How do I know ACLs are supported on my file system?

Question

Is it enough to see getfacl giving no error, or do I have to check some other place to see whether or not ACLs are supported by the file systems?

Solution 1

If you're talking about a mounted filesystem, I don't know of any intrinsic way to tell whether ACL are possible. Note that “are ACL supported?” isn't a very precise question since there are several types of ACL around (Solaris/Linux/not-POSIX-after-all, NFSv4, OSX, …). Note that getfacl is useless as a test since it will happily report Unix permissions if that's all there is: you need to try setting an ACL to test.

Still on mounted filesystem, you can check for the presence of acl in the mount options (which you can find in /proc/mount). Note that this isn't enough: you also need to take the kernel version and the filesystem type in consideration. Some filesystem types always have ACL available, regardless of mount options; this is the case for tmpfs, xfs and zfs. Some filesystems have ACL unless explicitly excluded; this is the case for ext4 since kernel 2.6.39.

Solution 2

To know if ACL is available you can:

1. Check current kernel version and filesystem:
   uname -r
df -T or mount | grep root
   
   Recent distro have ACL mount option included by default (since kernel 2.6). So it's not mandatory to redefine it in /etc/fstab (or similar). Non exhaustive list of filesystems concerned: ext3, ext4, tmpfs, xfs and zfs .
   
   If you have older setup then you may have to recompile the kernel and/or add acl in /etc/fstab.
   fstab example: /dev/root / ext4 acl,errors=remount-ro 0 1

2. Look for existing ACL settings (the "usual" config place is on /boot):
   sudo mount | grep -i acl #optionnal
cat /boot/config* | grep _ACL
   
   Depending of the system you could find the settings in /procinstead. Here is a way to extract the config from the .gz archive and then search for acl settings:
   cat /proc/config.gz | gunzip > running.config && grep -i 'acl' running.config
cat running.config | grep _ACL
   
   You should see something like:
   CONFIG_EXT3_FS_POSIX_ACL=y
CONFIG_EXT2_FS_POSIX_ACL=y
CONFIG_XFS_POSIX_ACL=y
   
   For the filesystem you can try to get more info with:
   sudo tune2fs -l /xxx/xxx| grep 'Default mount options:'
   (replace xxx/xxx by your filesystem)

--
Helpfull information can be found on:
- superuser.com,
- serverfault,
- bencane.com,
- wiki.archlinux.org

Solution 3

acl should be enabled as default if you are using ext2/3/4 or btrfs.

Check with:

tune2fs -l /dev/sdXY | grep "Default mount options:"

If it isn't in the output do a:

tune2fs -o acl /dev/sdXY

Comments

[Cthulhu Tentacles]

grep acl /etc/mke2fs.conf will do it too.

[0xC0000022L]

as for the getfacl test you're right. Except if I were able to find a non-default ACL (by suppressing default ones and header). Checking /proc/mount doesn't appear to be enough in cases where the acl option is a default option not coming from the mount command or fstab, though.

[CMCDragonkai]

I noticed when running ZFS on Linux, with acltype=posixacl, the /proc/mounts will show posixacl, but in another system with just ext4, there's nothing inside /proc/mounts, but acl was a default mount option for ext4.