[RFC 3/8] fs/buffer: restart block_read_full_folio() to avoid array overflow

[Luis Chamberlain <mcgrof@kernel.org>]

From: Hannes Reinecke <hare@suse.de>

block_read_full_folio() uses an on-stack array to hold any buffer_heads
which should be updated. The array is sized for the number of buffer_heads
per PAGE_SIZE, which of course will overflow for large folios.
So instead of increasing the size of the array (and thereby incurring
a possible stack overflow for really large folios) stop the iteration
when the array is filled up, submit these buffer_heads, and restart
the iteration with the remaining buffer_heads.

Signed-off-by: Hannes Reinecke <hare@suse.de>
---
 fs/buffer.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 1fc9a50def0b..818c9c5840fe 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2366,7 +2366,7 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 {
 	struct inode *inode = folio->mapping->host;
 	sector_t iblock, lblock;
-	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
+	struct buffer_head *bh, *head, *restart_bh = NULL, *arr[MAX_BUF_PER_PAGE];
 	size_t blocksize;
 	int nr, i;
 	int fully_mapped = 1;
@@ -2385,6 +2385,7 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 	iblock = div_u64(folio_pos(folio), blocksize);
 	lblock = div_u64(limit + blocksize - 1, blocksize);
 	bh = head;
+restart:
 	nr = 0;
 	i = 0;
 
@@ -2417,7 +2418,12 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 				continue;
 		}
 		arr[nr++] = bh;
-	} while (i++, iblock++, (bh = bh->b_this_page) != head);
+	} while (i++, iblock++, (bh = bh->b_this_page) != head && nr < MAX_BUF_PER_PAGE);
+
+	if (nr == MAX_BUF_PER_PAGE && bh != head)
+		restart_bh = bh;
+	else
+		restart_bh = NULL;
 
 	if (fully_mapped)
 		folio_set_mappedtodisk(folio);
@@ -2450,6 +2456,15 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 		else
 			submit_bh(REQ_OP_READ, bh);
 	}
+
+	/*
+	 * Found more buffers than 'arr' could hold,
+	 * restart to submit the remaining ones.
+	 */
+	if (restart_bh) {
+		bh = restart_bh;
+		goto restart;
+	}
 	return 0;
 }
 EXPORT_SYMBOL(block_read_full_folio);
-- 
2.43.0