On Fri, Apr 3, 2020 at 10:30 PM J. Bruce Fields <bfields@fieldses.org> wrote:
>
> On Fri, Apr 03, 2020 at 05:12:23PM +0200, Lennart Poettering wrote:
> > BTW, while we are at it: one more thing I'd love to see exposed by
> > statx() is a simple flag whether the inode is a mount point. There's
> > plenty code that implements a test like this all over the place, and
> > it usually isn't very safe. There's one implementation in util-linux
> > for example (in the /usr/bin/mountpoint binary), and another one in
> > systemd. Would be awesome to just have a statx() return flag for that,
> > that would make things *so* much easier and more robust. because in
> > fact most code isn't very good that implements this, as much of it
> > just compares st_dev of the specified file and its parent. Better code
> > compares the mount ID, but as mentioned that's not as pretty as it
> > could be so far...
>
> nfs-utils/support/misc/mountpoint.c:check_is_mountpoint() stats the file
> and ".." and returns true if they have different st_dev or the same
> st_ino.  Comparing mount ids sounds better.
>
> So anyway, yes, everybody reinvents the wheel here, and this would be
> useful.  (And, yes, we want to know for the vfsmount, we don't care
> whether the same inode is used as a mountpoint someplace else.)

Attaching a patch.

There's some ambiguity about what is a "mountpoint" and what these
tools are interested in.  My guess is that they are not interested in
an object being a mount point (something where another object is
mounted) but being a mount root (this is the object mounted at the
mount point).  I.e

fd = open("/mnt", O_PATH);
mount("/bin", "/mnt", NULL, MS_BIND, NULL);
statx(AT_FDCWD, "/mnt", 0, 0, &stx1);
statx(fd, "", AT_EMPTY_PATH, 0, &stx2);
printf("mount_root(/mnt) = %c, mount_root(fd) = %c\n",
    stx1.stx_attributes & STATX_ATTR_MOUNT_ROOT ? 'y' : 'n',
    stx2.stx_attributes & STATX_ATTR_MOUNT_ROOT ? 'y' : 'n');

Would print:
mount_root(/mnt) = y, mount_root(fd) = n

Thanks,
Miklos