[systemd-devel] [PATCH] path_is_mount_point: handle false positive on some fs

[Didier Roche]

Hey,

path_is_mount_point() can report some false positive that a file is a 
mount point on some file systems (like overlayfs).

The function tries to call name_to_handle_at(), if this one isn't 
supported by the file system, then a fallback using stat() is triggered 
on the entity (which can be a file). This compares the parent directory 
and entity st_dev.

However, some file systems (seems overlayfs at least) would report a 
major(st_dev) as 0 on directories and not on files. The current 
path_is_mount_point() fallback logic would thus reports that every files 
is a mount point. The enclosed patch introduces a second fallback to 
read /proc/mounts, and ensures there is no shadowing by later mounting 
of parent directories. This slower path is only used when the 2 first 
methods failed to determine with assurance if the path is a mount point 
or not.

Note that from what I heard, glib is going to use a similar mechanism. 
Also we could on the longer term maybe getting the whole 
path_is_mount_point() logic into libmount from util-linux, using 
mnt_get_mountpoint() (but this one only use st_dev comparison presently)?

As usual, I'm opened on any modification on this patch.
Cheers,
Didier
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