APFS Bug in macOS High Sierra Can Cause Data Loss When Writing to Disk Images
Posted on: 02/19/2018 12:06 PM

MacRumors posted a story that Apple's APFS file system suffers from a disk image vulnerability that in certain circumstances can lead to data loss

APFS Bug in macOS High Sierra Can Cause Data Loss When Writing to Disk Images


Earlier this week I noticed that an APFS-formatted sparsebundle disk image volume showed ample free space, despite that the underlying disk was completely full. Curious, I copied a video file to the disk image volume to see what would happen. The whole file copied without error! I opened the file, verified that the video played back start to finish, checksummed the file – as far as I could tell, the file was intact and whole on the disk image. When I unmounted and remounted the disk image, however, the video was corrupted. If you've ever lost data, you know the kick-in-the-gut feeling that would have ensued. Thankfully, I was just running some tests and the file that disappeared was just test data. Two related problems are identified by Bombich, above. The first is that the free space on the APFS-formatted sparse disk image doesn't update as it should when the free space on the underlying physical host disk is reduced. The second problem is the lack of error reports when write requests fail to dynamically grow the disk image, resulting in data being "written" into a void. Bombich tracks both bugs back to macOS's background "diskimages-helper" application service, which he has since reported to Apple.



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