5 files changed, 100 insertions, 17 deletions
diff --git a/configure.ac b/configure.ac
index ac059bf862..d39ad13677 100644
--- a/configure.ac
+++ b/configure.ac
@@ -360,7 +360,7 @@ test -z "$warnflags" ||
 AS_IF([test -z "${CFLAGS+set}"], [
     cflags=`echo " $cflags " | sed "$cflagspat;s/^ *//;s/ *$//"`
     orig_cflags="$cflags"
-    cflags="$cflags "'${optflags} ${debugflags} ${warnflags}'
+    cflags='${hardenflags} '"$cflags "'${optflags} ${debugflags} ${warnflags}'
 ])
 dnl AS_IF([test -z "${CXXFLAGS+set}"], [
 dnl     cxxflags=`echo " $cxxflags " | sed "$cflagspat;s/^ *//;s/ *$//"`
@@ -813,7 +813,7 @@ AS_IF([test "$GCC" = yes], [
 		  [fortify_source=$enableval])
     AS_IF([test "x$fortify_source" != xno], [
         RUBY_TRY_CFLAGS([$optflags -D_FORTIFY_SOURCE=2],
-                        [RUBY_APPEND_OPTION(XCFLAGS, -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2)], [],
+                        [RUBY_PREPEND_OPTION(hardenflags, -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2)], [],
                         [@%:@include <stdio.h>])
     ])
 
@@ -834,20 +834,24 @@ AS_IF([test "$GCC" = yes], [
     AC_MSG_CHECKING([for -fstack-protector])
     AC_MSG_RESULT(["$stack_protector"])
     AS_CASE(["$stack_protector"], [-*], [
-	RUBY_APPEND_OPTION(XCFLAGS, $stack_protector)
-	RUBY_APPEND_OPTION(XLDFLAGS, $stack_protector)
-	RUBY_APPEND_OPTION(LDFLAGS, $stack_protector)
+        RUBY_PREPEND_OPTION(hardenflags, $stack_protector)
+        RUBY_APPEND_OPTION(XLDFLAGS, $stack_protector)
+        RUBY_APPEND_OPTION(LDFLAGS, $stack_protector)
     ])
 
     # aarch64 branch protection
     AS_CASE(["$target_cpu"], [aarch64], [
 	AS_FOR(option, opt, [-mbranch-protection=pac-ret -msign-return-address=all], [
-            RUBY_TRY_CFLAGS(option, [branch_protection=yes], [branch_protection=no])
+            # Try these flags in the _prepended_ position - i.e. we want to try building a program
+            # with CFLAGS="-mbranch-protection=pac-ret $CFLAGS". If the builder has provided different
+            # branch protection flags in CFLAGS, we don't want to overwrite those. We just want to
+            # find some branch protection flags which work if none were provided.
+            RUBY_TRY_CFLAGS_PREPEND(option, [branch_protection=yes], [branch_protection=no])
             AS_IF([test "x$branch_protection" = xyes], [
-                # C compiler and assembler must be consistent for -mbranch-protection
-                # since they both check `__ARM_FEATURE_PAC_DEFAULT` definition.
-                RUBY_APPEND_OPTION(XCFLAGS, option)
-                RUBY_APPEND_OPTION(ASFLAGS, option)
+                # _prepend_ the options to CFLAGS, so that user-provided flags will overwrite them.
+                # These CFLAGS are used during the configure script to compile further test programs;
+                # however, $harden_flags is prepended separately to CFLAGS at the end of the script.
+                RUBY_PREPEND_OPTION(hardenflags, $opt)
                 break
             ])
         ])
@@ -995,6 +999,59 @@ AC_SUBST(incflags, "$INCFLAGS")
 test -z "${ac_env_CFLAGS_set}" -a -n "${cflags+set}" && eval CFLAGS="\"$cflags $ARCH_FLAG\""
 test -z "${ac_env_CXXFLAGS_set}" -a -n "${cxxflags+set}" && eval CXXFLAGS="\"$cxxflags $ARCH_FLAG\""
 
