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
