Fix permissions issues with archaic filesystems (ExFAT) on Linux

Here we are in the sixth decade of the Unix Epoch and the lowest common denominator supported filesystem (between Android, Linux, and the BSDs) is the archaic FAT family of DOS filesystems. The least shitty of the FAT family, ExFAT, is what most of my sdcards are formatted with, and its complete lack of Unix-native metadata occasionally poses problems when mounting such partitions on Linux.

Since these filesystems have no concept of users, groups, and r/w/x permissions, by default when you mount a partition formatted with ExFAT using mount(8), Linux provides some simulation of them inherited respectively from the user, group, and umask of the process which calls mount(2). Since mount must run as root and the filesystem doesn't technically support permissions, everything below the mountpoint will be owned as root and cannot be chown(1)'d. This is the case even if the mountpoint is owned by your user account. As you might imagine, this makes copying files to and from the disk rather annoying.

When mounting an ExFAT partition on my machine (OpenSuse Tumbleweed with Linux 6.9.1), mount(8) defaults to using the ExFAT module built into the kernel, which from what I can tell doesn't provide a way to specify ownership. Fortunately, there is an alternative ExFAT driver available that makes use of Fuse (this is the preferred one on FreeBSD and OpenBSD so this should apply to them too) which accepts options for setting custom uid and gid fields:

sudo mount -t exfat-fuse /dev/whence /mnt/whither -o uid=$(id -u),gid=$(id -g),fmask=137,dmask=027

This will mount the /dev/whence partition on the /mnt/whither directory with all files and directories in it being readable and writable and owned by the current user rather than root.

I've found this solution also works for VFAT (with the -t vfat driver on Linux) but haven't tested it with other FAT derivatives, nor on BSDs or other systems with different kernels.

To understand the foregoing mount options I'd recommend checking out the man pages for fstab(5), umask(1p), and mount(8), and this handy umask calculator.