+# The lines above expand out the $cflags/$optflags/$debugflags/$hardenflags variables into the
+# CFLAGS variable. So, at this point, we have a $CFLAGS var with the actual compiler flags we're
+# going to use.
+# That means this is the right time to check what branch protection flags are going to be in use
+# and define appropriate macros for use in Context.S based on this
+AS_CASE(["$target_cpu"], [aarch64], [
+    AC_CACHE_CHECK([whether __ARM_FEATURE_BTI_DEFAULT is defined],
+        rb_cv_aarch64_bti_enabled,
+        AC_COMPILE_IFELSE(
+            [AC_LANG_PROGRAM([[
+                @%:@ifndef __ARM_FEATURE_BTI_DEFAULT
+                @%:@error "__ARM_FEATURE_BTI_DEFAULT not defined"
+                @%:@endif
+            ]])],
+        [rb_cv_aarch64_bti_enabled=yes],
+        [rb_cv_aarch64_bti_enabled=no])
+    )
+    AS_IF([test "$rb_cv_aarch64_bti_enabled" = yes],
+          AC_DEFINE(RUBY_AARCH64_BTI_ENABLED, 1))
+    AC_CACHE_CHECK([whether __ARM_FEATURE_PAC_DEFAULT is defined],
+        rb_cv_aarch64_pac_enabled,
+        AC_COMPILE_IFELSE(
+            [AC_LANG_PROGRAM([[
+                @%:@ifndef __ARM_FEATURE_PAC_DEFAULT
+                @%:@error "__ARM_FEATURE_PAC_DEFAULT not defined"
+                @%:@endif
+            ]])],
+        [rb_cv_aarch64_pac_enabled=yes],
+        [rb_cv_aarch64_pac_enabled=no])
+    )
+    AS_IF([test "$rb_cv_aarch64_pac_enabled" = yes],
+          AC_DEFINE(RUBY_AARCH64_PAC_ENABLED, 1))
+    # Context.S will only ever sign its return address with the A-key; it doesn't support
+    # the B-key at the moment.
+    AS_IF([test "$rb_cv_aarch64_pac_enabled" = yes], [
+        AC_CACHE_CHECK([whether __ARM_FEATURE_PAC_DEFAULT specifies the b-key bit 0x02],
+            rb_cv_aarch64_pac_b_key,
+            AC_COMPILE_IFELSE(
+                [AC_LANG_PROGRAM([[
+                    @%:@ifdef __ARM_FEATURE_PAC_DEFAULT
+                    @%:@if __ARM_FEATURE_PAC_DEFAULT & 0x02
+                    @%:@error "__ARM_FEATURE_PAC_DEFAULT specifies B key"
+                    @%:@endif
+                    @%:@endif
+                ]])],
+            [rb_cv_aarch64_pac_b_key=no],
+            [rb_cv_aarch64_pac_b_key=yes])
+        )
+        AS_IF([test "$rb_cv_aarch64_pac_b_key" = yes],
+            AC_MSG_ERROR(-mbranch-protection flag specified b-key but Ruby's Context.S does not support this yet.))
+    ])
+])
+
 AC_CACHE_CHECK([whether compiler has statement and declarations in expressions],
   rb_cv_have_stmt_and_decl_in_expr,
   [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]],[[ __extension__ ({ int a = 0; a; }); ]])],
@@ -4226,12 +4283,13 @@ AS_IF([test "${ARCH_FLAG}"], [
 rb_cv_warnflags=`echo "$rb_cv_warnflags" | sed 's/^ *//;s/ *$//'`
 warnflags="$rb_cv_warnflags"
 AC_SUBST(cppflags)dnl
-AC_SUBST(cflags, ["${orig_cflags:+$orig_cflags }"'${optflags} ${debugflags} ${warnflags}'])dnl
+AC_SUBST(cflags, ['${hardenflags} '"${orig_cflags:+$orig_cflags }"' ${optflags} ${debugflags} ${warnflags}'])dnl
 AC_SUBST(cxxflags)dnl
 AC_SUBST(optflags)dnl
 AC_SUBST(debugflags)dnl
 AC_SUBST(warnflags)dnl
 AC_SUBST(strict_warnflags)dnl
+AC_SUBST(hardenflags)dnl
 AC_SUBST(XCFLAGS)dnl
 AC_SUBST(XLDFLAGS)dnl
 AC_SUBST(EXTLDFLAGS)dnl
@@ -4691,6 +4749,7 @@ config_summary "DLDFLAGS"            "$DLDFLAGS"
 config_summary "optflags"            "$optflags"
 config_summary "debugflags"          "$debugflags"
 config_summary "warnflags"           "$warnflags"
+config_summary "hardenflags"         "$hardenflags"
 config_summary "strip command"       "$STRIP"
 config_summary "install doc"         "$DOCTARGETS"
 config_summary "YJIT support"        "$YJIT_SUPPORT"
diff --git a/coroutine/arm64/Context.S b/coroutine/arm64/Context.S
index 5251ab214d..54611a247e 100644
--- a/coroutine/arm64/Context.S
+++ b/coroutine/arm64/Context.S
@@ -5,6 +5,8 @@
 ##  Copyright, 2018, by Samuel Williams.
 ##
 
+#include "ruby/config.h"
+
 #define TOKEN_PASTE(x,y) x##y
 #define PREFIXED_SYMBOL(prefix,name) TOKEN_PASTE(prefix,name)
 
@@ -27,10 +29,10 @@
 .global PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer)
 PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
 
