> > cat: /dev/fd/63: No such file or directory
> >
> > While on the normal system:
> > $ cat <(echo hello)
> > hello
> >
> > As I understand, the problem is that when fuse making getattr on file 
> > '/dev/fd/63' - it determines that it is symlink. The next step is 
> > readlink, which gives something like that: /25977/fd/63 => 
> > pipe:[3087676]. And at the final step getattr on file 
> > '/25977/fd/pipe:[3087676]' gives no such file or directory.
> >
> > Actually I encouter this issue while working on my own similar fuse proc 
> > (for jail environments). Then I find procfs, tried it out and get the 
> > same failure.
> >
> > I am tried to change logic of getattr in the next way. First make normal 
> > stat of file. If it is not S_ISFIFO - make lstat. In that way the 
> > process hangs forever on open syscall:
> > $ strace cat <(echo hello)
> > ...
> > open("/dev/fd/63", O_RDONLY
> >
> >
> > Is there any workaround of this problem?
> 
> Check to see if /dev/fd is a symlink to /proc/self/fd
> 
> If that symlink is missing magic using /dev/fd will fail.  I suspect
> that for some reason your /dev/fd symlink simply has not been created in
> /dev.

Eric,

I believe he is using a fuse fs called 'procfs' which isn't properly
handling readlink.

Timofey, can you confirm?

-serge