Re: [RFC PATCH] autofs: find_autofs_mount overmounted parent support

[Alexander Mikhalitsyn <alexander.mikhalitsyn@virtuozzo.com>]

On Tue, 9 Mar 2021 14:31:05 +0300
Alexander Mikhalitsyn <alexander.mikhalitsyn@virtuozzo.com> wrote:

> On Mon, 8 Mar 2021 00:12:22 +0000
> Al Viro <viro@zeniv.linux.org.uk> wrote:
> 
> > On Sun, Mar 07, 2021 at 11:51:20PM +0000, Al Viro wrote:
> > > On Thu, Mar 04, 2021 at 01:11:33PM +0300, Alexander Mikhalitsyn wrote:
> > > 
> > > > That problem connected with CRIU (Checkpoint-Restore in Userspace) project.
> > > > In CRIU we have support of autofs mounts C/R. To acheive that we need to use
> > > > ioctl's from /dev/autofs to get data about mounts, restore mount as catatonic
> > > > (if needed), change pipe fd and so on. But the problem is that during CRIU
> > > > dump we may meet situation when VFS subtree where autofs mount present was
> > > > overmounted as whole.
> > > > 
> > > > Simpliest example is /proc/sys/fs/binfmt_misc. This mount present on most
> > > > GNU/Linux distributions by default. For instance on my Fedora 33:
> > > > 
> > > > trigger automount of binfmt_misc
> > > > $ ls /proc/sys/fs/binfmt_misc
> > > > 
> > > > $ cat /proc/1/mountinfo | grep binfmt
> > > > 35 24 0:36 / /proc/sys/fs/binfmt_misc rw,relatime shared:16 - autofs systemd-1 rw,...,direct,pipe_ino=223
> > > > 632 35 0:56 / /proc/sys/fs/binfmt_misc rw,...,relatime shared:315 - binfmt_misc binfmt_misc rw
> > > > 
> > > > $ sudo unshare -m -p --fork --mount-proc sh
> > > > # cat /proc/self/mountinfo | grep "/proc"
> > > > 828 809 0:23 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
> > > > 829 828 0:36 / /proc/sys/fs/binfmt_misc rw,relatime - autofs systemd-1 rw,...,direct,pipe_ino=223
> > > > 943 829 0:56 / /proc/sys/fs/binfmt_misc rw,...,relatime - binfmt_misc binfmt_misc rw
> > > > 949 828 0:57 / /proc rw...,relatime - proc proc rw
> > > > 
> > > > As we can see now autofs mount /proc/sys/fs/binfmt_misc is inaccessible.
> > > > If we do something like:
> > > > 
> > > > struct autofs_dev_ioctl *param;
> > > > param = malloc(...);
> > > > devfd = open("/dev/autofs", O_RDONLY);
> > > > init_autofs_dev_ioctl(param);
> > > > param->size = size;
> > > > strcpy(param->path, "/proc/sys/fs/binfmt_misc");
> > > > param->openmount.devid = 36;
> > > > err = ioctl(devfd, AUTOFS_DEV_IOCTL_OPENMOUNT, param)
> > > > 
> > > > now we get err = -ENOENT.
> > > 
> > Where does that -ENOENT come from?  AFAICS, pathwalk ought to succeed and
> > return you the root of overmounting binfmt_misc.  Why doesn't the loop in
> > find_autofs_mount() locate anything it would accept?
> > 
> 
> Consider our mounts tree:
> > > > # cat /proc/self/mountinfo | grep "/proc"
> > > > 828 809 0:23 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
> > > > 829 828 0:36 / /proc/sys/fs/binfmt_misc rw,relatime - autofs systemd-1 rw,...,direct,pipe_ino=223
> > > > 943 829 0:56 / /proc/sys/fs/binfmt_misc rw,...,relatime - binfmt_misc binfmt_misc rw
> > > > 949 828 0:57 / /proc rw...,relatime - proc proc rw
> 
> ENOENT comes from here (current kernel code):
> /* Find the topmost mount satisfying test() */
> static int find_autofs_mount(const char *pathname,
> 			     struct path *res,
> 			     int test(const struct path *path, void *data),
> 			     void *data)
> {
> 	struct path path;
> 	int err;
> 
> 	err = kern_path(pathname, LOOKUP_MOUNTPOINT, &path);
> 	if (err)
> 		return err;
> <-------- here we successfuly open root dentry (/proc/sys/fs/binfmt_misc) of /proc (mnt_id = 949)
> 
> 	err = -ENOENT;
> <---- set err and start search autofs mount
> /*
>  here we use follow_up to move through upper dentries and find overmounted autofs.
>  But in our case we opened dentry from /proc (mnt_id = 949) and this concrete dentry is *NOT*
> overmounted (whole /proc overmounted).
> */
> 	while (path.dentry == path.mnt->mnt_root) {
> 		if (path.dentry->d_sb->s_magic == AUTOFS_SUPER_MAGIC) {
> 			if (test(&path, data)) {
> /*
>  we never get there
> */
> 				path_get(&path);
> 				*res = path;
> 				err = 0;
> 				break;
> 			}
> 		}
> 		if (!follow_up(&path))
> 			break;
> 	}
> /*
> loop finished. err stays as it was err = -ENOENT
