Re: [PATCH] [PATCH V4]ARM64: SCS: Add gcc plugin to support Shadow Call Stack

[Dan Li]

On 10/15/21 2:44 AM, Nick Desaulniers wrote:
>   On Wed, Oct 13, 2021 at 4:28 PM Dan Li <ashimida@xxxxxxxxxxxxxxxxx> wrote:
> > --- a/include/linux/compiler-gcc.h
> > +++ b/include/linux/compiler-gcc.h
> > @@ -50,6 +50,10 @@
> >   #define __latent_entropy __attribute__((latent_entropy))
> >   #endif
> >
> > +#if defined(SHADOW_CALL_STACK_PLUGIN) && !defined(__CHECKER__)
> > +#define __noscs __attribute__((no_shadow_call_stack))
> > +#endif
>
> Cool this is a nice addition, and something I don't think that clang
> has.  For any new feature, having a function attribute to disable it
> at the function granularity is nice, and plays better with LTO than -f
> group flags.  Though that begs the question: what happens if a __noscs
> callee is inlined into a non-__noscs caller, or vice versa?
Thanks Nick,

According to my understanding, all inline optimizations in gcc should
happen before inserting scs insns (scs and paciasp/autiasp use the
same insertion point). Therefore, the check for the __noscs attribute
will also occur after all inlining is completed.

As in the following example:
- Since __noscs attribute is specified, scs_test1 does not insert scs insns
- Since normal functions scs_test2/3 uses x30, it needs to insert scs insns
- Since __noscs attribute is specified, scs_test4 after inlining does not
need to insert scs insns

__always_inline __noscs void scs_test1(void)
{
    asm volatile("mov x1, x1\n\t":::"x30");
}

//scs insns inserted after function inline
void scs_test2(void)
{
    scs_test1();
}

__always_inline void scs_test3(void)
{
    asm volatile("mov x3, x3\n\t":::"x30");
}

//no scs insns inserted
__noscs void scs_test4(void)
{
    scs_test3();
}

ffff800010012900 <scs_test1>:
ffff800010012900:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
ffff800010012904:       910003fd        mov     x29, sp
ffff800010012908:       aa0103e1        mov     x1, x1
ffff80001001290c:       a8c17bfd        ldp     x29, x30, [sp], #16
ffff800010012910:       d65f03c0        ret

ffff800010012914 <scs_test2>:
ffff800010012914:       f800865e        str     x30, [x18], #8
ffff800010012918:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
ffff80001001291c:       910003fd        mov     x29, sp
ffff800010012920:       aa0103e1        mov     x1, x1
ffff800010012924:       a8c17bfd        ldp     x29, x30, [sp], #16
ffff800010012928:       f85f8e5e        ldr     x30, [x18, #-8]!
ffff80001001292c:       d65f03c0        ret

ffff800010012930 <scs_test3>:
ffff800010012930:       f800865e        str     x30, [x18], #8
ffff800010012934:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
ffff800010012938:       910003fd        mov     x29, sp
ffff80001001293c:       aa0303e3        mov     x3, x3
ffff800010012940:       a8c17bfd        ldp     x29, x30, [sp], #16
ffff800010012944:       f85f8e5e        ldr     x30, [x18, #-8]!
ffff800010012948:       d65f03c0        ret
ffff80001001294c:       d503201f        nop

ffff800010012950 <scs_test4>:
ffff800010012950:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
ffff800010012954:       910003fd        mov     x29, sp
ffff800010012958:       aa0303e3        mov     x3, x3
ffff80001001295c:       a8c17bfd        ldp     x29, x30, [sp], #16
ffff800010012960:       d65f03c0        ret
> I noticed that __noscs isn't actually applied anywhere in the kernel,
> yet, at least in this series.  Were there any places necessary that
> you've found thus far?
At present, I have not found a function that must use the __noscs
attribute in the kernel. I have only used this attribute in test cases.

> Overall, I'm happy with the patch and am ready to ack it, but I would
> like to see a link to to the upstream GCC feature request for SCS (and
> one created if it doesn't exist) cited explicitly in the commit
> message.  I think that would be a good demonstration that this can or
> will be upstreamed into the compiler proper for the compiler vendors
> to maintain, rather than the kernel folks.  The compiler vendors may
> have further feedback on the approach, such as my question above
> pertaining to inlining.
I have submitted a feature request to the gcc community, and waiting
for a follow-up response.

Is it fine to add the following description in [PATCH V5]?

A similar feature request has also been sent to gcc.
link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102768