[Qemu-devel] [PATCH v2 08/12] block/dmg: fix sector data offset calculation

[Peter Wu]

This patch addresses two issues:

 - The data fork offset was not taken into account, resulting in failure
   to read an InstallESD.dmg file (5164763151 bytes) which had a
   non-zero DataForkOffset field.
 - The offset of the previous block ("partition") was unconditionally
   added to the current block because older files would start the input
   offset of a new block at zero. Newer files (including vlc-2.1.5.dmg,
   tuxpaint-0.9.15-macosx.dmg and OS X Yosemite [MAS].dmg) failed in
   reads because these files have chunk offsets, relative to the begin
   of a data fork.

Now the data offset of the mish is taken into account. While we could
check that the data_offset is within the data fork, let's not do that
here as it would only result in parse failures on invalid files (rather
than gracefully handling such bad files). dmg_read will error out if
the offset is incorrect.

Signed-off-by: Peter Wu <address@hidden>
---
 v2: use sector and data offset as provided by the BLKX header. This
     allows us to drop last_in_offset. The previous heuristics to detect
     relative offsets is not needed anymore. Squashed the data fork
     offset length check into this patch.
---
 block/dmg.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/block/dmg.c b/block/dmg.c
index 57feb1b..130efac 100644
--- a/block/dmg.c
+++ b/block/dmg.c
@@ -188,7 +188,7 @@ static int64_t dmg_find_koly_offset(BlockDriverState 
*file_bs, Error **errp)
 typedef struct DmgHeaderState {
     /* used internally by dmg_read_mish_block to remember offsets of blocks
      * across calls */
-    uint64_t last_in_offset;
+    uint64_t data_fork_offset;
     uint64_t last_out_offset;
     /* exported for dmg_open */
     uint32_t max_compressed_size;
@@ -203,6 +203,8 @@ static int dmg_read_mish_block(BDRVDMGState *s, 
DmgHeaderState *ds,
     size_t new_size;
     uint32_t chunk_count;
     int64_t offset = 0;
+    uint64_t data_offset;
+    uint64_t in_offset = ds->data_fork_offset;
 
     type = buff_read_uint32(buffer, offset);
     /* skip data that is not a valid MISH block (invalid magic or too small) */
@@ -211,8 +213,12 @@ static int dmg_read_mish_block(BDRVDMGState *s, 
DmgHeaderState *ds,
         return 0;
     }
 
-    offset += 4;
-    offset += 200;
+    /* location in data fork for (compressed) blob (in bytes) */
+    data_offset = buff_read_uint64(buffer, offset + 0x18);
+    in_offset += data_offset;
+
+    /* move to begin of chunk entries */
+    offset += 204;
 
     chunk_count = (count - 204) / 40;
     new_size = sizeof(uint64_t) * (s->n_chunks + chunk_count);
@@ -228,7 +234,6 @@ static int dmg_read_mish_block(BDRVDMGState *s, 
DmgHeaderState *ds,
         if (s->types[i] != 0x80000005 && s->types[i] != 1 &&
             s->types[i] != 2) {
             if (s->types[i] == 0xffffffff && i > 0) {
-                ds->last_in_offset = s->offsets[i - 1] + s->lengths[i - 1];
                 ds->last_out_offset = s->sectors[i - 1] +
                                       s->sectorcounts[i - 1];
             }
@@ -255,7 +260,7 @@ static int dmg_read_mish_block(BDRVDMGState *s, 
DmgHeaderState *ds,
         }
 
         s->offsets[i] = buff_read_uint64(buffer, offset);
-        s->offsets[i] += ds->last_in_offset;
+        s->offsets[i] += in_offset;
         offset += 8;
 
         s->lengths[i] = buff_read_uint64(buffer, offset);
@@ -413,7 +418,7 @@ static int dmg_open(BlockDriverState *bs, QDict *options, 
int flags,
     s->n_chunks = 0;
     s->offsets = s->lengths = s->sectors = s->sectorcounts = NULL;
     /* used by dmg_read_mish_block to keep track of the current I/O position */
-    ds.last_in_offset = 0;
+    ds.data_fork_offset = 0;
     ds.last_out_offset = 0;
     ds.max_compressed_size = 1;
     ds.max_sectors_per_chunk = 1;
@@ -425,6 +430,15 @@ static int dmg_open(BlockDriverState *bs, QDict *options, 
int flags,
         goto fail;
     }
 
+    /* offset of data fork (DataForkOffset) */
+    ret = read_uint64(bs, offset + 0x18, &ds.data_fork_offset);
+    if (ret < 0) {
+        goto fail;
+    } else if (ds.data_fork_offset > offset) {
+        ret = -EINVAL;
+        goto fail;
+    }
+
     /* offset of resource fork (RsrcForkOffset) */
     ret = read_uint64(bs, offset + 0x28, &rsrc_fork_offset);
     if (ret < 0) {
-- 
2.2.1