> */
> 	path_put(&path);
> 	return err;
> }
> 
> Source: https://github.com/torvalds/linux/blob/master/fs/autofs/dev-ioctl.c#L194
> 
> > I really dislike the patch - the whole "normalize path" thing is fundamentally
> > bogus, not to mention the iterator over all mounts, etc., so I would like to
> > understand what the hell is going on before even thinking of *not* NAKing
> > it on sight.
> 
> I'm not trying to break current API or something similar. I'm just prepared
> RFC patch with my proposal. I'm ready to rework all of that to make it good.
> But without maintainers/community comments/suggestions it's unreal :)
> 
> Please, explain what do you mean by "normalize path thing"?
> I'm not changing semantics of current ioctl() I've just trying to extend it to make
> it work in case when parent mount of autofs mount is overmounted.
> 
> > 
> > 	Wait, so you have /proc overmounted, without anything autofs-related on
> > /proc/sys/fs/binfmt_misc and still want to have the pathname resolved, just
> > because it would've resolved with that overmount of /proc removed?
> 
> Something like that.
> 
> 1. I don't expect that /proc/sys/fs/binfmt_misc path will be resolved
> (because, for instance we can overmount /proc by empty tmpfs and in this case after
> overmounting we can't even open /proc/sys/fs/binfmt_misc and that's okay).
> 
> 2. We talking about autofs specific function which is used in several autofs-specific
> ioctls. One of that ioctl(AUTOFS_DEV_IOCTL_OPENMOUNT) which is designed to open
> overmounted autofs mounts. Because it's frequent case when autofs mount is overmounted
> (when we talk about direct mounts). This ioctl allows to open file desciptor
> of autofs root dentry and later, autofs daemon use it to manage mount (call another autofs
> ioctls on that fd).
> 
> I've just meet problem, that this API not works when parent mount of autofs mount is overmounted.
> For example:
> tmpfs     /some-dir
> autofs    /some-dir/autofs1 <-autofs direct mount
> nfs       /some-dir/autofs1 <-automounted fs on top of autofs
> 
> ioctl(AUTOFS_DEV_IOCTL_OPENMOUNT) will work in this case. Because loop
> with follow_up() starts from /some-dir/autofs1 dentry of nfs, then follow_up()
> and move to /some-dir/autofs1 dentry of autofs.
> 
> But if we change picture to:
> tmpfs1     /some-dir
> autofs     /some-dir/autofs1 <-autofs direct mount
> nfs        /some-dir/autofs1 <-automounted fs on top of autofs
> tmpfs2     /some-dir
> 
> This will breaks API. Because know we can't even open /some-dir/autofs1
> dentry.
> 
> Ok. We can create this dentry at first by mkdir /some-dir/autofs1.
> But it will not help because our loop:
> 	while (path.dentry == path.mnt->mnt_root) {
> 		if (path.dentry->d_sb->s_magic == AUTOFS_SUPER_MAGIC) {
> 			if (test(&path, data)) {
> ...
> 		if (!follow_up(&path))
> 			break;
> 	}
> will start from dentry /some-dir/autofs1 from tmpfs2. And after follow_up
> on that dentry we will move to / dentry => loop finishes => user get ENOENT.
> 
> > 
> > 	I hope I'm misreading you; in case I'm not, the ABI is extremely
> > tasteless and until you manage to document the exact semantics you want
> > for param->path, consider it NAKed.
> > 
> > 	BTW, if that thing would be made to work, what's to stop somebody from
> > doing ...at() syscalls with the resulting fd as a starting point and pathnames
> > starting with ".."?  "/foo is overmounted, but we can get to anything under
> > /foo/bar/ in the underlying tree since there's an autofs mount somewhere in
> > /foo/bar/splat/puke/*"?
> 
> Interesting point. Thank you!
> I'm not sure. But is it serious problem for us? What stop somebody to open
> and hold fd to any directory before overmounting?
> 
> > 
> > 	IOW, the real question (aside of "WTF?") is what are you using the
> > resulting descriptor for and what do you need to be able to do with it.
> > Details, please.
> 
> Sure. I've covered use cases of file descriptor returned by ioctl(AUTOFS_DEV_IOCTL_OPENMOUNT)
> above.
> 
> Thanks for your reply!
> I'm sorry If my patch description is unclear. I'm newbie here :)
> 
> Regards,
> Alex

Gentle ping.

Thanks,
Alex