btrfs: fix incorrect readahead expansion length

The intent of btrfs_readahead_expand() was to expand to the length of
the current compressed extent being read. However, "ram_bytes" is *not*
that, in the case where a single physical compressed extent is used for
multiple file extents.

Consider this case with a large compressed extent C and then later two
non-compressed extents N1 and N2 written over C, leaving C1 and C2
pointing to offset/len pairs of C:
[               C                 ]
[ N1 ][     C1     ][ N2 ][   C2  ]

In such a case, ram_bytes for both C1 and C2 is the full uncompressed
length of C. So starting readahead in C1 will expand the readahead past
the end of C1, past N2, and into C2. This will then expand readahead
again, to C2_start + ram_bytes, way past EOF. First of all, this is
totally undesirable, we don't want to read the whole file in arbitrary
chunks of the large underlying extent if it happens to exist. Secondly,
it results in zeroing the range past the end of C2 up to ram_bytes. This
is particularly unpleasant with fs-verity as it can zero and set
uptodate pages in the verity virtual space past EOF. This incorrect
readahead behavior can lead to verity verification errors, if we iterate
in a way that happens to do the wrong readahead.

Fix this by using em->len for readahead expansion, not em->ram_bytes,
resulting in the expected behavior of stopping readahead at the extent
boundary.

Reported-by: Max Chernoff <git@maxchernoff.ca>
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2399898
Fixes: 9e9ff87 ("btrfs: use readahead_expand() on compressed extents")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: boryas

fs/btrfs/extent_io.c

@@ -972,7 +972,7 @@ static void btrfs_readahead_expand(struct readahead_control *ractl,
 {
 	const u64 ra_pos = readahead_pos(ractl);
 	const u64 ra_end = ra_pos + readahead_length(ractl);
-	const u64 em_end = em->start + em->ram_bytes;
+	const u64 em_end = em->start + em->len;
 
 	/* No expansion for holes and inline extents. */
 	if (em->disk_bytenr > EXTENT_MAP_LAST_BYTE)