Re: new flag: AT_NO_JUMPS

From: Al Viro
Date: Thu May 04 2017 - 23:01:07 EST

On Thu, May 04, 2017 at 06:27:10PM -0700, Linus Torvalds wrote:

> As mentioned last time, at least for the git usage, even relative
> symlinks are a no-no - not because they'd escape, but simply because
> git wants to see the *unique* name, and resolve relative symlinks to
> either the symlink, or to the actual file it points to.
> So I think that we'd want an additional flag that says "no symlinks at all".

OK, that's easily done.

> And I think the "no mountpoint" traversal might be splittable too.
> Yes, sometimes you'd probably want to say "stay exactly inside this
> filesystem" (like find -xdev). So no arguments against AT_XDEV that
> refuses any mount traversal (kind of like my "no symlink traversal"
> thing).
> But at other points you might want to just guarantee that the walk
> stays below a certain starting point and doesn't escape.
> That could still allow crossing mount-points, but only if they are
> non-bind mounts and cannot let us escape.
> I'm not sure if that's testable, though.

This one isn't, unfortunately - there is no difference between bind and
no-bind; vfsmounts form a tree and both normal mount and bind add leaves
to it. Moreover, mount -t ext2 /dev/sdc7 /mnt; mount -t ext2 /dev/sdc7 /tmp/a
yield the same state as mount -t ext2 /dev/sdc7; mount --bind /mnt /tmp/a.
There is no way to tell the difference, simply because there *is* no
difference. Moreover, either can be followed by umount /mnt and you'll get
the same state as you would have after a solitary mount of the same fs on

Ho-hum... So:

absolute pathname: EXDEV
non-relative symlink: EXDEV ? ELOOP
relative symlink: ELOOP
.. from starting point: EXDEV
.. crossing mountpoint: EXDEV
crossing into mountpoint: EXDEV

1) What should AT_XDEV do about absolute symlinks? Nothing special? EXDEV?
EXDEV if we are not on root?
2) What should AT_BENEATH | AT_NO_SYMLINKS do on absolute symlinks? My
preference would be "AT_NO_SYMLINKS wins, ELOOP for you", but that's based
mostly upon the convenience of implementation.
3) What effect should AT_NO_SYMLINKS have upon the final component? Same