[PATCH] [RFC/RFT] AARCH64: Add gcc Shadow Call Stack support

[Dan Li <ashimida@linux.alibaba.com>]

I tried to submit a patch[1] to add compiler's SCS support on gcc-11.1.0.

Kernel can enable SCS under gcc based on this patch, commands as follows:

make ARCH=arm64 defconfig
./scripts/config -e CONFIG_SHADOW_CALL_STACK
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-

[1] https://gcc.gnu.org/pipermail/gcc-patches/2021-November/583062.html

Signed-off-by: Dan Li <ashimida@linux.alibaba.com>

---
This function can be used to test whether the shadow stack is effective:
//noinline void __noscs scs_test(void)
noinline void scs_test(void)
{
            register unsigned long *sp asm("sp");
                unsigned long * lr = sp + 1;

                    asm volatile("":::"x30");
                        *lr = 0;
}

ffff800010012710 <scs_test>:
ffff800010012710:       d503245f        bti     c
ffff800010012714:       d503233f        paciasp
ffff800010012718:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
ffff80001001271c:       910003fd        mov     x29, sp
ffff800010012720:       910003e0        mov     x0, sp
ffff800010012724:       f900041f        str     xzr, [x0, #8]
ffff800010012728:       a8c17bfd        ldp     x29, x30, [sp], #16
ffff80001001272c:       d50323bf        autiasp
ffff800010012730:       d65f03c0        ret

 arch/Kconfig                 | 6 +++---
 arch/arm64/Kconfig           | 2 +-
 include/linux/compiler-gcc.h | 6 ++++++
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 98db63496bab..35b27be0d7ee 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -593,11 +593,11 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
 	  switching.
 
 config SHADOW_CALL_STACK
-	bool "Clang Shadow Call Stack"
-	depends on CC_IS_CLANG && ARCH_SUPPORTS_SHADOW_CALL_STACK
+	bool "Shadow Call Stack"
+	depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
 	depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
 	help
-	  This option enables Clang's Shadow Call Stack, which uses a
+	  This option enables Clang/GCC's Shadow Call Stack, which uses a
 	  shadow stack to protect function return addresses from being
 	  overwritten by an attacker. More information can be found in
 	  Clang's documentation:
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 62c3c1d2190f..5d49c0c89645 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1062,7 +1062,7 @@ config ARCH_HAS_FILTER_PGPROT
 
 # Supported by clang >= 7.0
 config CC_HAVE_SHADOW_CALL_STACK
-	def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
+	def_bool CC_IS_CLANG || $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
 
 config PARAVIRT
 	bool "Enable paravirtualization code"
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index cb9217fc60af..917c3bb6aa43 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -104,6 +104,12 @@
 #define KASAN_ABI_VERSION 3
 #endif
 
+#if __has_attribute(__no_sanitize_shadow_call_stack__)
+#define __noscs __attribute__((no_sanitize_shadow_call_stack))
+#else
+#define __noscs
+#endif
+
 #if __has_attribute(__no_sanitize_address__)
 #define __no_sanitize_address __attribute__((no_sanitize_address))
 #else
-- 
2.17.1