losetup bug in kernel

[NXvVGEBaq4]

Branch:R1
Failed to install xposed module of magisk when mounting magisk.img/magisk_merge.img using twrp recorvery/smartisan os 6.6.5.2.
The module used mount_magisk_img function which is in magisk framework like this:

when using recovery, the log is like that :

but i can using cat to this img:

To figure out that problem,i am using smartisan OS ,using new busybox just downloaded from magisk app:

Here is the strace result in smartisan OS:
miao.txt
Here is the strace result in recovery:

[NXvVGEBaq4]

miao2.txt
（looks like the strace binary file download from the internet works not well , i am too lazy to compile a new one）