Merge bitcoin-core/secp256k1#1517: autotools: Disable eager MSan in ctime_tests

ebfb82ee2f ci: Add job with -fsanitize-memory-param-retval (Tim Ruffing)
e1bef0961c configure: Move "experimental" warning to bottom (Tim Ruffing)
55e5d975db autotools: Disable eager MSan in ctime_tests (Tim Ruffing)

Pull request description:

  This is the autotools solution for #1516.

  Alternatively, we could have a full-blown `--enable-msan` option, but it's more work, and I'm not convinced that it's necessary or at least much better.

  hebasto If you're Concept ACK, are you willing to work on an equivalent PR for CMake?

ACKs for top commit:
  hebasto:
    ACK ebfb82ee2f, tested on Ubuntu 24.04 with different clang versions (from 15 to 18) and different build configurations. CI changes look OK as well.

Tree-SHA512: c083d778fd50bd35c2e29b7fe0d92b98d912ee5ac7809ae73067d050a0d3c42b3483260f1286d0023cdb802a3c3006bf932ecf60ce81b942de1c9824374c0132

.github/workflows/ci.yml

@@ -485,18 +485,24 @@ jobs:
       matrix:
         configuration:
           - env_vars:
+              CTIMETESTS: 'yes'
               CFLAGS: '-fsanitize=memory -fsanitize-recover=memory -g'
           - env_vars:
               ECMULTGENKB: 2
               ECMULTWINDOW: 2
+              CTIMETESTS: 'yes'
               CFLAGS: '-fsanitize=memory -fsanitize-recover=memory -g -O3'
+          - env_vars:
+              # -fsanitize-memory-param-retval is clang's default, but our build system disables it
+              # when ctime_tests when enabled.
+              CFLAGS: '-fsanitize=memory -fsanitize-recover=memory -fsanitize-memory-param-retval -g'
+              CTIMETESTS: 'no'
 
     env:
       ECDH: 'yes'
       RECOVERY: 'yes'
       SCHNORRSIG: 'yes'
       ELLSWIFT: 'yes'
-      CTIMETESTS: 'yes'
       CC: 'clang'
       SECP256K1_TEST_ITERS: 32
       ASM: 'no'

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"
@@ -426,12 +440,7 @@ fi
 ### Check for --enable-experimental if necessary
 ###
 
-if test x"$enable_experimental" = x"yes"; then
-  AC_MSG_NOTICE([******])
-  AC_MSG_NOTICE([WARNING: experimental build])
-  AC_MSG_NOTICE([Experimental features do not have stable APIs or properties, and may not be safe for production use.])
-  AC_MSG_NOTICE([******])
-else
+if test x"$enable_experimental" = x"no"; then
   if test x"$set_asm" = x"arm32"; then
     AC_MSG_ERROR([ARM32 assembly is experimental. Use --enable-experimental to allow.])
   fi
@@ -492,3 +501,17 @@ 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
+
+if test x"$enable_experimental" = x"yes"; then
+  echo
+  echo "WARNING: Experimental build"
+  echo "  Experimental features do not have stable APIs or properties, and may not be safe for"
+  echo "  production use."
+fi