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.