Re: [RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C startup and thread creation (v15)

[Florian Weimer]

* Mathieu Desnoyers:

> ----- 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");
> +

Something like it would have to go into the *public* header.

Inside glibc, you can assume __attribute__ support.

>>>> 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.

Yuck.  Even with larger-than-16 alignment?

>> 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() ?

It would not be valid, but I don't think we have diagnostics for C
like we have them for C++'s operator new.

>>> 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.

The kernel ABI doesn't change.  The kernel cannot use the alignment
information anyway.  Userspace struct layout may change in subtle
ways, though.