overlayfs-setup: Fallback to read only mount when overlay not supported

[dbnicholson]

Overlayfs doesn't support using case insensitive filesystems such as vfat and exfat as a lower directory. Change the mount to read only so changes can't be made there.

In theory this would simply be mount -o remount,ro. Unfortunately, if systemd has setup an automount, autofs is not our friend. In that case, the mount options need to be persisted into the mount unit so that a subsequent remounting retains the ro option.

https://phabricator.endlessm.com/T35641

[wjt]

LGTM.

I wonder what happens if you have both rw and ro in the mount options. Which one wins? Is it the last one specified? If so the mount flag change could be simplified to unconditionally appending ,ro.

[dbnicholson]

I wonder what happens if you have both rw and ro in the mount options. Which one wins? Is it the last one specified? If so the mount flag change could be simplified to unconditionally appending ,ro.

I'm not sure. I think probably last one wins and just appending was my first thought. Lemme try that out...

[dbnicholson]

I wonder what happens if you have both rw and ro in the mount options. Which one wins? Is it the last one specified? If so the mount flag change could be simplified to unconditionally appending ,ro.

I'm not sure. I think probably last one wins and just appending was my first thought. Lemme try that out...

Yeah, that seems to work. I added an override to a test mount unit that set Options=umask=0077,noauto,rw,ro. After mounting the options were displayed as ro,... with no rw anywhere. I don't know if mount(8) or the kernel is doing the canonicalizing. Similary, I tried mount -o remount,rw,ro and it similary became ro,.... I suspect it's the kernel that does this since it always lists ro or rw as the first option regardless of what order they were specified.

I'll change that since the mount option editing was definitely my least favorite part of this patch.

[dbnicholson]

I suspect it's the kernel that does this since it always lists ro or rw as the first option regardless of what order they were specified.

Oh, actually it is mount(8). I ran strace mount -o umask=0077,rw,ro /dev/vda1 /mnt and the resulting syscall was mount("/dev/vda1", "/mnt", "vfat", MS_RDONLY, "umask=0077"). Since read only is a mount(2) flag, it has to extract it from the options. Running it again with ro,rw or neither ro or rw resulted in mount("/dev/vda1", "/mnt", "vfat", 0, "umask=0077"). In other words, rw is the default of no flags specified.

[dbnicholson]

Updated now to unconditionally append ro. I tested in a VM and it seems to work correctly.