Hello! This is the third version of the patches to add config option to not allow writing to mounted block devices. The new API for block device opening has been merged so hopefully this patchset can progress towards being merged. We face some issues with necessary btrfs changes (review bandwidth) so this series is modified to enable restricting of writes for all other filesystems. Once btrfs can merge necessary device scanning changes, enabling the support for restricting writes for it is trivial. For motivation why restricting writes to mounted block devices is interesting see patch 3/7. I've been testing the patches more extensively and I've found couple of things that get broken by disallowing writes to mounted block devices: 1) "mount -o loop" gets broken because util-linux keeps the loop device open read-write when attempting to mount it. Hopefully fixable within util-linux. 2) resize2fs online resizing gets broken because it tries to open the block device read-write only to call resizing ioctl. Trivial to fix within e2fsprogs. 3) Online e2label will break because it directly writes to the ext2/3/4 superblock while the FS is mounted to set the new label. Ext4 driver will have to implement the SETFSLABEL ioctl() and e2label will have to use it, matching what happens for online labelling of btrfs and xfs. Likely there will be other breakage I didn't find yet but overall the breakage looks minor enough that the option might be useful. Definitely good enough for syzbot fuzzing and likely good enough for hardening of systems with more tightened security. This patch set is based on the VFS tree as of yesterday. Changes since v2: * Rebased on top of current VFS tree * Added missing conversion of bcachefs to new bdev opening API * Added patch to drop old bdev opening API * Dropped support for restricting writes to btrfs to avoid patch dependencies and unblock merging of the patches Changes since v1: * Added kernel cmdline argument to toggle whether writing to mounted block devices is allowed or not * Fixed handling of partitions * Limit write blocking only to devices open with explicit BLK_OPEN_BLOCK_WRITES flag Honza Previous versions: Link: https://lore.kernel.org/all/20230612161614.10302-1-jack@xxxxxxx #v1 Link: https://lore.kernel.org/all/20230704122727.17096-1-jack@xxxxxxx #v2
