[Ocfs2-devel] [PATCH] ocfs2: don't evaluate buffer head to NULL managed by caller

[Changwei Ge]

ocfs2_read_blocks() is used to read several blocks from disk.
Currently, the input argument *bhs* can be NULL or NOT. It depends on
the caller's behavior. If the function fails in reading blocks from
disk, the corresponding bh will be assigned to NULL and put.

Obviously, above process for non-NULL input bh is not appropriate.
Because the caller doesn't even know its bhs are put and re-assigned.

If buffer head is managed by caller, ocfs2_read_blocks should not
evaluate it to NULL. It will cause caller accessing illegal memory,
thus crash.

Signed-off-by: Changwei Ge <ge.changwei at h3c.com>
---
 fs/ocfs2/buffer_head_io.c | 31 +++++++++++++++++++++++++------
 1 file changed, 25 insertions(+), 6 deletions(-)

diff --git a/fs/ocfs2/buffer_head_io.c b/fs/ocfs2/buffer_head_io.c
index d9ebe11..17329b6 100644
--- a/fs/ocfs2/buffer_head_io.c
+++ b/fs/ocfs2/buffer_head_io.c
@@ -188,6 +188,7 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
 	int i, ignore_cache = 0;
 	struct buffer_head *bh;
 	struct super_block *sb = ocfs2_metadata_cache_get_super(ci);
+	int new_bh = 0;
 
 	trace_ocfs2_read_blocks_begin(ci, (unsigned long long)block, nr, flags);
 
@@ -213,6 +214,18 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
 		goto bail;
 	}
 
+	/* Use below trick to check if all bhs are NULL or assigned.
+	 * Basically, we hope all bhs are consistent so that we can
+	 * handle exception easily.
+	 */
+	new_bh = (bhs[0] == NULL);
+	for (i = 1 ; i < nr ; i++) {
+		if ((new_bh && bhs[i]) || (!new_bh && !bhs[i])) {
+			WARN(1, "Not all bhs are consistent\n");
+			break;
+		}
+	}
+
 	ocfs2_metadata_cache_io_lock(ci);
 	for (i = 0 ; i < nr ; i++) {
 		if (bhs[i] == NULL) {
@@ -324,8 +337,10 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
 		if (!(flags & OCFS2_BH_READAHEAD)) {
 			if (status) {
 				/* Clear the rest of the buffers on error */
-				put_bh(bh);
-				bhs[i] = NULL;
+				if (new_bh) {
+					put_bh(bh);
+					bhs[i] = NULL;
+				}
 				continue;
 			}
 			/* We know this can't have changed as we hold the
@@ -342,8 +357,10 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
 				 * for this bh as it's not marked locally
 				 * uptodate. */
 				status = -EIO;
-				put_bh(bh);
-				bhs[i] = NULL;
+				if (new_bh) {
+					put_bh(bh);
+					bhs[i] = NULL;
+				}
 				continue;
 			}
 
@@ -355,8 +372,10 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
 				clear_buffer_needs_validate(bh);
 				status = validate(sb, bh);
 				if (status) {
-					put_bh(bh);
-					bhs[i] = NULL;
+					if (new_bh) {
+						put_bh(bh);
+						bhs[i] = NULL;
+					}
 					continue;
 				}
 			}
-- 
2.7.4