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Date:   Sun, 27 Feb 2022 23:50:57 -0800
From:   Dan Li <>
To:     Kees Cook <>
Subject: Re: [PATCH] [PATCH v2] AARCH64: Add gcc Shadow Call Stack support

On 2/25/22 12:58, Kees Cook wrote:
> On Thu, Feb 24, 2022 at 07:24:10PM -0800, Dan Li wrote:
>> Signed-off-by: Dan Li <>
> Thanks for the tweaks!
>> ---
>> FYI:
>> This function can be used to test if the shadow call stack works:
>> //noinline void __noscs scs_test(void)
>> noinline void scs_test(void)
>> {
>>      unsigned long * lr = (unsigned long *)__builtin_frame_address(0) + 1;
>>      asm volatile("str xzr, [%0]\n\t": : "r"(lr) : "x30");
>> }
> Not a big deal, but just FYI, there's a lot of whitespace trailing the
> "}" above...

Ah, sorry for the mistake.
>> If SCS protection is enabled, this function will return normally.
>> If the function has __noscs attribute (scs disabled), it will crash due to 0
>> address access.
> It would be cool to turn this into an LKDTM test... (see things like the
> CFI_FORWARD_PROTO test). I imagine this should be CFI_BACKWARD_SHADOW or
> something...

OK, I'll add it in the next version.

> Also, I assume you're using real hardware to test this? It'd be nice to
> see if qemu can be convinced to run with the needed features. Whenever
> I've tried this it becomes impossibly slow. :)

I also use qemu to test the patch (qemu 6.1.0 with command "-cpu max"),
and can feel the performance drop.

Maybe because my test environment only has simple busybox and ltp,
the feeling of a slow system running is not that strong for me :)

For comparison, I simply tested the difference in kernel boot time
in my test environment:
//run qemu with "-cpu cortex-a57",
[    1.254481] Run /linuxrc as init process
//run qemu with "-cpu max"
[    3.566091] Run /linuxrc as init process

>> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
>> index ccbbd31b3aae..deff5b308470 100644
>> --- a/include/linux/compiler-gcc.h
>> +++ b/include/linux/compiler-gcc.h
>> @@ -97,6 +97,10 @@
>>   #define KASAN_ABI_VERSION 4
>>   #endif
>> +#define __noscs __attribute__((__no_sanitize__("shadow-call-stack")))
>> +#endif
> I initially wondered if we need a separate __no_sanitize(STUFF) patch to
> make the compiler-clang.h macros easier, but I see there are places
> where we do multiple ("address", "hwaddress") and have specialized
> macros, so I think this is fine. And since GCC doesn't support
> "__has_feature", I think this is the correct location for this.

As in:

I think maybe we could use "#ifdef CONFIG_SHADOW_CALL_STACK"
instead of "#if __has_attribute(__no_sanitize_address__)" here,
then move it to `compiler_types.h`.

 From my current test results, __noscs seems to work fine in
clang compilation.


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