[PATCH] fs/remap: constrain dedupe of EOF blocks

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

If dedupe of an EOF block is not constrainted to match against only
other EOF blocks with the same EOF offset into the block, it can
match against any other block that has the same matching initial
bytes in it, even if the bytes beyond EOF in the source file do
not match.

Fix this by constraining the EOF block matching to only match
against other EOF blocks that have identical EOF offsets and data.
This allows "whole file dedupe" to continue to work without allowing
eof blocks to randomly match against partial full blocks with the
same data.

Reported-by: Ansgar Lößer <ansgar.loesser@tu-darmstadt.de>
Fixes: 1383a7ed6749 ("vfs: check file ranges before cloning files")
Link: https://lore.kernel.org/linux-fsdevel/a7c93559-4ba1-df2f-7a85-55a143696405@tu-darmstadt.de/
Signed-off-by: Dave Chinner <dchinner@redhat.com>
---

This is tested against the case provided in the initial report. Old
kernel:

$ ./dedupe.sh |less
secret
$

Patched kernel:

$ ./dedupe.sh
dedupe-bug: t.c:90: main: Assertion `status != FILE_DEDUPE_RANGE_DIFFERS' failed.
./dedupe.sh: line 11:  4831 Aborted /home/dave/dedupe-bug $MNT/writeonly.txt $MNT/test.tmp
$

So now it fails with FILE_DEDUPE_RANGE_DIFFERS because it can't use
short files to discover the dedupe character match one byte at a
time.

It also passes fstests ismoke tests via running the './check -g
dedupe' test group, so the fix doesn't obviously break anything.

 fs/remap_range.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/remap_range.c b/fs/remap_range.c
index e112b5424cdb..881a306ee247 100644
--- a/fs/remap_range.c
+++ b/fs/remap_range.c
@@ -71,7 +71,8 @@ static int generic_remap_checks(struct file *file_in, loff_t pos_in,
 	 * Otherwise, make sure the count is also block-aligned, having
 	 * already confirmed the starting offsets' block alignment.
 	 */
-	if (pos_in + count == size_in) {
+	if (pos_in + count == size_in &&
+	    (!(remap_flags & REMAP_FILE_DEDUP) || pos_out + count == size_out)) {
 		bcount = ALIGN(size_in, bs) - pos_in;
 	} else {
 		if (!IS_ALIGNED(count, bs))