From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
paulmck <paulmck@linux.ibm.com>,
Boqun Feng <boqun.feng@gmail.com>,
"H. Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
linux-api <linux-api@vger.kernel.org>,
Christian Brauner <christian.brauner@ubuntu.com>,
carlos <carlos@redhat.com>
Subject: Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq
Date: Tue, 14 Jul 2020 09:19:51 -0400 (EDT) [thread overview]
Message-ID: <2452161.11491.1594732791558.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <87a7028d5u.fsf@oldenburg2.str.redhat.com>
----- On Jul 14, 2020, at 9:00 AM, Florian Weimer fweimer@redhat.com wrote:
> * Mathieu Desnoyers:
>
>>> How are extensions going to affect the definition of struct rseq,
>>> including its alignment?
>>
>> The alignment will never decrease. If the structure becomes large enough
>> its alignment could theoretically increase. Would that be an issue ?
>
> Telling the compiler that struct is larger than it actually is, or that
> it has more alignment than in memory, results in undefined behavior,
> even if only fields are accessed in the smaller struct region.
>
> An increase in alignment from 32 to 64 is perhaps not likely to have
> this effect. But the undefined behavior is still there, and has been
> observed for mismatches like 8 vs 16.
Good points.
>
>>> As things stand now, glibc 2.32 will make the size and alignment of
>>> struct rseq part of its ABI, so it can't really change after that.
>>
>> Can the size and alignment of a structure be defined as minimum alignment
>> and size values ? For instance, those would be invariant for a given glibc
>> version (if we always use the internal struct rseq declaration), but could
>> be increased in future versions.
>
> Not if we are talking about a global (TLS) data symbol. No such changes
> are possible there. We have some workarounds for symbols that live
> exclusively within glibc, but they don't work if there are libraries out
> there which interpose the symbol.
OK
>
>>> With a different approach, we can avoid making the symbol size part of
>>> the ABI, but then we cannot use the __rseq_abi TLS symbol. As a result,
>>> interoperability with early adopters would be lost.
>>
>> Do you mean with a function "getter", and then keeping that pointer around
>> in a per-user TLS ? I would prefer to avoid that because it adds an extra
>> pointer dereference on a fast path.
>
> My choice would have been a function that returns the offset from the
> thread pointer (which has to be unchanged regarding all threads).
So AFAIU we would have glibc expose a symbol, e.g.:
off_t rseq_tls_offset(void);
Which would be typically called by user libraries and applications at initialization
to get the offset of the struct rseq. They should store it in a static variable so
rseq critical sections can use that offset.
Is there an arch-agnostic way to get the thread pointer from user-space code ? That
would be needed by all rseq critical section implementations.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2020-07-14 13:19 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-14 3:03 [RFC PATCH 0/4] rseq: Introduce extensible struct rseq Mathieu Desnoyers
2020-07-14 3:03 ` [RFC PATCH 1/4] selftests: rseq: Use fixed value as rseq_len parameter Mathieu Desnoyers
2020-07-14 3:03 ` [RFC PATCH 2/4] rseq: Allow extending struct rseq Mathieu Desnoyers
2020-07-14 9:58 ` Florian Weimer
2020-07-14 12:50 ` Mathieu Desnoyers
2020-07-14 13:00 ` Florian Weimer
2020-07-14 13:19 ` Mathieu Desnoyers [this message]
2020-07-14 21:30 ` Carlos O'Donell
2020-07-15 13:12 ` Mathieu Desnoyers
2020-07-15 13:22 ` Florian Weimer
2020-07-15 13:31 ` Mathieu Desnoyers
2020-07-15 13:42 ` Florian Weimer
2020-07-15 13:55 ` Christian Brauner
2020-07-15 14:20 ` Mathieu Desnoyers
2020-07-15 14:54 ` Mathieu Desnoyers
2020-07-15 14:58 ` Florian Weimer
2020-07-15 15:26 ` Mathieu Desnoyers
2020-07-14 17:24 ` Peter Oskolkov
2020-07-14 17:43 ` Mathieu Desnoyers
2020-07-14 18:33 ` Peter Oskolkov
2020-07-15 2:34 ` Chris Kennelly
2020-07-15 6:31 ` Florian Weimer
2020-07-15 10:59 ` Christian Brauner
2020-07-15 14:38 ` Mathieu Desnoyers
2020-07-15 14:50 ` Mathieu Desnoyers
2020-07-15 11:38 ` Christian Brauner
2020-07-15 12:33 ` Christian Brauner
2020-07-15 15:10 ` Mathieu Desnoyers
2020-07-15 15:33 ` Christian Brauner
2020-07-14 3:03 ` [RFC PATCH 3/4] selftests: rseq: define __rseq_abi with extensible size Mathieu Desnoyers
2020-07-14 3:03 ` [RFC PATCH 4/4] selftests: rseq: print rseq extensible size in basic test Mathieu Desnoyers
2020-07-14 20:55 ` [RFC PATCH 0/4] rseq: Introduce extensible struct rseq Carlos O'Donell
2020-07-15 13:02 ` Mathieu Desnoyers
2020-07-16 13:39 ` Carlos O'Donell
2020-07-16 14:45 ` Mathieu Desnoyers
2020-07-15 15:12 ` Florian Weimer
2020-07-15 15:32 ` Mathieu Desnoyers
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
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=2452161.11491.1594732791558.JavaMail.zimbra@efficios.com \
--to=mathieu.desnoyers@efficios.com \
--cc=boqun.feng@gmail.com \
--cc=carlos@redhat.com \
--cc=christian.brauner@ubuntu.com \
--cc=fweimer@redhat.com \
--cc=hpa@zytor.com \
--cc=linux-api@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=paulmck@linux.ibm.com \
--cc=peterz@infradead.org \
--cc=pjt@google.com \
--cc=tglx@linutronix.de \
/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 NNTP newsgroup(s).