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Unbelievable, just lost 6h of my life to Linux fs API "misdesign". So let's say you have a disk image file with an ext4 fs, it's read-only, you want to mount it read-only. So you read-only attach it to a loopback device (i.e. O_RDONLY open + LO_FLAGS_READ_ONLY). Because you are…
…extra careful you even check with BLKROGET and yes, it tells you: this block device is read-only. So you go on, and mount it read-only, i.e. mount() with MS_RDONLY. One would assume because everything in this chain was explicitly done read-only frickin' ext4 wouldn't modify…
… the original file. Ha, you are so naive, of course it does! All these read-only flags don't matter a thing, ext4 writes whenever it wants to. Read-only access is for wussies, real kernel code doesn't care about explicitly specified flags on every level. ext4 doesn't, …
