[Android Q&A, Help & Troubleshooting] tmartin_nihonsuki:
Thanks for the pointer. What is the intention of the line:

adb shell "-c '/data/local/tmp/su'"

?

When I run that, I get a reply that the option is invalid, and this is confirmed in the --help output. I can confirm that the steps up to that point have been successful:

adb shell "ls -la /data/local/tmp"
total 82
drwxrwx--x 2 shell shell 3488 2020-08-20 08:10 .
drwxr-x--x 5 root root 3488 2008-01-01 00:00 ..
-rwxrwxrwx 1 shell shell 75352 2020-04-01 00:58 su

p.s. I've guessed at:

adb shell "/data/local/tmp/su -c setenforce 0"

but that returns 1. The next step fails as mount /system is not in /proc/mounts

p.p.s

I've managed to disable SELinux. The stumbling block is remounting /system as rw. Even if I can remount /system, will I not have permission problems moving su to /system/bin as shell user? fstab appears to be:

adb shell "cat /vendor/etc/fstab.s9863a1h10"
# Android fstab file.
# The filesystem that contains the filesystem checker binary (typically /system) cannot
# specify MF_CHECK, and must come before any filesystems that do specify MF_CHECK
# <src> <mnt_point> <type> <mnt_flags and options> <fs_mgr_flags>

system /system ext4 ro,barrier=1 wait,logical,first_stage_mount,avb_keys=/avb/q-gsi.avbpubkey:/avb/r-gsi.avbpubkey:/avb/s-gsi.avbpubkey
vendor /vendor ext4 ro,barrier=1 wait,logical,first_stage_mount
product /product ext4 ro,barrier=1 wait,logical,first_stage_mount
/dev/block/platform/soc/soc:ap-ahb/20600000.sdio/by-name/userdata /data f2fs noatime,nosuid,nodev,discard,inline_xattr,inline_data wait,check,fileencryption=aes-256-xts,reservedsize=128M
/dev/block/platform/soc/soc:ap-ahb/20600000.sdio/by-name/userdata /data ext4 noatime,nosuid,nodev,nomblk_io_submit,noauto_da_alloc wait,check,fileencryption=aes-256-xts
/dev/block/platform/soc/soc:ap-ahb/20600000.sdio/by-name/metadata /metadata ext4 nodev,noatime,nosuid,errors=panic wait,formattable,first_stage_mount
/devices/platform/soc/soc:ap-ahb/20200000.usb/musb-hdrc.*.auto/usb* auto vfat defaults voldmanaged=usbdisk:auto
/devices/platform/soc/soc:ap-ahb/20300000.sdio/mmc_host/mmc1/mmc1:*/block/mmcblk1 auto vfat defaults voldmanaged=sdcard0:auto,noemulatedsd,encryptable=footer
/dev/block/platform/soc/soc:ap-ahb/20600000.sdio/by-name/prodnv /mnt/vendor ext4 noatime,nosuid,nodev,nomblk_io_submit,noauto_da_alloc wait,check
/dev/block/platform/soc/soc:ap-ahb/20600000.sdio/by-name/cache /cache ext4 noatime,nosuid,nodev,nomblk_io_submit,noauto_da_alloc wait,check
# Should after mount prodnv for prodnv wholly occupying /mnt/vendor
/dev/block/platform/soc/soc:ap-ahb/20600000.sdio/by-name/socko /mnt/vendor/socko ext4 ro,noatime,nosuid,nodev,nomblk_io_submit,noauto_da_alloc wait,avb=socko
/dev/block/platform/soc/soc:ap-ahb/20600000.sdio/by-name/odmko /mnt/vendor/odmko ext4 ro,noatime,nosuid,nodev,nomblk_io_submit,noauto_da_alloc wait,avb=odmko
/dev/block/platform/soc/soc:ap-ahb/20600000.sdio/by-name/misc /misc emmc defaults defaults
#/dev/block/memdisk.0 /system ext4 rw,barrier=1 wait
#/dev/block/memdisk.1 /data ext4 noatime,nosuid,nodev,noauto_da_alloc,journal_async_commit,errors=panic wait

p.p.p.s. [MENTION=1890170]jwoegerbauer[/MENTION] 's method of dropping su will not work on this device as Android 10 will not allow the mounting of /system rw.

Best hint I've seen to circumnavigate the problem so far is here:

https://xiaomi.eu/community/threads/mount-system-not-possible.53011/