avoid fuzzer false positive

strukturag · Mar 2, 2025 · dc49550 · dc49550
1 parent 9daa4b9
commit dc49550
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Showing 2 changed files with 17 additions and 6 deletions.
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -584,6 +584,7 @@ if (WITH_FUZZERS)
     separate_arguments(FUZZING_COMPILE_OPTIONS UNIX_COMMAND "${FUZZING_COMPILE_OPTIONS}")
     separate_arguments(FUZZING_LINKER UNIX_COMMAND "${FUZZING_LINKER_OPTIONS}")
     add_compile_options(${FUZZING_COMPILE_OPTIONS})
+    add_definitions(-DAVOID_FUZZER_FALSE_POSITIVE=1)
     add_link_options(${FUZZING_COMPILE_OPTIONS} ${FUZZING_LINKER_OPTIONS})
 
     add_subdirectory(fuzzing)

diff --git a/libheif/bitstream.cc b/libheif/bitstream.cc
@@ -32,8 +32,6 @@
 
 #define MAX_UVLC_LEADING_ZEROS 20
 
-#define AVOID_FUZZER_FALSE_POSITIVE 0
-
 
 StreamReader_istream::StreamReader_istream(std::unique_ptr<std::istream>&& istr)
     : m_istr(std::move(istr))
@@ -512,7 +510,10 @@ uint32_t BitReader::get_bits(int n)
   uint64_t val = nextbits;
   val >>= 64 - n;
 
-  if (AVOID_FUZZER_FALSE_POSITIVE) nextbits &= (0xffffffffffffffffULL >> n);
+#if AVOID_FUZZER_FALSE_POSITIVE
+  // Shifting an unsigned integer left such that some MSBs fall out is well defined in C++ despite the fuzzer claiming otherwise.
+  nextbits &= (0xffffffffffffffffULL >> n);
+#endif
 
   nextbits <<= n;
   nextbits_cnt -= n;
@@ -600,14 +601,20 @@ void BitReader::skip_bits(int n)
     refill();
   }
 
-  if (AVOID_FUZZER_FALSE_POSITIVE) nextbits &= (0xffffffffffffffffULL >> n);
+#if AVOID_FUZZER_FALSE_POSITIVE
+  nextbits &= (0xffffffffffffffffULL >> n);
+#endif
+
   nextbits <<= n;
   nextbits_cnt -= n;
 }
 
 void BitReader::skip_bits_fast(int n)
 {
-  if (AVOID_FUZZER_FALSE_POSITIVE) nextbits &= (0xffffffffffffffffULL >> n);
+#if AVOID_FUZZER_FALSE_POSITIVE
+  nextbits &= (0xffffffffffffffffULL >> n);
+#endif
+
   nextbits <<= n;
   nextbits_cnt -= n;
 }
@@ -616,7 +623,10 @@ void BitReader::skip_to_byte_boundary()
 {
   int nskip = (nextbits_cnt & 7);
 
-  if (AVOID_FUZZER_FALSE_POSITIVE) nextbits &= (0xffffffffffffffffULL >> nskip);
+#if AVOID_FUZZER_FALSE_POSITIVE
+  nextbits &= (0xffffffffffffffffULL >> nskip);
+#endif
+
   nextbits <<= nskip;
   nextbits_cnt -= nskip;
 }