gh-104282: Fix null pointer dereference in lzma._decode_filter_properties (GH-104283)

Author: chgnrdv

Lib/test/test_lzma.py

@@ -1401,6 +1401,14 @@ def test__decode_filter_properties(self):
         self.assertEqual(filterspec["lc"], 3)
         self.assertEqual(filterspec["dict_size"], 8 << 20)
 
+        # see gh-104282
+        filters = [lzma.FILTER_X86, lzma.FILTER_POWERPC,
+                   lzma.FILTER_IA64, lzma.FILTER_ARM,
+                   lzma.FILTER_ARMTHUMB, lzma.FILTER_SPARC]
+        for f in filters:
+            filterspec = lzma._decode_filter_properties(f, b"")
+            self.assertEqual(filterspec, {"id": f})
+
     def test_filter_properties_roundtrip(self):
         spec1 = lzma._decode_filter_properties(
                 lzma.FILTER_LZMA1, b"]\x00\x00\x80\x00")

Misc/NEWS.d/next/Library/2023-05-08-09-30-00.gh-issue-104282.h4c6Eb.rst

@@ -0,0 +1,3 @@
+Fix null pointer dereference in :func:`lzma._decode_filter_properties`
+due to improper handling of BCJ filters with properties of zero length.
+Patch by Radislav Chugunov.

Modules/_lzmamodule.c

@@ -492,7 +492,9 @@ build_filter_spec(const lzma_filter *f)
         case LZMA_FILTER_ARMTHUMB:
         case LZMA_FILTER_SPARC: {
             lzma_options_bcj *options = f->options;
-            ADD_FIELD(options, start_offset);
+            if (options) {
+                ADD_FIELD(options, start_offset);
+            }
             break;
         }
         default: