Hey,
This is a mechanism that allows the holder of a block device to yield
device access before actually closing the block device.
If a someone yields a device then any concurrent opener claiming the
device exclusively with the same blk_holder_ops as the current owner can
wait for the device to be given up. Filesystems by default use
fs_holder_ps and so can wait on each other.
This mechanism allows us to simplify superblock handling quite a bit at
the expense of requiring filesystems to yield devices. A filesytems must
yield devices under s_umount. This allows costly work to be done outside
of s_umount.
There's nothing wrong with the way we currently do things but this does
allow us to simplify things and kills a whole class of theoretical UAF
when walking the superblock list.
I had originally considered doing it this way but wasn't able
(time-wise) to code that up but since we recently had that discussion
again here it is.
Survives both xfstests and blktests. Also tested this by introducing
custom delays into kill_block_super() to widen the race where a
superblock is removed from the instance list and the device is fully
closed and synced.
Based on on vfs.super and the freezer work sent out earlier.
Very barebones commit messages and less analyzed then usually for
possible side-effects.
Thanks!
Christian
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
---
Christian Brauner (6):
fs: simplify setup_bdev_super() calls
xfs: simplify device handling
ext4: simplify device handling
bdev: simplify waiting for concurrent claimers
block: mark device as about to be released
fs: add ->yield_devices()
block/bdev.c | 54 +++++++++++++++++++++++++++-----------
fs/ext4/super.c | 15 ++++++++---
fs/super.c | 67 ++++++++++++++---------------------------------
fs/xfs/xfs_super.c | 46 +++++++++++++++++++++-----------
include/linux/blk_types.h | 8 +++++-
include/linux/blkdev.h | 1 +
include/linux/fs.h | 1 +
7 files changed, 109 insertions(+), 83 deletions(-)
---
base-commit: c6cc4b13e95115c13433136a17150768d562a54c
change-id: 20231024-vfs-super-rework-ca447a3240c9
