[PATCH v2 3/7] buffer: Fix grow_buffers() for block size > PAGE_SIZE

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

We must not shift by a negative number so work in terms of a byte
offset to avoid the awkward shift left-or-right-depending-on-sign
option.  This means we need to use check_mul_overflow() to ensure
that a large block number does not result in a wrap.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 fs/buffer.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 44e0c0b7f71f..9c3f49cf8d28 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1085,26 +1085,21 @@ static bool grow_dev_folio(struct block_device *bdev, sector_t block,
 static bool grow_buffers(struct block_device *bdev, sector_t block,
 		unsigned size, gfp_t gfp)
 {
-	pgoff_t index;
-	int sizebits;
-
-	sizebits = PAGE_SHIFT - __ffs(size);
-	index = block >> sizebits;
+	loff_t pos;
 
 	/*
-	 * Check for a block which wants to lie outside our maximum possible
-	 * pagecache index.  (this comparison is done using sector_t types).
+	 * Check for a block which lies outside our maximum possible
+	 * pagecache index.
 	 */
-	if (unlikely(index != block >> sizebits)) {
-		printk(KERN_ERR "%s: requested out-of-range block %llu for "
-			"device %pg\n",
+	if (check_mul_overflow(block, size, &pos) || pos > MAX_LFS_FILESIZE) {
+		printk(KERN_ERR "%s: requested out-of-range block %llu for device %pg\n",
 			__func__, (unsigned long long)block,
 			bdev);
 		return false;
 	}
 
 	/* Create a folio with the proper size buffers */
-	return grow_dev_folio(bdev, block, index, size, gfp);
+	return grow_dev_folio(bdev, block, pos / PAGE_SIZE, size, gfp);
 }
 
 static struct buffer_head *
-- 
2.42.0