Linus Torvalds <[email protected]> writes: > On Wed, Aug 16, 2017 at 12:56 PM, Linus Torvalds > <[email protected]> wrote: >> >> So the fact that we _don't_ get the right pathname for the pts entry >> here means that something got screwed up in setting filp->f_path to >> the right thing. We have all the code in place that _tries_ to do it, >> but it clearly has a bug somewhere. > > Ok, I think I see what the bug is, although I don't have a fix for it yet. > > We generate the path largely correctly: the path has a nice dentry > that contains the right pts number, and has the right parent pointer > that points to the root of the pts mount. > > And we also fill in the path 'mnt' field. Everything should be fine. > > Except when we actually hit that root dentry of the pts mount, the > code in prepend_path() hits this condition: > > if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) { > struct mount *parent = ACCESS_ONCE(mnt->mnt_parent); > /* Escaped? */ > if (dentry != vfsmnt->mnt_root) { > > and we break out, and reset the path to '/' because we think it > somehow escaped out of the user namespace.
Escaped it's bind mount actually. There should always be a path from mnt_root to a dentry under that mount point. In some rare caseses involving bind mounts a rename that moves a dentry from one directory to another can result in dentries that are not reachable from mnt_root. As those entries do not have a path in a meaningful sense setting the path to '/' is the best we can do. This condition should be limited to bind mounts as any dentry on a filesystem is descendent from the filesystems root directory. The rest of your analysis below is correct. My apologies for the pendantic reply. I am repling just so that someone doesn't find this in an email archive 20 years from now and become impossibly confused. > So it looks like we filled in the path with the *wrong* mount information. > > And THAT in turn is because we fill the path with the mount > information for the "/dev/ptmx" field - which is *not* in the > /dev/pts/ mount - that's the mount for '/dev'. > > So we have a dentry and a mnt, but they simply aren't paired up correctly. > > And you can see this with your test program: if you open /dev/pts/ptmx > for the master, it actually works correctly (but you need to make sure > the permissions for that ptmx node allow that). > > Anyway, I know what's wrong, next step is to figure out what the fix is. > > Linus
