4 ms·

MAC and ACL are two different things. On Linux, ACL is the "facl" family of functions functions (and the set/getfacl programs). These supplement the tradition

by LukeShu 10y ago

MAC and ACL are two different things. On Linux, ACL is the "facl" family of functions functions (and the set/getfacl programs). These supplement the traditional owner/group and mode bits. Essentially, it allows you to set a separate set of read/write/execute bits on a file for an arbitrary number of specific users or groups, rather than being limited to simply "owner", "group", "everyone-else". If anything is set beyond the normal owner/group/world, GNU `ls -l` indicates this with a "+" next to the normal mode bits. I'm unsure how much this changes for other kernels.

rleigh 10y ago
Linux supports POSIX.1e DRAFT ACLs. These are a de-facto standard by implementation, but were never formally standardised by POSIX. They are also supported by other kernels. However, other systems support the more modern and featureful NFSv4 ACLs. NFSv4 ACLs are supported by the NFSv4 filesytem and also used by ZFS. They are part of the NFSv4 formal specification. On systems supporting ZFS (other than Linux), you get the full NFSv4 ACL support. Linux doesn't have VFS support for NFSv4 ACLs, and doesn't expose them even for NFSv4 mounts. In contrast, a FreeBSD mount of an NFSv4 filesystem served from ZFS exposes the full permissions model to the client. FreeBSD has a separate set of richer acl_* functions rather than the get/setfacl functions on Linux. Linux: % getfacl ~/.zshrc getfacl: Removing leading '/' from absolute path names # file: home/rleigh/.zshrc # owner: rleigh # group: rleigh user::rw- group::r-- other::r-- FreeBSD: % getfacl ~/.zshrc # file: /home/rleigh/.zshrc # owner: rleigh # group: rleigh owner@:rw-p--aARWcCos:-------:allow group@:r-----a-R-c--s:-------:allow everyone@:r-----a-R-c--s:-------:allow Linux really needs to get proper support for NFSv4 ACLs.
- vetinari 10y ago
  > Linux really needs to get proper support for NFSv4 ACLs. Not to nitpick, but: why would it need to do that? Nobody (in Joel Spolsky definition of nobody) cares about NFSv4. The only place where ACL matters for Linux users is Samba. And Samba works fine with Linux ACLs as it is.
  - rleigh 10y ago
    Most people aren't aware of what they are missing and how it could bring a great deal of benefit. This isn't about NFSv4 (the filesystem), it's about the rich permissions model it standardised which is now used by more than just NFS. It's used by ZFS, and it's used by the FreeBSD and Solaris VFS, and likely others as well. It's the only rich ACL model in real use today on open platforms, and being a superset of both POSIX.1e DRAFT and Windows ACLs, it's needed for full interoperability between platforms. Samba doesn't "work fine" except for the simple case. Using it with groups and group permissions can be exceedingly painful. When I export a ZFS filesystem on FreeBSD over Samba, it exposes the full ACL model to the Windows client which is accessible and manipulable via the standard security properties as well as the command-line on the server or and NFSv4 clients. When working with group permissions locally or over NFSv4 or CIFS, you aren't restricted to the pitiful group and setgid permissions offered by the standard UNIX permissions or even POSIX.1e ACLs. You can add an @group ACE detailing the group access permissions and their inheritance, or custom ones per user, which can make working on shared data much simpler. No more pain when someone with a broken umask adds something and strips off the group permissions, or they drop the setgid bit and the inheritance is lost, or they access it via samba and it sets the wrong group ownership or permissions. All of which I see regularly on shares exported from Linux. We often have to export the same share multiple times with different configuration to work around the limitations (read-only, group and group perms), where if it was using proper ACLs just one would work for all cases.
    - vetinari 10y ago
      Thank you for your detailed answer. However, your first sentence is valid for many things, not just ACLs. For example, there are people (ehm, me) who value being able to have Kerberos tickets for multiple principals from multiple KDCs at the same time. This is something that the most popular OS (Windows) is not able to do - and nobody minds. Because for most people, being part of one domain is good enough. With ACLs, I find it similar. For most people, the complexity is too high, the reasoning about them so complicated, so there's no benefit to have them available, they won't use them anyway. Also, in our case, we just have multiple groups at the root of samba share and that's it. It is good enough. So maybe it is yet another case of worse is better.

