Q When repairing permissions recently using Disk Utility, it reported a load of permissions that differed and had to be repaired, issued warnings of SUID files being modified and not repaired, and three ACLs were found but not expected. Is there any way to complete repair without performing a clean installation?
A You should not need to: although they may seem alarming, these messages have been common after applying a software update, such as one of Apple’s security updates.
SUID files are quite normal, and are normal background processes running services for your Mac, which need to gain root privileges to do their job properly. To do this, they set their user ID (SUID) to that of the root user, and Disk Utility cannot check or repair their permissions as a result.
ACLs are special extended permissions that are being used increasingly, and again are quite normal and nothing to worry about.
Disk Utility has thus performed the repairs that were needed, and your Mac is healthy again. If you want it to check and repair absolutely everything, you could restart from your recovery partition or an external bootable drive (such as a suitable USB stick), and run the repair from that. This should avoid the SUID issue, as no services will be running from your hard disk, but you may still see warnings about ACLs, of course.
Comments Older versions of OS X tended to run into permissions problems more often, and repairing permissions was a popular panacea for many problems. This is no longer the case, and repairing permissions now is not likely to help much, unless it is specifically indicated.
Updated from the original, which was first published in MacUser volume 27 issue 5, 2011.