On Thu, Aug 01, 2024 at 08:32:18AM +0200, Johannes Bauer wrote: > Am 01.08.24 um 08:18 schrieb Johannes Bauer: > > > But my point is, that is what I am doing -- creating the losetup mapping > > R/O: > > > > # losetup --read-only --show -f image.img > > /dev/loop35 > > > > # echo foo >/dev/loop35 > > bash: echo: write error: Operation not permitted Oh, I missed that critical detail buried in the example code. Most people who report "writes occuring on RO mounts" have no clue that writes are actually allowed on RO mounts, and you made no mention that you were also setting the block device read only and that's why you were expecting writes to fail.... As it is, I don't think the filesystem is actually writing anything to disk. There's a couple of individual metadata fields that have been changed in the block device page cache in some metadata at offset 0x8000: $ diff -u img.t bdev.t --- img.t 2024-08-09 11:41:20.358217508 +1000 +++ bdev.t 2024-08-09 11:41:29.966289011 +1000 @@ -103,10 +103,11 @@ 007ff0 0000 0000 0000 0000 000c de00 0eb9 b0b0 008000 3bc0 9839 0000 0400 0000 0000 0000 0004 008010 0000 0004 0000 0100 0000 0100 0000 0000 -008020 0000 0000 0000 0000 0000 0000 0000 0000 +008020 0000 0000 0000 0000 0000 1200 0000 0000 008030 9f03 d923 77f8 9748 64af 1176 1b28 9959 008040 0000 0100 0000 0000 0000 0000 0000 0000 -008050 0000 0000 0000 0000 0000 0000 0000 0000 +008050 0004 0000 0000 0000 0000 0000 0000 0000 +008060 0000 0000 0000 0000 0000 0000 0000 0000 * 008800 0fff 0000 0000 0000 0000 0000 0000 0000 008810 0000 0000 0000 0000 0000 0000 0000 0000 This does not mean a write was done, however. The filesystem may have changed metadata directly in the page cache so that it is correct for a RO mount on a RO block device without actually writing anything to disk. There's nothing wrong with doing this - it isn't a bug at all even though such modifications will be visible to userspace via block device page cache reads. You'll need someone who actually knows what metadata ext4 needs to modify directly on read-only mounts to tell you exactly what the mount is modifying and whether or not it should actually be doing that or not. However, I personally wouldn't consider this behaviour a bug if it necessary to allow read-only mounts on read-only block devices to work reliably... -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
