Messages in this thread | | | Date | Sun, 4 Apr 2021 15:56:02 +0000 | From | Al Viro <> | Subject | Re: [syzbot] WARNING in mntput_no_expire (2) |
| |
On Sun, Apr 04, 2021 at 01:34:45PM +0200, Christian Brauner wrote:
> Sorry for not replying to your earlier mail but I've been debugging this > too. My current theory is that it's related to LOOKUP_ROOT_GRABBED when > LOOKUP_CACHED is specified _possibly_ with an interaction how > create_io_thread() is created with CLONE_FS. The reproducer requires you > either have called pivot_root() or chroot() in order for the failure to > happen. So I think the fact that we skip legitimize_root() when > LOOKUP_CACHED is set might figure into this. I can keep digging. >
> Funny enough I already placed a printk statement into the place you > wanted one too so I just amended mine. Here's what you get: > > If pivot pivot_root() is used before the chroot() you get: > > [ 637.464555] AAAA: count(-1) | mnt_mntpoint(/) | mnt->mnt.mnt_root(/) | id(579) | dev(tmpfs) > > if you only call chroot, i.e. make the pivot_root() branch a simple > if (true) you get: > > [ 955.206117] AAAA: count(-2) | mnt_mntpoint(/) | mnt->mnt.mnt_root(/) | id(580) | dev(tmpfs)
Very interesting. What happens if you call loop() twice? And now I wonder whether it's root or cwd, actually... Hmm...
How about this: fd = open("/proc/self/mountinfo", 0); mkdir("./newroot/foo", 0777); mount("./newroot/foo", "./newroot/foo", 0, MS_BIND, NULL); chroot("./newroot"); chdir("/foo"); while (1) { static char buf[4096]; int n = read(fd, buf, 4096); if (n <= 0) break; write(1, buf, n); } close(fd); drop_caps(); loop(); as the end of namespace_sandbox_proc(), instead of chroot("./newroot"); chdir("/"); drop_caps(); loop(); sequence we have there?
> The cat /proc/self/mountinfo is for the id(579) below: ... and it misses the damn thing, since we call it before the mount in question had been created ;-/ So we'd probably be better off not trying to be clever and just doing that as explicit (and completely untested) read-write loop above.
| |