[Magisk] Deic:
Input/output of updater-binary, find_boot_image function part, it is where fail

+ find_boot_image
+ [ -z ]
+ find /dev -iname kern-a
+ BOOTIMAGE=
+ [ ! -z ]
+ find /dev -iname android_boot
+ BOOTIMAGE=
+ [ ! -z ]
+ find /dev -iname kernel
+ BOOTIMAGE=
+ [ ! -z ]
+ find /dev -iname boot
+ BOOTIMAGE=/dev/block/platform/mtk-msdc.0/by-name/boot
/dev/BOOT
+ [ ! -z /dev/block/platform/mtk-msdc.0/by-name/boot /dev/BOOT ]
sh: /dev/BOOT: unknown operand
+ find /dev -iname lnx
+ BOOTIMAGE=
+ [ ! -z ]
+ [ -z ]
+ grep -oE /dev/[a-zA-Z0-9_./-]*
+ grep -E \b/boot\b /etc/fstab
+ BOOTIMAGE=
+ [ ! -z ]
+ grep -E \b/boot\b /etc/recovery.fstab
+ grep -oE /dev/[a-zA-Z0-9_./-]*
+ BOOTIMAGE=
+ [ ! -z ]
+ grep -E \b/boot\b /etc/recovery.fstab.bak
+ grep -oE /dev/[a-zA-Z0-9_./-]*
+ BOOTIMAGE=
+ [ ! -z ]
+ [ -L ]
+ [ -z ]
+ ui_print ! Unable to detect boot image
+ false
+ echo -n -e ui_print ! Unable to detect boot image\n
+ echo -n -e ui_print\n
+ exit 1

It is searching for "boot" but as is case insensitive and I have got a file called "BOOT" in /dev, there is 2 results and the test fail.

EDIT

I edited to be case sensitive (-name option) and anyways fails due all loop devices are busy (and chcon fail to /data/magisk, IDK if is relevant)...

