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 08:50:35 -0400 (EDT) [thread overview]
Message-ID: <131549905.11442.1594731035989.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <87mu42bepq.fsf@oldenburg2.str.redhat.com>
----- On Jul 14, 2020, at 5:58 AM, Florian Weimer fweimer@redhat.com wrote:
> * Mathieu Desnoyers:
>
>> + /*
>> + * Very last field of the structure, to calculate size excluding padding
>> + * with offsetof().
>> + */
>> + char end[];
>> } __attribute__((aligned(4 * sizeof(__u64))));
>
> This makes the header incompatible with standard C++.
One alternative would be to add a helper to compute the effective size on c++, e.g.:
/* Always updated with struct rseq_cs declaration. */
#define rseq_last_field kernel_size
static inline size_t rseq_effective_size(void)
{
return offsetof(struct rseq, rseq_last_field) + sizeof(((struct rseq *)NULL)->rseq_last_field);
}
>
> 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 ?
> 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.
> 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.
> One way to avoid this problem would be for every library to register its
> own rseq area, of the appropriate size. Then process-wide coordination
> in userspace would not be needed.
I did propose the code to do just that in my initial rseq implementations, but
the idea was shutdown by kernel maintainers because it required the kernel to
handle a linked-list of rseq areas per thread, which was more complex within
the kernel.
Thanks,
Mathieu
>
> Thanks,
> Florian
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2020-07-14 12:50 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 [this message]
2020-07-14 13:00 ` Florian Weimer
2020-07-14 13:19 ` Mathieu Desnoyers
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
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