On Fri, Oct 15, 2021 at 11:29 AM Dan Li <ashimida@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> 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();
> }
That may be surprising to developers. Perhaps __always_inline on
scs_test1 is distracting this test case, but I suspect it may not make
a difference. This particular issue comes up time and again with
stack protectors; ie. the callee is marked no stack protector, then
gets inlined into a caller and suddenly gets a stack protector.
>
> __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.
--
Thanks,
~Nick Desaulniers
