autotools: Disable eager MSan in ctime_tests

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

build-aux/m4/bitcoin_secp.m4

@@ -45,6 +45,18 @@ fi
 AC_MSG_RESULT($has_valgrind)
 ])
 
+AC_DEFUN([SECP_MSAN_CHECK], [
+AC_MSG_CHECKING(whether MemorySanitizer is enabled)
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+  #if defined(__has_feature)
+  #  if __has_feature(memory_sanitizer)
+  #    error "MemorySanitizer is enabled."
+  #  endif
+  #endif
+  ]])], [msan_enabled=no], [msan_enabled=yes])
+AC_MSG_RESULT([$msan_enabled])
+])
+
 dnl SECP_TRY_APPEND_CFLAGS(flags, VAR)
 dnl Append flags to VAR if CC accepts them.
 AC_DEFUN([SECP_TRY_APPEND_CFLAGS], [

configure.ac

@@ -247,6 +247,20 @@ if test x"$enable_ctime_tests" = x"auto"; then
     enable_ctime_tests=$enable_valgrind
 fi
 
+print_msan_notice=no
+if test x"$enable_ctime_tests" = x"yes" && test x"$GCC" = x"yes"; then
+  SECP_MSAN_CHECK
+  # MSan on Clang >=16 reports unitialized memory in function parameters and return values, even if
+  # the uninitalized variable is never actually "used". This is called "eager" checking, and it's
+  # sounds like good idea for normal use of MSan. However, it yields many false positives in the
+  # ctime_tests because many return values depend on secret (i.e., "uninitialized") values, and
+  # we're only interested in detecting branches (which count as "uses") on secret data.
+  if test x"$msan_enabled" = x"yes"; then
+    SECP_TRY_APPEND_CFLAGS([-fno-sanitize-memory-param-retval], SECP_CFLAGS)
+    print_msan_notice=yes
+  fi
+fi
+
 if test x"$enable_coverage" = x"yes"; then
     SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DCOVERAGE=1"
     SECP_CFLAGS="-O0 --coverage $SECP_CFLAGS"
@@ -492,3 +506,10 @@ echo "  CPPFLAGS                = $CPPFLAGS"
 echo "  SECP_CFLAGS             = $SECP_CFLAGS"
 echo "  CFLAGS                  = $CFLAGS"
 echo "  LDFLAGS                 = $LDFLAGS"
+
+if test x"$print_msan_notice" = x"yes"; then
+  echo
+  echo "Note:"
+  echo "  MemorySanitizer detected, tried to add -fno-sanitize-memory-param-retval to SECP_CFLAGS"
+  echo "  to avoid false positives in ctime_tests. Pass --disable-ctime-tests to avoid this."
+fi