A journey in obscure MacOS permissions



I’m in the process of rebuilding my primary Mac (stuff broke and it was easier to do a clean install than try to fix them). One of the things that broke was my normal backups to my Time Capsule, and while I’d successfully done a temporary backup to a local USB drive, that data wasn’t available in the Time Machine interface. I reverted to the old skool approach and did the restoration directly, following this pattern (in the case below, restoring my ~/bin directory):

mercury:~ root# cd /Volumes/Offsite Set 1/Backups.backupdb/mercury/2015-07-05-074244/Macintosh HD/Users/esteele/
mercury:esteele root# tar -cf - bin/ | (cd /Users/esteele/; tar -xvf -)

When I tried to edit one of the transferred files, I got permissions problems even though, at first glance, filesystem permissions suggest I shouldn’t have problems at all:

mercury:~ esteele$ touch bin/testfile
touch: bin/testfile: Permission denied
mercury:~ esteele$ ls -l bin/testfile
-rw-r--r--@ 1 esteele  staff  159  4 Jul 08:52 bin/testfile

Most of my day is spent on Linux machines, with occasional ventures onto OpenBSD, but I knew that the @ in the listing was significant and that MacOS made use of flags and extended attributes on files. After fumbling around in chflags (1) and xattr (1) man pages and then the ls (1) man page, I could see that the @ meant that extended attributes had been set, and that there were no flags on the file (the dash to the left of 159). I thought it unlikely that those extended attributes were responsible for the permissions problems that I was seeing, so I moved on:

mercury:~ esteele$ ls -lO bin/testfile
-rw-r--r--@ 1 esteele  staff  - 159  4 Jul 08:52 bin/testfile
mercury:~ esteele$ ls -l@ bin/testfile
-rw-r--r--@ 1 esteele  staff  159  4 Jul 08:52 bin/testfile
com.apple.metadata:_kTimeMachineNewestSnapshot   50
com.apple.metadata:_kTimeMachineOldestSnapshot   50

But any operation to modify the file was impossible, and the messages from rm were quite confusing

mercury:~ esteele$ rm bin/testfile
override rw-r--r--  esteele/staff for bin/testfile? y
rm: bin/testfile: Permission denied

I knew that more granular permissions existed past the usual owner, group, other paradigm, but didn’t think they were in play here because of a comment in the ls man page.

If the file or directory has extended attributes, the permissions field printed by the -l option is followed by a ‘@’ character. Otherwise, if the file or directory has extended security information (such as an access control list), the permissions field printed by the -l option is followed by a ‘+’ character.

After quite some gnashing of teeth, I discovered that they were in play…

mercury:~ esteele$ ls -le bin/testfile
-rw-r--r--@ 1 esteele  staff  159  4 Jul 08:52 bin/testfile
0: group:everyone deny write,delete,append,writeattr,writeextattr,chown

But why didn’t I see a + when I ran ls? It seems extended attributes get display priority over ACLs (notice how the @ is replaced by a + when the extended attributes are removed)

mercury:~ esteele$ ls -l@ bin/testfile
-rw-r--r--@ 1 esteele  staff  159  4 Jul 08:52 bin/testfile
mercury:~ esteele$ sudo xattr -d com.apple.metadata:_kTimeMachineNewestSnapshot bin/testfile
mercury:~ esteele$ sudo xattr -d com.apple.metadata:_kTimeMachineOldestSnapshot bin/testfile
mercury:~ esteele$ ls -l@ bin/testfile
-rw-r--r--+ 1 esteele  staff  159  4 Jul 08:52 bin/testfile

How frustrating. In retrospect I can see how the man page is alluding to this behaviour, but it’d be easy to make this more explicit e.g.

If the file or directory has extended attributes, the permissions field printed by the -l option is followed by a ‘@’ character. If the file or directory has extended security information (such as an access control list), the permissions field printed by the -l option is followed by a ‘+’ character, unless the file or directory also has extended attributes, in which case only the ‘@’ character will be shown. In this case, view extended security information using the -e option.

Or better still, if you’re going to jam several flags into a single character and there aren’t many combinations, use another character to represent combinations of the two e.g.

If the file or directory has extended attributes, the permissions field printed by the -l option is followed by a ‘@’ character. If the file or directory has extended security information (such as an access control list), the permissions field printed by the -l option is followed by a ‘+’ character, unless the file or directory also has extended attributes, in which case the permissions field printed by the -l option is followed by a ‘#’ character

So finally I was able to remove the ACLs, and everything was back to normal

mercury:~ esteele$ chmod -a# 0 bin/testfile
mercury:~ esteele$ ls -le bin/testfile
-rw-r--r--  1 esteele  staff  159  4 Jul 08:52 bin/testfile
mercury:~ esteele$ touch bin/testfile
mercury:~ esteele$

So, this was a small journey into less frequently used file attributes made necessary by a failure but complicated by unintuitive documentation. I don’t know why the failure happened but it makes me appreciate good documentation all the more, and particularly the effort that the OpenBSD guys put into their man pages.

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