lkdtm: cfi: Make PAC test work with GCC 7 and 8

The CFI test uses the branch-protection=none compiler attribute to
disable PAC return address protection on a function. While newer GCC
versions support this attribute, older versions (GCC 7 and 8) instead
supported the sign-return-address=none attribute, leading to a build
failure when the test is built with older compilers. Fix it by checking
which attribute is supported and using the correct one.

Fixes: 2e53b877 ("lkdtm: Add CFI_BACKWARD to test ROP mitigations")
Reported-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/CAEUSe78kDPxQmQqCWW-_9LCgJDFhAeMoVBFnX9QLx18Z4uT4VQ@mail.gmail.com/

drivers/misc/lkdtm/cfi.c

@@ -54,7 +54,11 @@ static void lkdtm_CFI_FORWARD_PROTO(void)
 # ifdef CONFIG_ARM64_BTI_KERNEL
 #  define __no_pac             "branch-protection=bti"
 # else
-#  define __no_pac             "branch-protection=none"
+#  ifdef CONFIG_CC_HAS_BRANCH_PROT_PAC_RET
+#   define __no_pac            "branch-protection=none"
+#  else
+#   define __no_pac            "sign-return-address=none"
+#  endif
 # endif
 # define __no_ret_protection   __noscs __attribute__((__target__(__no_pac)))
 #else