[PATCH 23/23] build: initialize stack variables to zero by default

Darrick J. Wong
2025-01-16T23:31:10Z

Newer versions of gcc and clang can include the ability to zero stack
variables by default. Let's enable it so that we (a) reduce the risk of
writing stack contents to disk somewhere and (b) try to reduce
unpredictable program behavior based on random stack contents. The
kernel added this 6 years ago, so I think it's mature enough for
fstests.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
---
configure.ac | 1 +
include/builddefs.in | 3 ++-
m4/package_libcdev.m4 | 14 ++++++++++++++
3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index c81411e735a90d..f3c8c643f0eb9e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -72,6 +72,7 @@ AC_HAVE_NFTW
AC_HAVE_RLIMIT_NOFILE
AC_NEED_INTERNAL_XFS_IOC_EXCHANGE_RANGE
AC_HAVE_FICLONE
+AC_HAVE_TRIVIAL_AUTO_VAR_INIT

AC_CHECK_FUNCS([renameat2])
AC_CHECK_FUNCS([reallocarray])
diff --git a/include/builddefs.in b/include/builddefs.in
index 7274cde8d0814c..5b5864278682a4 100644
--- a/include/builddefs.in
+++ b/include/builddefs.in
@@ -76,6 +76,7 @@ NEED_INTERNAL_XFS_IOC_EXCHANGE_RANGE = @need_internal_xfs_ioc_exchange_range@
HAVE_FICLONE = @have_ficlone@

GCCFLAGS = -funsigned-char -fno-strict-aliasing -Wall
+SANITIZER_CFLAGS += @autovar_init_cflags@

ifeq ($(PKG_PLATFORM),linux)
PCFLAGS = -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 $(GCCFLAGS)
@@ -90,7 +91,7 @@ GCFLAGS = $(OPTIMIZER) $(DEBUG) $(CPPFLAGS) \
-I$(TOPDIR)/include -DVERSION=\"$(PKG_VERSION)\"

# Global, Platform, Local CFLAGS
-CFLAGS += $(GCFLAGS) $(PCFLAGS) $(LCFLAGS)
+CFLAGS += $(GCFLAGS) $(PCFLAGS) $(LCFLAGS) $(SANITIZER_CFLAGS)

include $(TOPDIR)/include/buildmacros

diff --git a/m4/package_libcdev.m4 b/m4/package_libcdev.m4
index a0d50f4d9b68e4..ed8fe6e32ae00a 100644
--- a/m4/package_libcdev.m4
+++ b/m4/package_libcdev.m4
@@ -72,3 +72,17 @@ AC_DEFUN([AC_HAVE_FICLONE],
AC_MSG_RESULT(yes)],[AC_MSG_RESULT(no)])
AC_SUBST(have_ficlone)
])
+
+# Check if we have -ftrivial-auto-var-init=zero
+AC_DEFUN([AC_HAVE_TRIVIAL_AUTO_VAR_INIT],
+ [ AC_MSG_CHECKING([if C compiler supports zeroing automatic vars])
+ OLD_CFLAGS="$CFLAGS"
+ TEST_CFLAGS="-ftrivial-auto-var-init=zero"
+ CFLAGS="$CFLAGS $TEST_CFLAGS"
+ AC_LINK_IFELSE([AC_LANG_PROGRAM([])],
+ [AC_MSG_RESULT([yes])]
+ [autovar_init_cflags=$TEST_CFLAGS],
+ [AC_MSG_RESULT([no])])
+ CFLAGS="${OLD_CFLAGS}"
+ AC_SUBST(autovar_init_cflags)
+ ])