On Jan 1, 2020, at 11:44 PM, Aleksa Sarai [off-list ref] wrote:

On 2020-01-01, Al Viro [off-list ref] wrote:

quoted

quoted

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@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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 <redacted>
Tested-by: Aleksa Sarai <redacted>

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.