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Date:   Thu, 19 Mar 2020 14:55:26 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fw@deneb.enyo.de>
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Subject: Re: [RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C
 startup and thread creation (v15)
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Thread-Topic: glibc: Perform rseq(2) registration at C startup and thread creation (v15)
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----- On Mar 19, 2020, at 2:34 PM, Florian Weimer fw@deneb.enyo.de wrote:

> * Mathieu Desnoyers:
> 
>> ----- On Mar 19, 2020, at 2:16 PM, Florian Weimer fw@deneb.enyo.de 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.

OK, so the _Static_assert () could sit in sys/rseq.h

> 
>>>>> 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 are two:

target_core_user.h
45:#define ALIGN_SIZE 64 /* Should be enough for most CPUs */
58:	__u32 cmd_tail __attribute__((__aligned__(ALIGN_SIZE)));

netfilter_bridge/ebtables.h:90:	char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
netfilter_bridge/ebtables.h:132:	unsigned char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
netfilter_bridge/ebtables.h:145:	unsigned char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
netfilter_bridge/ebtables.h:158:	unsigned char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
netfilter_bridge/ebtables.h:191:	unsigned char elems[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));


> 
>>> 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.

We could at least make an effort to let people know that alignment is
required here when allocating struct rseq and struct rseq_cs on the
heap by adding some comments to that effect in linux/rseq.h ?

> 
>>>> 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.

Considering the amount of pain this can cause in user-space, and because
it can break userspace, this is not a UAPI change I am willing to consider.
I'm not sure why we are even discussing the possibility of breaking a Linux
UAPI considering that those are set in stone.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com