Messages in this thread | | | From | Florian Weimer <> | Subject | Re: [RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C startup and thread creation (v15) | Date | Thu, 19 Mar 2020 19:16:12 +0100 |
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* Mathieu Desnoyers:
>> You also need to add an assert that the compiler supports >> __attribute__ ((aligned)) because ignoring it produces an >> ABI-incompatible header. > > Are you aware of some helper macro I should use to do this, or > is it done elsewhere in glibc ?
I don't think we have any such GCC-only types yet. max_align_t is provided by GCC itself.
>> The struct rseq/struct rseq_cs definitions >> are broken, they should not try to change the alignment. > > AFAIU, this means we should ideally not have used __attribute__((aligned)) > in the uapi headers in the first place. Why is it broken ?
Compilers which are not sufficiently GCC-compatible define __attribute__(X) as the empty expansion, so you silently get a different ABI.
There is really no need to specify 32-byte alignment here. Is not even the size of a standard cache line. It can result in crashes if these structs are heap-allocated using malloc, when optimizing for AVX2.
For example, clang turns
void clear (struct rseq *p) { memset (p, 0, sizeof (*p)); }
into:
vxorps %xmm0, %xmm0, %xmm0 vmovaps %ymm0, (%rdi) vzeroupper retq
My understanding is that vmovaps will trap if the pointer is misaligned (“When the source or destination operand is a memory operand, the operand must be aligned on a 32-byte boundary or a general-protection exception (#GP) will be generated.”).
> However, now that it is in the wild, it's a bit late to change that.
I had forgotten about the alignment crashes. I think we should seriously consider changing the types. 8-(
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