From: Mathieu Desnoyers via Libc-alpha <libc-alpha@sourceware.org>
To: Florian Weimer <fw@deneb.enyo.de>
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 14:55:26 -0400 (EDT) [thread overview]
Message-ID: <900536577.4062.1584644126425.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <87k13ggpmf.fsf@mid.deneb.enyo.de>
----- 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
next prev parent reply other threads:[~2020-03-19 18:55 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
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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: https://www.gnu.org/software/libc/involved.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=900536577.4062.1584644126425.JavaMail.zimbra@efficios.com \
--to=libc-alpha@sourceware.org \
--cc=bmaurer@fb.com \
--cc=boqun.feng@gmail.com \
--cc=dalias@libc.org \
--cc=davejwatson@fb.com \
--cc=fw@deneb.enyo.de \
--cc=joseph@codesourcery.com \
--cc=linux-api@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=paulmck@linux.vnet.ibm.com \
--cc=peterz@infradead.org \
--cc=pjt@google.com \
--cc=tglx@linutronix.de \
--cc=will.deacon@arm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).