POSIX requires that on ftruncate() expansion, the new bytes must read as zeroes. If someone's mmap()ed the file and stored past EOF, for most filesystems the bytes in that page will be not-zero. It's a pretty minor violation; someone could race you and write to the file between the ftruncate() call and you reading from it, but it's a bit of a QOI violation. I've tested xfs (passes before & after), ext4 and tmpfs (both fail before, pass after). Testing from other FS developers appreciated. fstest to follow; not sure how to persuade git-send-email to work on multiple repositories Matthew Wilcox (Oracle) (5): truncate: Zero bytes after 'oldsize' if we're expanding the file ext4: Zero bytes after 'oldsize' if we're expanding the file tmpfs: Zero bytes after 'oldsize' if we're expanding the file afs: Zero bytes after 'oldsize' if we're expanding the file btrfs: Zero bytes after 'oldsize' if we're expanding the file fs/afs/inode.c | 2 ++ fs/btrfs/inode.c | 1 + fs/ext4/inode.c | 1 + mm/shmem.c | 2 ++ mm/truncate.c | 7 +++++-- 5 files changed, 11 insertions(+), 2 deletions(-) -- 2.35.1
