[PATCH v2 4/4] udf: don't use eloc after the end of the unallocated space table

[Matthias Goergens <matthias.goergens@gmail.com>]

When udf_next_aext() returns 0, its eloc, elen and etype outputs hold
no defined value.  Two walks of the unallocated space table still use
eloc after the walk has reached the end of the table.

udf_table_prealloc_blocks() looks for a free extent starting at
first_block and decides whether it found one by comparing eloc with
first_block after the walk.  If the table's list ends in a continuation
descriptor that leads to an empty allocation extent, eloc is then the
block of that allocation extent, which is in use, and if that is
first_block, the block is handed out as preallocation.  Once
udf_discard_prealloc() no longer frees the wrong block, this is what
still makes fsx in generic/091 and generic/263 read back bad data on a
table, in about one run in four.  Only take the extent if the walk
stopped at it.

When udf_table_free_blocks() cannot merge the freed blocks into an
existing extent, it appends a new one and sets only its block number,
so the partition reference comes from whatever eloc held after the
walk, and with long allocation descriptors it is written to disk.  For
a table without extents, eloc was never set.  Take the partition
reference from the freed blocks instead.  Short allocation descriptors
do not store the partition reference, which is why this half remains a
latent long-descriptor fix rather than a reproduced failure.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
---
Measured on 2-3 October on the for_next base 4ad934e7a85e by running
fstests generic/075, 091 and 263 as a group, each run on a fresh 2 GiB
"mkudffs --space=unalloctable" test filesystem.  With patch 3 alone, 8
of 34 runs read back bad data in 091 or 263.  In 32 of these runs a
temporary debug message (not part of this series) reported when the
comparison above matched after the walk had reached the end of the
table; it fired in all 7 failing runs among them, each time on a
continuation descriptor (type 3, one block long), and in one passing
run.  With patches 3 and 4, none of 18 runs failed.  The
udf_table_free_blocks() case has not been reproduced.

 fs/udf/balloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/udf/balloc.c b/fs/udf/balloc.c
index 2ec577b..7512418 100644
--- a/fs/udf/balloc.c
+++ b/fs/udf/balloc.c
@@ -457,6 +457,7 @@ static void udf_table_free_blocks(struct super_block *sb,
 
 		int adsize;
 
+		eloc.partitionReferenceNum = bloc->partitionReferenceNum;
 		eloc.logicalBlockNum = start;
 		elen = EXT_RECORDED_ALLOCATED |
 			(count << sb->s_blocksize_bits);
@@ -532,7 +533,7 @@ static int udf_table_prealloc_blocks(struct super_block *sb,
 			  eloc.logicalBlockNum, elen, first_block);
 	}
 
-	if (first_block == eloc.logicalBlockNum) {
+	if (ret > 0 && first_block == eloc.logicalBlockNum) {
 		epos.offset -= adsize;
 
 		alloc_count = (elen >> sb->s_blocksize_bits);
-- 
2.56.0