ffu_extract: pick store by blkSize sanity

tools/ffu_extract.py

@@ -24,11 +24,20 @@ def load(ffu):
     sec = f.read(32)
     _, _, chunk_kb, _, cat, hsh = struct.unpack("<I12sIIII", sec)
     assert chunk_kb * 1024 == CHUNK
-    # store header: ASCII PlatformId inside it; header starts 12B before
+    # store header: ASCII PlatformId inside it; header starts 12B before.
-    st = f.find(b"Microsoft.MSM") - 12
+    # (manifest has one too — pick the occurrence with sane blkSize)
-    assert st > 0
+    st = -1
-    vals = struct.unpack("<11I", f[st + 204:st + 248])
+    i = 0
-    assert vals[0] == CHUNK, vals  # blkSize sanity
+    while True:
+        i = f.find(b"Microsoft.MSM", i)
+        if i < 0:
+            break
+        vals = struct.unpack("<11I", f[i - 12 + 204:i - 12 + 248])
+        if vals[0] == CHUNK:
+            st = i - 12
+            break
+        i += 1
+    assert st > 0, "store header not found"
     wcnt = vals[1]
     pos = st + 248
     disk = {}   # diskchunk -> datachunk