inflate/pack_exe: Document and check the assumption that packed_entry…

… is 24 bytes
keirf · Nov 30, 2023 · b09a395 · b09a395
1 parent 9477599
commit b09a395
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Showing 2 changed files with 6 additions and 2 deletions.
diff --git a/inflate/depacker_main.S b/inflate/depacker_main.S
@@ -46,7 +46,7 @@ unpack_next_seg:
         lsr.l   #2,d0
         cmp.l   (sp),a3         /* First segment?... */
         jne     1f
-        subq.l  #6,d0           /* ...Then adjust for depacker_entry */
+        subq.l  #6,d0           /* ...Then clip depacker_entry (24 bytes) */
 1:      move.l  -(a0),-(a5)
         subq.l  #1,d0
         jne     1b

diff --git a/inflate/pack_exe.py b/inflate/pack_exe.py
@@ -154,7 +154,11 @@ def process_hunk(f, i):
                 if i == 0:
                     # First hunk must be code: we inject our entry/exit code
                     assert id == HUNK_CODE
-                    packed = get_code('depacker_entry') + packed
+                    # Prepend the depacker entry code. This must be exactly
+                    # 24 bytes in size, as this is assumed by depacker_main.
+                    depacker_entry = get_code('depacker_entry')
+                    assert len(depacker_entry) == 24
+                    packed = depacker_entry + packed
                     # Allocate explicit extra space for the final exit code
                     # This must always extend the allocation as these bytes
                     # will not be zeroed before we jump to the original exe.