+ find_boot_image
+ [ -z ]
+ find /dev -name kern-a
+ BOOTIMAGE=
+ [ ! -z ]
+ find /dev -name android_boot
+ BOOTIMAGE=
+ [ ! -z ]
+ find /dev -name kernel
+ BOOTIMAGE=
+ [ ! -z ]
+ find /dev -name boot
+ BOOTIMAGE=/dev/block/platform/mtk-msdc.0/by-name/boot
+ [ ! -z /dev/block/platform/mtk-msdc.0/by-name/boot ]
+ break
+ [ -z /dev/block/platform/mtk-msdc.0/by-name/boot ]
+ [ -L /dev/block/platform/mtk-msdc.0/by-name/boot ]
+ readlink /dev/block/platform/mtk-msdc.0/by-name/boot
+ BOOTIMAGE=/dev/block/mmcblk0p7
+ [ -z /dev/block/mmcblk0p7 ]
+ ui_print - Constructing environment
+ false
+ echo -n -e ui_print - Constructing environment\n
+ echo -n -e ui_print\n
+ is_mounted /data
+ [ ! -z ]
+ cat /proc/mounts
+ grep /data
+ return 0
+ MAGISKBIN=/data/magisk
+ rm -rf /data/magisk
+ mkdir -p /data/magisk
+ cp -af /dev/tmp/magisk/arm64/. /dev/tmp/magisk/common/. /data/magisk
+ chmod -R 755 /data/magisk
+ chcon -hR u:object_r:system_file:s0 /data/magisk
chcon: Could not label u:object_r:system_file:s0 with -hR: No such file or directory
+ false
+ is_mounted /data
+ [ ! -z ]
+ cat /proc/mounts
+ grep /data
+ return 0
+ IMG=/data/magisk.img
+ [ -f /data/magisk.img ]
+ ui_print - /data/magisk.img detected!
+ false
+ echo -n -e ui_print - /data/magisk.img detected!\n
+ echo -n -e ui_print\n
+ mount_image /data/magisk.img /magisk
+ [ ! -d /magisk ]
+ mount -o rw,remount rootfs /
+ mkdir -p /magisk
+ false
+ [ ! -d /magisk ]
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ LOOPDEVICE=
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ LOOPDEVICE=/dev/block/loop0
+ [ ! -f /dev/block/loop0 ]
+ mknod /dev/block/loop0 b 7 0
+ losetup /dev/block/loop0 /data/magisk.img
+ [ 0 -eq 0 ]
+ mount -t ext4 -o loop /dev/block/loop0 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toolbox mount -t ext4 -o loop /dev/block/loop0 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toybox mount -t ext4 -o loop /dev/block/loop0 /magisk
/tmp/updater: line 1: /system/bin/toybox: not found
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ LOOPDEVICE=/dev/block/loop1
+ [ ! -f /dev/block/loop1 ]
+ mknod /dev/block/loop1 b 7 1
+ losetup /dev/block/loop1 /data/magisk.img
+ [ 0 -eq 0 ]
+ mount -t ext4 -o loop /dev/block/loop1 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toolbox mount -t ext4 -o loop /dev/block/loop1 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toybox mount -t ext4 -o loop /dev/block/loop1 /magisk
/tmp/updater: line 1: /system/bin/toybox: not found
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ LOOPDEVICE=/dev/block/loop2
+ [ ! -f /dev/block/loop2 ]
+ mknod /dev/block/loop2 b 7 2
+ losetup /dev/block/loop2 /data/magisk.img
+ [ 0 -eq 0 ]
+ mount -t ext4 -o loop /dev/block/loop2 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toolbox mount -t ext4 -o loop /dev/block/loop2 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toybox mount -t ext4 -o loop /dev/block/loop2 /magisk
/tmp/updater: line 1: /system/bin/toybox: not found
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ LOOPDEVICE=/dev/block/loop3
+ [ ! -f /dev/block/loop3 ]
+ mknod /dev/block/loop3 b 7 3
+ losetup /dev/block/loop3 /data/magisk.img
+ [ 0 -eq 0 ]
+ mount -t ext4 -o loop /dev/block/loop3 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toolbox mount -t ext4 -o loop /dev/block/loop3 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toybox mount -t ext4 -o loop /dev/block/loop3 /magisk
/tmp/updater: line 1: /system/bin/toybox: not found
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ LOOPDEVICE=/dev/block/loop4
+ [ ! -f /dev/block/loop4 ]
+ mknod /dev/block/loop4 b 7 4
+ losetup /dev/block/loop4 /data/magisk.img
+ [ 0 -eq 0 ]
+ mount -t ext4 -o loop /dev/block/loop4 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toolbox mount -t ext4 -o loop /dev/block/loop4 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toybox mount -t ext4 -o loop /dev/block/loop4 /magisk
/tmp/updater: line 1: /system/bin/toybox: not found
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ LOOPDEVICE=/dev/block/loop5
+ [ ! -f /dev/block/loop5 ]
+ mknod /dev/block/loop5 b 7 5
+ losetup /dev/block/loop5 /data/magisk.img
+ [ 0 -eq 0 ]
+ mount -t ext4 -o loop /dev/block/loop5 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toolbox mount -t ext4 -o loop /dev/block/loop5 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toybox mount -t ext4 -o loop /dev/block/loop5 /magisk
/tmp/updater: line 1: /system/bin/toybox: not found
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ LOOPDEVICE=/dev/block/loop6
+ [ ! -f /dev/block/loop6 ]
+ mknod /dev/block/loop6 b 7 6
+ losetup /dev/block/loop6 /data/magisk.img
+ [ 0 -eq 0 ]
+ mount -t ext4 -o loop /dev/block/loop6 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toolbox mount -t ext4 -o loop /dev/block/loop6 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toybox mount -t ext4 -o loop /dev/block/loop6 /magisk
/tmp/updater: line 1: /system/bin/toybox: not found
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ LOOPDEVICE=/dev/block/loop7
+ [ ! -f /dev/block/loop7 ]
+ mknod /dev/block/loop7 b 7 7
+ losetup /dev/block/loop7 /data/magisk.img
+ [ 0 -eq 0 ]
+ mount -t ext4 -o loop /dev/block/loop7 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ grep /magisk
+ cat /proc/mounts
+ return 1
+ /system/bin/toolbox mount -t ext4 -o loop /dev/block/loop7 /magisk
ioctl LOOP_SET_FD failed: Device or resource busy
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ /system/bin/toybox mount -t ext4 -o loop /dev/block/loop7 /magisk
/tmp/updater: line 1: /system/bin/toybox: not found
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ is_mounted /magisk
+ [ ! -z ]
+ cat /proc/mounts
+ grep /magisk
+ return 1
+ ui_print ! Magisk image mount failed...
+ false
+ echo -n -e ui_print ! Magisk image mount failed...\n
+ echo -n -e ui_print\n
+ exit 1

With busybox and toybox the mount command works, but I haven't got toybox in /system/bin and the script doesn't run busybox mount command.