From: Florian Weimer <fw@deneb.enyo.de>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Rich Felker <dalias@libc.org>,
libc-alpha <libc-alpha@sourceware.org>,
linux-api <linux-api@vger.kernel.org>,
Boqun Feng <boqun.feng@gmail.com>,
Will Deacon <will.deacon@arm.com>,
linux-kernel <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Ben Maurer <bmaurer@fb.com>, Dave Watson <davejwatson@fb.com>,
Thomas Gleixner <tglx@linutronix.de>,
Paul <paulmck@linux.vnet.ibm.com>, Paul Turner <pjt@google.com>,
Joseph Myers <joseph@codesourcery.com>
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 [thread overview]
Message-ID: <87sgi4gqhf.fsf@mid.deneb.enyo.de> (raw)
In-Reply-To: <1302331358.3965.1584641354569.JavaMail.zimbra@efficios.com> (Mathieu Desnoyers's message of "Thu, 19 Mar 2020 14:09:14 -0400 (EDT)")
* 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-(
next prev parent reply other threads:[~2020-03-19 18:17 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-19 14:41 [RFC PATCH glibc 0/8] Restartable Sequences enablement Mathieu Desnoyers via Libc-alpha
2020-03-19 14:41 ` [RFC PATCH glibc 1/8] Introduce <elf_machine_sym_no_match.h> Mathieu Desnoyers via Libc-alpha
2020-03-19 14:41 ` [RFC PATCH glibc 2/8] Implement __libc_early_init Mathieu Desnoyers via Libc-alpha
2020-03-19 14:41 ` [RFC PATCH glibc 3/8] nptl: Start new threads with all signals blocked [BZ #25098] Mathieu Desnoyers via Libc-alpha
2020-03-19 14:41 ` [RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C startup and thread creation (v15) Mathieu Desnoyers via Libc-alpha
2020-03-19 14:53 ` Florian Weimer
2020-03-19 15:56 ` Mathieu Desnoyers via Libc-alpha
2020-03-19 16:03 ` Florian Weimer
2020-03-19 18:09 ` Mathieu Desnoyers via Libc-alpha
2020-03-19 18:16 ` Florian Weimer [this message]
2020-03-19 18:28 ` Mathieu Desnoyers via Libc-alpha
2020-03-19 18:34 ` Florian Weimer
2020-03-19 18:55 ` Mathieu Desnoyers via Libc-alpha
2020-03-19 19:05 ` Florian Weimer
2020-03-19 19:46 ` Mathieu Desnoyers via Libc-alpha
2020-03-20 13:44 ` Mathieu Desnoyers via Libc-alpha
2020-03-20 14:47 ` Mathieu Desnoyers via Libc-alpha
2020-03-19 14:41 ` [RFC PATCH glibc 5/8] glibc: sched_getcpu(): use rseq cpu_id TLS on Linux (v6) Mathieu Desnoyers via Libc-alpha
2020-03-19 14:41 ` [RFC PATCH glibc 6/8] support record failure: allow use from constructor Mathieu Desnoyers via Libc-alpha
2020-03-19 14:41 ` [RFC PATCH glibc 7/8] support: implement xpthread key create/delete (v4) Mathieu Desnoyers via Libc-alpha
2020-03-19 14:41 ` [RFC PATCH glibc 8/8] rseq registration tests (v8) Mathieu Desnoyers via Libc-alpha
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