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John Mee
Asked: 2024-01-26 17:25:50 +0800 CST

How to remount a drive without root?

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I want to allow a user script to remount a drive as "ro" or "rw".

By using the user option in the fstab, I can successfully mount and unmount the drive as a user but the -o remount,rw fails.

Is it not possible for the user to use the remount option?!?

/etc/fstab reads...

UUID=789f-4b3a-b0b2-c05955d7c5ad /mnt/nasbackup auto rw,user,noexec 0 2

but...

john@johnbox /mnt> mount UUID=789f-4b3a-b0b2-c05955d7c5ad
john@johnbox /mnt> mount | grep nvme
/dev/nvme0n1 on /mnt/nasbackup type ext4 (rw,nosuid,nodev,noexec,relatime,stripe=4,user=john)
john@johnbox /mnt> mount -o remount,ro UUID=789f-4b3a-b0b2-c05955d7c5ad
mount: /mnt/nasbackup: must be superuser to use mount.
       dmesg(1) may have more information after failed mount system call.
john@johnbox /mnt [32]> umount UUID=789f-4b3a-b0b2-c05955d7c5ad
john@johnbox /mnt>
mount

1 Answers

  1. Best Answer

    This is a architectural limitation of mount. As a workaround you can use C to write some suid wrappers. Shell scripts won't work because Bash refuses to keep suid.

    1. sudo apt install build-essential
    2. Paste the following into a remountro.c
    #include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
 if (setuid(0) != 0) abort();
 puts("Will remount nasbackup as ro");
 execl("/bin/mount", "mount", "-o", "remount,ro", "UUID=789f-4b3a-b0b2-c05955d7c5ad", NULL);
 abort();
}
    1. Paste the following into a remountrw.c:
    #include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
 if (setuid(0) != 0) abort();
 puts("Will remount nasbackup as rw");
 execl("/bin/mount", "mount", "-o", "remount,rw", "UUID=789f-4b3a-b0b2-c05955d7c5ad", NULL);
 abort();
}
    1. Run make remountrw remountro
    2. Run in a root shell:
    chown root:root remountrw remountro
chmod 4755 remountrw remountro # 4 = suid
mv -t /usr/local/bin remountrw remountro
    1. Now any user can run remountro or remountrw to fix those exact two commands

    You still need the user part in /etc/fstab so the non-remount commands keep working like before. Alternatively, you can edit /etc/sudoers, but it's more complicated to ensure that the parameter restriction rules you write for sudo remain secure.

    • 2