[PULL v2 2/9] parallels: read header/BAT table in bounded chunks

parallels_open() read the whole header+BAT table with a single
bdrv_pread() call sized s->header_size. For an image whose catalog
approaches the "Catalog too large" bound (INT_MAX / sizeof(uint32_t)
entries), that size approaches BDRV_REQUEST_MAX_BYTES, and the block
layer legitimately refuses a single request that large, so the image
failed to open with a generic I/O error even though the catalog size
itself is within the format's documented limit.

Read the header and BAT table in fixed-size chunks instead, so the
maximum catalog size parallels_open() can actually address matches
the bound it already enforces, independent of the file's block-layer
alignment requirements.

Signed-off-by: Denis V. Lunev <[email protected]>
CC: Thomas Huth <[email protected]>
CC: Stefan Hajnoczi <[email protected]>
---
 block/parallels.c                             | 17 ++++++++++----
 tests/qemu-iotests/tests/parallels-checks     | 23 +++++++++++++++++++
 tests/qemu-iotests/tests/parallels-checks.out | 11 +++++++++
 3 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/block/parallels.c b/block/parallels.c
index 59f00c64a6..0f655b58d5 100644
--- a/block/parallels.c
+++ b/block/parallels.c
@@ -52,6 +52,7 @@
 #define HEADER_VERSION 2
 #define HEADER_INUSE_MAGIC  (0x746F6E59)
 #define MAX_PARALLELS_IMAGE_FACTOR (1ull << 32)
+#define PARALLELS_HEADER_READ_CHUNK (64 * 1024 * 1024)
 
 static QEnumLookup prealloc_mode_lookup = {
     .array = (const char *const[]) {
@@ -1241,7 +1242,7 @@ static int parallels_open(BlockDriverState *bs, QDict *options, int flags,
     BDRVParallelsState *s = bs->opaque;
     ParallelsHeader ph;
     int ret, i;
-    uint32_t size;
+    uint32_t size, header_off;
     int64_t file_nb_sectors, sector;
     uint32_t data_start;
     bool need_check = false;
@@ -1311,9 +1312,17 @@ static int parallels_open(BlockDriverState *bs, QDict *options, int flags,
         return -ENOMEM;
     }
 
-    ret = bdrv_pread(bs->file, 0, s->header_size, s->header, 0);
-    if (ret < 0) {
-        goto fail;
+    /* A single request s->header_size large exceeds BDRV_REQUEST_MAX_BYTES. */
+    for (header_off = 0; header_off < s->header_size;
+         header_off += PARALLELS_HEADER_READ_CHUNK) {
+        uint32_t chunk = MIN(s->header_size - header_off,
+                             PARALLELS_HEADER_READ_CHUNK);
+
+        ret = bdrv_pread(bs->file, header_off, chunk,
+                         (uint8_t *)s->header + header_off, 0);
+        if (ret < 0) {
+            goto fail;
+        }
     }
     s->bat_bitmap = (uint32_t *)(s->header + 1);
 
diff --git a/tests/qemu-iotests/tests/parallels-checks b/tests/qemu-iotests/tests/parallels-checks
index b281246a42..9535024885 100755
--- a/tests/qemu-iotests/tests/parallels-checks
+++ b/tests/qemu-iotests/tests/parallels-checks
@@ -44,6 +44,7 @@ _supported_os Linux
 SIZE=$((4 * 1024 * 1024))
 IMGFMT=parallels
 CLUSTER_SIZE_OFFSET=28
+BAT_ENTRIES_OFFSET=32
 DATA_OFF_OFFSET=48
 BAT_OFFSET=64
 
@@ -199,6 +200,28 @@ _check_test_img -r all
 echo "== check first cluster =="
 { $QEMU_IO -r -c "read -P 0x55 0 $CLUSTER_SIZE" "$TEST_IMG"; } 2>&1 | _filter_qemu_io | _filter_testdir
 
+# Clear image
+_make_test_img $SIZE
+
+echo "== TEST HUGE BAT TABLE OPEN =="
+
+# Overflows a single read request, but stays under parallels_open()'s
+# own catalog-size cap.
+BAT_ENTRIES=536870896
+HEADER_SIZE=$((64 + 4 * BAT_ENTRIES))
+
+echo "== advertise a BAT table larger than BDRV_REQUEST_MAX_BYTES =="
+poke_file "$TEST_IMG" "$BAT_ENTRIES_OFFSET" "\xf0\xff\xff\x1f"
+
+echo "== grow the file to match, without writing real data =="
+truncate -s $HEADER_SIZE "$TEST_IMG"
+
+echo "== open must succeed: the header/BAT read is chunked =="
+_img_info
+
+echo "== an unallocated cluster still reads as zeroes =="
+{ $QEMU_IO -r -c "read -P 0x00 0 $CLUSTER_SIZE" "$TEST_IMG"; } 2>&1 | _filter_qemu_io | _filter_testdir
+
 # success, all done
 echo "*** done"
 rm -f $seq.full
diff --git a/tests/qemu-iotests/tests/parallels-checks.out b/tests/qemu-iotests/tests/parallels-checks.out
index 9793423111..6fb2014e8e 100644
--- a/tests/qemu-iotests/tests/parallels-checks.out
+++ b/tests/qemu-iotests/tests/parallels-checks.out
@@ -129,4 +129,15 @@ No errors were found on the image.
 == check first cluster ==
 read 1048576/1048576 bytes at offset 0
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4194304
+== TEST HUGE BAT TABLE OPEN ==
+== advertise a BAT table larger than BDRV_REQUEST_MAX_BYTES ==
+== grow the file to match, without writing real data ==
+== open must succeed: the header/BAT read is chunked ==
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 4 MiB (4194304 bytes)
+== an unallocated cluster still reads as zeroes ==
+read 1048576/1048576 bytes at offset 0
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 *** done
-- 
2.53.0