by LukeShu 10y ago

rleigh 10y ago

Linux supports POSIX.1e DRAFT ACLs. These are a de-facto standard by implementation, but were never formally standardised by POSIX. They are also supported by other kernels. However, other systems support the more modern and featureful NFSv4 ACLs. NFSv4 ACLs are supported by the NFSv4 filesytem and also used by ZFS. They are part of the NFSv4 formal specification. On systems supporting ZFS (other than Linux), you get the full NFSv4 ACL support. Linux doesn't have VFS support for NFSv4 ACLs, and doesn't expose them even for NFSv4 mounts. In contrast, a FreeBSD mount of an NFSv4 filesystem served from ZFS exposes the full permissions model to the client. FreeBSD has a separate set of richer acl_* functions rather than the get/setfacl functions on Linux. Linux: % getfacl ~/.zshrc getfacl: Removing leading '/' from absolute path names # file: home/rleigh/.zshrc # owner: rleigh # group: rleigh user::rw- group::r-- other::r-- FreeBSD: % getfacl ~/.zshrc # file: /home/rleigh/.zshrc # owner: rleigh # group: rleigh owner@:rw-p--aARWcCos:-------:allow group@:r-----a-R-c--s:-------:allow everyone@:r-----a-R-c--s:-------:allow Linux really needs to get proper support for NFSv4 ACLs.

vetinari 10y ago
> Linux really needs to get proper support for NFSv4 ACLs. Not to nitpick, but: why would it need to do that? Nobody (in Joel Spolsky definition of nobody) cares about NFSv4. The only place where ACL matters for Linux users is Samba. And Samba works fine with Linux ACLs as it is.
- rleigh 10y ago
  Most people aren't aware of what they are missing and how it could bring a great deal of benefit. This isn't about NFSv4 (the filesystem), it's about the rich permissions model it standardised which is now used by more than just NFS. It's used by ZFS, and it's used by the FreeBSD and Solaris VFS, and likely others as well. It's the only rich ACL model in real use today on open platforms, and being a superset of both POSIX.1e DRAFT and Windows ACLs, it's needed for full interoperability between platforms. Samba doesn't "work fine" except for the simple case. Using it with groups and group permissions can be exceedingly painful. When I export a ZFS filesystem on FreeBSD over Samba, it exposes the full ACL model to the Windows client which is accessible and manipulable via the standard security properties as well as the command-line on the server or and NFSv4 clients. When working with group permissions locally or over NFSv4 or CIFS, you aren't restricted to the pitiful group and setgid permissions offered by the standard UNIX permissions or even POSIX.1e ACLs. You can add an @group ACE detailing the group access permissions and their inheritance, or custom ones per user, which can make working on shared data much simpler. No more pain when someone with a broken umask adds something and strips off the group permissions, or they drop the setgid bit and the inheritance is lost, or they access it via samba and it sets the wrong group ownership or permissions. All of which I see regularly on shares exported from Linux. We often have to export the same share multiple times with different configuration to work around the limitations (read-only, group and group perms), where if it was using proper ACLs just one would work for all cases.
  - vetinari 10y ago
    Thank you for your detailed answer. However, your first sentence is valid for many things, not just ACLs. For example, there are people (ehm, me) who value being able to have Kerberos tickets for multiple principals from multiple KDCs at the same time. This is something that the most popular OS (Windows) is not able to do - and nobody minds. Because for most people, being part of one domain is good enough. With ACLs, I find it similar. For most people, the complexity is too high, the reasoning about them so complicated, so there's no benefit to have them available, they won't use them anyway. Also, in our case, we just have multiple groups at the root of samba share and that's it. It is good enough. So maybe it is yet another case of worse is better.