> On Jan 1, 2020, at 11:44 PM, Aleksa Sarai <cyphar@xxxxxxxxxx> wrote: > > On 2020-01-01, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: >>> On Wed, Jan 01, 2020 at 12:54:46AM +0000, Al Viro wrote: >>> Note, BTW, that lookup_last() (aka walk_component()) does just >>> that - we only hit step_into() on LAST_NORM. The same goes >>> for do_last(). mountpoint_last() not doing the same is _not_ >>> intentional - it's definitely a bug. >>> >>> Consider your testcase; link points to . here. So the only >>> thing you could expect from trying to follow it would be >>> the directory 'link' lives in. And you don't have it >>> when you reach the fscker via /proc/self/fd/3; what happens >>> instead is nd->path set to ./link (by nd_jump_link()) *AND* >>> step_into() called, pushing the same ./link onto stack. >>> It violates all kinds of assumptions made by fs/namei.c - >>> when pushing a symlink onto stack nd->path is expected to >>> contain the base directory for resolving it. >>> >>> I'm fairly sure that this is the cause of at least some >>> of the insanity you've caught; there always could be >>> something else, of course, but this hole needs to be >>> closed in any case. >> >> ... and with removal of now unused local variable, that's >> >> mountpoint_last(): fix the treatment of LAST_BIND >> >> step_into() should be attempted only in LAST_NORM >> case, when we have the parent directory (in nd->path). >> We get away with that for LAST_DOT and LOST_DOTDOT, >> since those can't be symlinks, making step_init() and >> equivalent of path_to_nameidata() - we do a bit of >> useless work, but that's it. For LAST_BIND (i.e. >> the case when we'd just followed a procfs-style >> symlink) we really can't go there - result might >> be a symlink and we really can't attempt following >> it. >> >> lookup_last() and do_last() do handle that properly; >> mountpoint_last() should do the same. >> >> Cc: stable@xxxxxxxxxxxxxxx >> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > > Thanks, this fixes the issue for me (and also fixes another reproducer I > found -- mounting a symlink on top of itself then trying to umount it). > > Reported-by: Aleksa Sarai <cyphar@xxxxxxxxxx> > Tested-by: Aleksa Sarai <cyphar@xxxxxxxxxx> > > As for the original topic of bind-mounting symlinks -- given this is a > supported feature, would you be okay with me sending an updated > O_EMPTYPATH series? FWIW, I have an actual use case for mounting over a symlink: replacing /etc/resolv.conf. My virtme tool is presented with somewhat arbitrary crud in /etc, where /etc/resolv.conf might be a plain file or a symlink, but, regardless, has inappropriate contents. If it’s a file, I can mount a new file over it. If it’s a symlink and the kernel properly supported it, I could also mount over it. Yes, I could also use overlayfs. Maybe I should regardless.
