block/dmg: properly detect the UDIF trailer

DMG files have a variable length with a UDIF trailer at the end of a
file. This UDIF trailer is essential as it describes the contents of
the image. At the moment however, the start of this trailer is almost
always incorrect as bdrv_getlength() returns a multiple of the block
size (rounded up). This results in a failure to recognize DMG files,
resulting in Invalid argument (EINVAL) errors.

As there is no API to retrieve the real file size, look for the magic
header in the last two sectors to find the start of this 512-byte UDIF
trailer (the "koly" block).

The resource fork offset ("info_begin") has its offset adjusted as the
initial value of offset does not mean "end of file" anymore, but "begin
of UDIF trailer".

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: John Snow <jsnow@redhat.com>

1 files changed, 45 insertions, 4 deletions

diff --git a/block/dmg.c b/block/dmg.c
index e455886d2b..918345954b 100644
--- a/block/dmg.c
+++ b/block/dmg.c
@@ -131,6 +131,48 @@ static void update_max_chunk_size(BDRVDMGState *s, uint32_t chunk,
     }
 }
 
+static int64_t dmg_find_koly_offset(BlockDriverState *file_bs, Error **errp)
+{
+    int64_t length;
+    int64_t offset = 0;
+    uint8_t buffer[515];
+    int i, ret;
+
+    /* bdrv_getlength returns a multiple of block size (512), rounded up. Since
+     * dmg images can have odd sizes, try to look for the "koly" magic which
+     * marks the begin of the UDIF trailer (512 bytes). This magic can be found
+     * in the last 511 bytes of the second-last sector or the first 4 bytes of
+     * the last sector (search space: 515 bytes) */
+    length = bdrv_getlength(file_bs);
+    if (length < 0) {
+        error_setg_errno(errp, -length,
+            "Failed to get file size while reading UDIF trailer");
+        return length;
+    } else if (length < 512) {
+        error_set(errp, ERROR_CLASS_GENERIC_ERROR,
+            "dmg file must be at least 512 bytes long");
+        return -EINVAL;
+    }
+    if (length > 511 + 512) {
+        offset = length - 511 - 512;
+    }
+    length = length < 515 ? length : 515;
+    ret = bdrv_pread(file_bs, offset, buffer, length);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret, "Failed while reading UDIF trailer");
+        return ret;
+    }
+    for (i = 0; i < length - 3; i++) {
+        if (buffer[i] == 'k' && buffer[i+1] == 'o' &&
+            buffer[i+2] == 'l' && buffer[i+3] == 'y') {
+            return offset + i;
+        }
+    }
+    error_set(errp, ERROR_CLASS_GENERIC_ERROR,
+        "Could not locate UDIF trailer in dmg file");
+    return -EINVAL;
+}
+
 static int dmg_open(BlockDriverState *bs, QDict *options, int flags,
                     Error **errp)
 {
@@ -145,15 +187,14 @@ static int dmg_open(BlockDriverState *bs, QDict *options, int flags,
     s->n_chunks = 0;
     s->offsets = s->lengths = s->sectors = s->sectorcounts = NULL;
 
-    /* read offset of info blocks */
-    offset = bdrv_getlength(bs->file);
+    /* locate the UDIF trailer */
+    offset = dmg_find_koly_offset(bs->file, errp);
     if (offset < 0) {
         ret = offset;
         goto fail;
     }
-    offset -= 0x1d8;
 
-    ret = read_uint64(bs, offset, &info_begin);
+    ret = read_uint64(bs, offset + 0x28, &info_begin);
     if (ret < 0) {
         goto fail;
     } else if (info_begin == 0) {