EXT: don't reserve any block for root

[ydirson]

There is only root in dom0, and df erroneously substracts the default 5% of disk space as "reserved for root", which is just meaningless here.

It could be worth going further with doing the same for all FileSR types, but I thought I'd ask for feedback first.

If this becomes a thing, it could be useful then to find some way to get tune2fs -m 0 run on existing SR to make them even.

[MarkSymsCtx]

As you say, all access is performed as root anyway so I'm not sure what actual difference this will make?

[MarkSymsCtx]

Seems reasonable

[rosslagerwall]

There is only root in dom0

This assertion is not correct for XenServer. In XenServer's dom0 various processes run as different users (e.g. chrony, pvsproxy, QEMU, swtpm, ...). A vulnerability without a privilege escalation in one of those components could then more easily lead to a DoS if the reserve is removed, i.e. it would be a loss of defence-in-depth. Whether this is important enough to keep the reserve is debatable but the discussion at least needs to be had.

[ydirson]

There is only root in dom0

This assertion is not correct for XenServer. In XenServer's dom0 various processes run as different users (e.g. chrony, pvsproxy, QEMU, swtpm, ...). A vulnerability without a privilege escalation in one of those components could then more easily lead to a DoS if the reserve is removed, i.e. it would be a loss of defence-in-depth. Whether this is important enough to keep the reserve is debatable but the discussion at least needs to be had.

OK, my statement was really too broad. However, other users will only matter for system partitions, right?
This PR is about EXT SRs, where none of those extra users would have a permission to write.