I'm trying to get devt of mountpoint (path*) in one of the LSM hooks. Assuming I have the following mount:
/dev/mmcblk0p5 is mounted to /mnt/photos
I would expect that calling
kern_path("/mnt/photos", ...)
will return a struct path with path.mnt->mnt_sb->s_dev = 0x0b300005 since /dev/mmcblk0p5 is 179:5,
but I always get 0x0b300002 (i.e. 179:2) - which is the root-fs device.
Only after mounting again /dev/mmcblk0p5 to /mnt/photos, kern_path returns 0x0b300005.
Why is that? How can I fix it?
It happens with both kernel 4.14 and 5.15.
some more info,
after 1st mount, printing address of path.mnt->mnt_sb is the same as of "/" (0x0b300002).
after 2nd mount, printing address of path.mnt->mnt_sb is different.
i guess that mnt_sb is being updated sometime between the 1st and the 2nd mount.
so, when it happens? how can i get notification/hook after this update?