Re: [RFC][PATCHSET] sanitized pathwalk machinery (v2)

Al Viro
2020-02-28T03:02:05Z

On Fri, Feb 28, 2020 at 12:24:51PM +1100, Aleksa Sarai wrote:

> > Another one is about LOOKUP_NO_XDEV again: suppose you have process'
> > root directly overmounted and cwd in the root of whatever's overmounting
> > it. Resolution of .. will stay in cwd - we have no parent within the
> > chroot jail we are in, so we move to whatever's overmounting that root.
> > Which is the original location. Should we fail on LOOKUP_NO_XDEV here?
> > Plain .. in the root of chroot jail (not overmounted by anything) does
> > *not*...
>
> I think LOOKUP_NO_XDEV should block that since you end up crossing a
> mountpoint.

You are not. Your process' root is overmounted and your current directory
is on that overmount. You attempt to resolve ".." there.

# cd /
# unshare -m
# stat .
File: .
Size: 4096 Blocks: 8 IO Block: 4096 directory
Device: 801h/2049d Inode: 2 Links: 25
Access: (0755/drwxr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2020-02-26 20:51:06.623409892 -0500
Modify: 2020-02-26 20:43:51.284020000 -0500
Change: 2020-02-26 20:43:51.284020000 -0500
Birth: -
# mkdir /tmp/foo
# mount -t tmpfs none /tmp/foo/
# for i in *; do test -d $i && mkdir /tmp/foo/$i && mount --rbind $i /tmp/foo/$i; done
# cd /tmp/foo/
# mount --move . /
# /bin/pwd
/
# ls
bin dev home lib32 libx32 ltp mnt proc run srv tmp var
boot etc lib lib64 lost+found media opt root sbin sys usr
# ls /
253_metadump etc lib32 media run usr
315.full home lib64 mnt sbin var
bin initrd.img libx32 opt srv vmlinuz
boot initrd.img.old lost+found proc sys vmlinuz.old
dev lib ltp root tmp
# stat ..
File: ..
Size: 500 Blocks: 0 IO Block: 4096 directory
Device: 19h/25d Inode: 1875746 Links: 25
Access: (1777/drwxrwxrwt) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2020-02-27 21:48:45.649705410 -0500
Modify: 2020-02-27 21:46:40.829607188 -0500
Change: 2020-02-27 21:46:40.829607188 -0500
Birth: -
# stat .
File: .
Size: 500 Blocks: 0 IO Block: 4096 directory
Device: 19h/25d Inode: 1875746 Links: 25
Access: (1777/drwxrwxrwt) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2020-02-27 21:48:45.649705410 -0500
Modify: 2020-02-27 21:46:40.829607188 -0500
Change: 2020-02-27 21:46:40.829607188 -0500
Birth: -

See what's going on? We have a tmpfs overmounting root; current directory
is the root of tmpfs; process root is on ext3. Lookups for . and .. yield
exactly the same result here - we stay in the root of tmpfs.

Anyway, see #work.dotdot in vfs.git