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Date:   Thu, 19 Mar 2020 14:28:51 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fw@deneb.enyo.de>
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        Rich Felker <dalias@libc.org>,
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        Thomas Gleixner <tglx@linutronix.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 2:16 PM, Florian Weimer fw@deneb.enyo.de wrote:

> * Mathieu Desnoyers:
>=20
>>> 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 ?
>=20
> 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) >=3D 4 * sizeof(uint64_t),
+                 "alignment");
+  _Static_assert (__alignof__ (struct rseq) >=3D 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 ?
>=20
> 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.

>=20
> 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() ?

>=20
> For example, clang turns
>=20
> void
> clear (struct rseq *p)
> {
>  memset (p, 0, sizeof (*p));
> }
>=20
> into:
>=20
>=09vxorps=09%xmm0, %xmm0, %xmm0
>=09vmovaps=09%ymm0, (%rdi)
>=09vzeroupper
>=09retq
>=20
> My understanding is that vmovaps will trap if the pointer is
> misaligned (=E2=80=9CWhen 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.=E2=80=9D).
>=20
>> However, now that it is in the wild, it's a bit late to change that.
>=20
> 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

--=20
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com