Merge #18413: script: prevent UB when computing abs value for num opcode serialize

2748e8793267126c5b40621d75d1930e358f057e script: prevent UB when computing abs value for num opcode serialize (pierrenn)

Pull request description:

  This was reported by practicalswift here #18046

  It seems that the original author of the line used a reference to glibc `abs`: https://github.com/lattera/glibc/blob/master/stdlib/abs.c

  However depending on some implementation details this can be undefined behavior for unusual values.

  A detailed explanation of the UB is provided here : https://stackoverflow.com/questions/17313579/is-there-a-safe-way-to-get-the-unsigned-absolute-value-of-a-signed-integer-with (by [Billy O'Neal](https://twitter.com/malwareminigun))

  Simple relevant godbolt example :  https://godbolt.org/z/yRwtCG

  Thanks!

ACKs for top commit:
  sipa:
    ACK 2748e8793267126c5b40621d75d1930e358f057e
  MarcoFalke:
    ACK 2748e8793267126c5b40621d75d1930e358f057e, only checked that the bitcoind binary does not change with clang -O2 🎓
  practicalswift:
    ACK 2748e8793267126c5b40621d75d1930e358f057e

Tree-SHA512: 539a34c636c2674c66cb6e707d9d0dfdce63f59b5525610ed88da10c9a8d59d81466b111ad63b850660cef3750d732fc7755530c81a2d61f396be0707cd86dec

2 files changed, 2 insertions, 10 deletions

diff --git a/src/test/fuzz/integer.cpp b/src/test/fuzz/integer.cpp
index d5b5ec3c9a..35d6804d4f 100644
--- a/src/test/fuzz/integer.cpp
+++ b/src/test/fuzz/integer.cpp
@@ -148,11 +148,7 @@ void test_one_input(const std::vector<uint8_t>& buffer)
 
     const CScriptNum script_num{i64};
     (void)script_num.getint();
-    // Avoid negation failure:
-    // script/script.h:332:35: runtime error: negation of -9223372036854775808 cannot be represented in type 'int64_t' (aka 'long'); cast to an unsigned type to negate this value to itself
-    if (script_num != CScriptNum{std::numeric_limits<int64_t>::min()}) {
-        (void)script_num.getvch();
-    }
+    (void)script_num.getvch();
 
     const arith_uint256 au256 = UintToArith256(u256);
     assert(ArithToUint256(au256) == u256);
diff --git a/src/test/fuzz/scriptnum_ops.cpp b/src/test/fuzz/scriptnum_ops.cpp
index 42b1432f13..f4e079fb89 100644
--- a/src/test/fuzz/scriptnum_ops.cpp
+++ b/src/test/fuzz/scriptnum_ops.cpp
@@ -129,10 +129,6 @@ void test_one_input(const std::vector<uint8_t>& buffer)
             break;
         }
         (void)script_num.getint();
-        // Avoid negation failure:
-        // script/script.h:332:35: runtime error: negation of -9223372036854775808 cannot be represented in type 'int64_t' (aka 'long'); cast to an unsigned type to negate this value to itself
-        if (script_num != CScriptNum{std::numeric_limits<int64_t>::min()}) {
-            (void)script_num.getvch();
-        }
+        (void)script_num.getvch();
     }
 }