Hi all, While auditing the file permission dropping for fallocate, I reached the conclusion that fallocate can modify file contents, and therefore should be treated as a file write. As such, it needs to update the file modification and file (metadata) change timestamps, and it needs to drop file privileges such as setuid and capabilities, just like a regular write. Moreover, if the inode is configured for synchronous writes, then all the fallocate changes really ought to be persisted to disk before fallocate returns to userspace. Unfortunately, the XFS fallocate implementation doesn't do this correctly. setgid without group-exec is a mandatory locking mark and is left alone by write(), which means that we shouldn't drop it unconditionally. Furthermore, file capabilities are another vector for setuid to be set on a program file, and XFS ignores these. I also noticed that fallocate doesn't flush the log to disk after fallocate when the fs is mounted with -o sync or if the DIFLAG_SYNC flag is set on the inode. Therefore, refactor the XFS fallocate implementation to use the VFS helper file_modified to update file metadata instead of open-coding it incorrectly. Refactor it further to use xfs_file_sync_writes to decide if we need to flush the log; and then fix the log flushing so that it flushes after we've made /all/ the changes. v2: fix some bisection problems If you're going to start using this mess, you probably ought to just pull from my git trees, which are linked below. This is an extraordinary way to destroy everything. Enjoy! Comments and questions are, as always, welcome. --D kernel git tree: https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=falloc-fix-perm-updates-5.17 --- fs/xfs/xfs_file.c | 72 ++++++++++++++++++++++++++++++++++++++--------------- 1 file changed, 52 insertions(+), 20 deletions(-)
