[PATCH 2/2] DONOTMERGE: arm64: module: Test module for BTI veneers

[Ard Biesheuvel]

This module has a tiny .text section with a static function, which is
called by the module init hook, which is placed in a ~128M init code
section, which is therefore guaranteed to be place out of direct
branching range when CONFIG_RANDOMIZE_MODULE_REGION_FULL is disabled.

When CONFIG_ARM64_BTI_KERNEL is enabled, this forces the use of a BTI
veneer, as recent GCCs and Clangs will elide BTI landing pads for static
functions that never have their address taken, even in the presence of
cross-section calls to those functions.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/kernel/Makefile          |  1 +
 arch/arm64/kernel/bti_veneer_test.c | 26 ++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index d2690c3ec528..b74e2dbd2d5f 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -72,6 +72,7 @@ obj-$(CONFIG_ARM64_MPAM)		+= mpam.o
 obj-$(CONFIG_ARM64_MTE)			+= mte.o
 obj-y					+= vdso-wrap.o
 obj-$(CONFIG_COMPAT_VDSO)		+= vdso32-wrap.o
+obj-m					+= bti_veneer_test.o
 
 # Force dependency (vdso*-wrap.S includes vdso.so through incbin)
 $(obj)/vdso-wrap.o: $(obj)/vdso/vdso.so
diff --git a/arch/arm64/kernel/bti_veneer_test.c b/arch/arm64/kernel/bti_veneer_test.c
new file mode 100644
index 000000000000..62c20e960d8d
--- /dev/null
+++ b/arch/arm64/kernel/bti_veneer_test.c
@@ -0,0 +1,26 @@
+
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/types.h>
+
+static int noinline static_noninit_function(void)
+{
+	static volatile int ret;
+	return ret;
+}
+
+static int __init bti_veneer_test_init(void)
+{
+	return static_noninit_function();
+}
+module_init(bti_veneer_test_init);
+
+static void bti_veneer_test_exit(void)
+{
+}
+module_exit(bti_veneer_test_exit);
+
+/* Make the init code region too big to fit in 128M 'near' module region */
+asm(".section .init.padding, \"ax\", %progbits; .space (128 << 20) - 64; .previous");
+
+MODULE_LICENSE("GPL");
-- 
2.47.3