ext4: partial zero eof block on unaligned inode size extension

ext4: partial zero eof block on unaligned inode size extension

Using mapped writes, it's technically possible to expose stale
post-eof data on a truncate up operation. Consider the following
example:

$ xfs_io -fc "pwrite 0 2k" -c "mmap 0 4k" -c "mwrite 2k 2k" \
	-c "truncate 8k" -c "pread -v 2k 16" <file>
...
00000800:  58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58  XXXXXXXXXXXXXXXX
...

This shows that the post-eof data written via mwrite lands within
EOF after a truncate up. While this is deliberate of the test case,
behavior is somewhat unpredictable because writeback does post-eof
zeroing, and writeback can occur at any time in the background. For
example, an fsync inserted between the mwrite and truncate causes
the subsequent read to instead return zeroes. This basically means
that there is a race window in this situation between any subsequent
extending operation and writeback that dictates whether post-eof
data is exposed to the file or zeroed.

To prevent this problem, perform partial block zeroing as part of
the various inode size extending operations that are susceptible to
it. For truncate extension, zero around the original eof similar to
how truncate down does partial zeroing of the new eof. For extension
via writes and fallocate related operations, zero the newly exposed
range of the file to cover any partial zeroing that must occur at
the original and new eof blocks.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://patch.msgid.link/20240919160741.208162-2-bfoster@redhat.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>