[PATCH] iomap: adjust read range correctly for non-block-aligned positions

[Joanne Koong <joannelkoong@gmail.com>]

iomap_adjust_read_range() assumes that the position and length passed in
are block-aligned. This is not always the case however, as shown in the
syzbot generated case for erofs. This causes too many bytes to be
skipped for uptodate blocks, which results in returning the incorrect
position and length to read in. If all the blocks are uptodate, this
underflows length and returns a position beyond the folio.

Fix the calculation to also take into account the block offset when
calculating how many bytes can be skipped for uptodate blocks.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Tested-by: syzbot@syzkaller.appspotmail.com
---
 fs/iomap/buffered-io.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 8b847a1e27f1..1c95a0a7b302 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -240,17 +240,24 @@ static void iomap_adjust_read_range(struct inode *inode, struct folio *folio,
 	 * to avoid reading in already uptodate ranges.
 	 */
 	if (ifs) {
-		unsigned int i;
+		unsigned int i, blocks_skipped;
 
 		/* move forward for each leading block marked uptodate */
-		for (i = first; i <= last; i++) {
+		for (i = first; i <= last; i++)
 			if (!ifs_block_is_uptodate(ifs, i))
 				break;
-			*pos += block_size;
-			poff += block_size;
-			plen -= block_size;
-			first++;
+
+		blocks_skipped = i - first;
+		if (blocks_skipped) {
+			unsigned long block_offset = *pos & (block_size - 1);
+			unsigned bytes_skipped =
+				(blocks_skipped << block_bits) - block_offset;
+
+			*pos += bytes_skipped;
+			poff += bytes_skipped;
+			plen -= bytes_skipped;
 		}
+		first = i;
 
 		/* truncate len if we find any trailing uptodate block(s) */
 		while (++i <= last) {
-- 
2.47.3