buffer: fix grow_buffers() for block size > PAGE_SIZE

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.

Link: https://lkml.kernel.org/r/20231109210608.2252323-4-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
[nathan@kernel.org: add cast in grow_buffers() to avoid a multiplication libcall]
  Link: https://lkml.kernel.org/r/20231128-avoid-muloti4-grow_buffers-v1-1-bc3d0f0ec483@kernel.orgSigned-off-by: Andrew Morton <akpm@linux-foundation.org>

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, (sector_t)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 *