Messages in this thread | | | Subject | Re: [PATCH] [PATCH V4]ARM64: SCS: Add gcc plugin to support Shadow Call Stack | From | Dan Li <> | Date | Sat, 16 Oct 2021 02:28:56 +0800 |
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On 10/15/21 2:44 AM, Nick Desaulniers wrote: > On Wed, Oct 13, 2021 at 4:28 PM Dan Li <ashimida@linux.alibaba.com> 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
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