check xattr namespace behaviour

[ThomasWaldmann]

there was a strange issue in #6988 and the root cause was a strange xattr named user.security.selinux (the correct name would be security.selinux).

the processing (or rather ignoring) of such invalid stuff was solved by #6990.

but the open question is still: where did that invalid xattr initially come from?

we should make sure it did not come from borg.

[ThomasWaldmann]

@rudd-o if you have some time, you could maybe test that, like:

• locate some file that has a correct security.selinux xattr and not any invalid xattrs
• backup that file with borg create ...
• restore that file with borg extract ... - anything strange to see in the extracted file's xattrs?
• restore that file with borg mount ... and copying it from within the mounted fs - anything strange to see in the extracted file's xattrs?

[ThomasWaldmann]

Notes:

• tests in borg.testsuite.xattr work ok on macOS (I also added some with security. namespace and didn't spot anything unusual)
• the freebsd api for xattrs is a bit different as it has a separate argument for the namespace

[ThomasWaldmann]

@rudd-o do you use freebsd? if so, did you restore a linux backup on a freebsd machine?

[ThomasWaldmann]

It looks like the borg.platform.freebsd.setxattr code is problematic when used cross-platform:

• when backing up on freebsd, borg only queries the xattrs from the EXTATTR_NAMESPACE_USER (and writes them with a user. prefix into the archive) - check if we also need to back up other namespaces, like EXTATTR_NAMESPACE_SYSTEM.
• so when restoring such an archive on freebsd, only user.* xattrs are expected and processed correctly (as far as i could see)
• but, if an archive has security.* xattrs, borg.platform.freebsd.setxattr is misbehaved:
  • it does not find the usual user. prefix (and thus does not strip it)
  • it does not expect a security. prefix (and thus does not strip it)
  • but it still calls the freebsd api with EXTATTR_NAMESPACE_USER, so the result could be something like user.security.*. :-(
• on freebsd, there is no security namespace and guess extracting something like security.selinux does not make sense anyway as it is platform-specific to "Linux with SElinux".

[Rudd-O]

Only Linux. No FreeBSD. The attributes must have been set by either Syncthing or some other tool. Right now all I want is those attributes gone. user.security.selinux is ridiculous and meaningless.

[ThomasWaldmann]

I am not familiar with xattr usage on freebsd, so guess this needs somebody with knowledge about this, see my previous comment.

[oikumene]

FreeBSD manual of extattr(3) says

Although a file system may implement arbitrary namespaces, these functions only support the EXTATTR_NAMESPACE_USER ("user") and EXTATTR_NAMESPACE_SYSTEM ("system") namespaces, which are defined in extattr(9).

does this help?

[ThomasWaldmann]

@oikumene for what are xattrs typically used for under freebsd? Is there some list of the "usual stuff" (keys)?

From WP: "The user namespace has no restrictions with regard to naming or contents. The system namespace is primarily used by the kernel for access control lists and mandatory access control."

[ThomasWaldmann]

@oikumene can you review #7435?

[oikumene]

for what are xattrs typically used for under freebsd? Is there some list of the "usual stuff" (keys)?

I do not know well, but I think there are no "usual stuff" for xattr in FreeBSD. I have never encountered one. For Mandatory Access Control (MAC) (which is similar to SELinux), they use maclabel which seems different from xattr (as far as reading manpages.)

[ThomasWaldmann]

^ fixed in 1.2-maint branch, hopefully.