----- On Mar 19, 2020, at 2:16 PM, Florian Weimer fw@xxxxxxxxxxxxx wrote:
> * 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.
I was thinking of adding the following to
sysdeps/unix/sysv/linux/rseq-internal.h: rseq_register_current_thread()
+ /* Ensure the compiler supports __attribute__ ((aligned)). */
+ _Static_assert (__alignof__ (struct rseq_cs) >= 4 * sizeof(uint64_t),
+ "alignment");
+ _Static_assert (__alignof__ (struct rseq) >= 4 * sizeof(uint64_t),
+ "alignment");
+
>>> 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.
It is worth noting that rseq.h is not the only Linux uapi header
which uses __attribute__ ((aligned)), so this ABI problem exists today
anyway for those compilers.
>
> 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.
Why would it be valid to allocate those with malloc ? Isn't it the
purpose of posix_memalign() ?
>
> 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-(
I don't think this is an option at this stage given that it is part
of the Linux kernel UAPI. I am not convinced that it is valid at all
to allocate struct rseq or struct rseq_cs with malloc(), because it
does not guarantee any alignment.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