-#if defined(__ARM_FEATURE_PAC_DEFAULT) && (__ARM_FEATURE_PAC_DEFAULT != 0)
+#if defined(RUBY_AARCH64_PAC_ENABLED)
 	# paciasp (it also acts as BTI landing pad, so no need to insert BTI also)
 	hint #25
-#elif defined(__ARM_FEATURE_BTI_DEFAULT) && (__ARM_FEATURE_BTI_DEFAULT != 0)
+#elif defined(RUBY_AARCH64_BTI_ENABLED)
 	# For the case PAC is not enabled but BTI is.
 	# bti c
 	hint #34
@@ -73,7 +75,7 @@ PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
 	# Pop stack frame
 	add sp, sp, 0xa0
 
-#if defined(__ARM_FEATURE_PAC_DEFAULT) && (__ARM_FEATURE_PAC_DEFAULT != 0)
+#if defined(RUBY_AARCH64_PAC_ENABLED)
 	# autiasp: Authenticate x30 (LR) with SP and key A
 	hint #29
 #endif
@@ -85,18 +87,18 @@ PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer):
 .section .note.GNU-stack,"",%progbits
 #endif
 
-#if __ARM_FEATURE_BTI_DEFAULT != 0 || __ARM_FEATURE_PAC_DEFAULT != 0
+#if defined(RUBY_AARCH64_BTI_ENABLED) || defined(RUBY_AARCH64_PAC_ENABLED)
 /*  See "ELF for the Arm 64-bit Architecture (AArch64)"
     https://github.com/ARM-software/abi-aa/blob/2023Q3/aaelf64/aaelf64.rst#program-property */
 #  define GNU_PROPERTY_AARCH64_FEATURE_1_BTI (1<<0)
 #  define GNU_PROPERTY_AARCH64_FEATURE_1_PAC (1<<1)
 
-#  if __ARM_FEATURE_BTI_DEFAULT != 0
+#  if defined(RUBY_AARCH64_BTI_ENABLED)
 #    define BTI_FLAG GNU_PROPERTY_AARCH64_FEATURE_1_BTI
 #  else
 #    define BTI_FLAG 0
 #  endif
-#  if __ARM_FEATURE_PAC_DEFAULT != 0
+#  if defined(RUBY_AARCH64_PAC_ENABLED)
 #    define PAC_FLAG GNU_PROPERTY_AARCH64_FEATURE_1_PAC
 #  else
 #    define PAC_FLAG 0
diff --git a/template/Makefile.in b/template/Makefile.in
index 033ac56cb3..abb4469777 100644
--- a/template/Makefile.in
+++ b/template/Makefile.in
@@ -89,6 +89,7 @@ cflags = @cflags@
 optflags = @optflags@
 debugflags = @debugflags@
 warnflags = @warnflags@ @strict_warnflags@
+hardenflags = @hardenflags@
 cppflags = @cppflags@
 incflags = @incflags@
 RUBY_DEVEL = @RUBY_DEVEL@ # "yes" or empty
diff --git a/tool/m4/ruby_append_option.m4 b/tool/m4/ruby_append_option.m4
index ff828d2162..98359fa1f9 100644
--- a/tool/m4/ruby_append_option.m4
+++ b/tool/m4/ruby_append_option.m4
@@ -3,3 +3,7 @@ AC_DEFUN([RUBY_APPEND_OPTION],
 	[# RUBY_APPEND_OPTION($1)
 	AS_CASE([" [$]{$1-} "],
 	[*" $2 "*], [], ['  '], [ $1="$2"], [ $1="[$]$1 $2"])])dnl
+AC_DEFUN([RUBY_PREPEND_OPTION],
+	[# RUBY_APPEND_OPTION($1)
+	AS_CASE([" [$]{$1-} "],
+	[*" $2 "*], [], ['  '], [ $1="$2"], [ $1="$2 [$]$1"])])dnl
diff --git a/tool/m4/ruby_try_cflags.m4 b/tool/m4/ruby_try_cflags.m4
index b74718fe5e..b397642aad 100644
--- a/tool/m4/ruby_try_cflags.m4
+++ b/tool/m4/ruby_try_cflags.m4
@@ -22,3 +22,20 @@ AC_DEFUN([RUBY_TRY_CFLAGS], [
 	AC_MSG_RESULT(no)],
 	[$4], [$5])
 ])dnl
+
+AC_DEFUN([_RUBY_TRY_CFLAGS_PREPEND], [
+    RUBY_WERROR_FLAG([
+    CFLAGS="$1 [$]CFLAGS"
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[$4]], [[$5]])],
+	[$2], [$3])
+    ])dnl
+])dnl
+AC_DEFUN([RUBY_TRY_CFLAGS_PREPEND], [
+    AC_MSG_CHECKING([whether ]$1[ is accepted as CFLAGS])dnl
+    _RUBY_TRY_CFLAGS_PREPEND([$1],
+	[$2
+	AC_MSG_RESULT(yes)],
+	[$3
+	AC_MSG_RESULT(no)],
+	[$4], [$5])
+])dnl