linux-kernel - Re: [RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C startup and thread creation (v15)

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Message-ID: <82259847.4696.1584715631625.JavaMail.zimbra@efficios.com>
Date:   Fri, 20 Mar 2020 10:47:11 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     libc-alpha <libc-alpha@...rceware.org>, carlos <carlos@...hat.com>,
        Rich Felker <dalias@...c.org>,
        linux-api <linux-api@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ben Maurer <bmaurer@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul <paulmck@...ux.vnet.ibm.com>, Paul Turner <pjt@...gle.com>,
        Joseph Myers <joseph@...esourcery.com>
Subject: Re: [RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C
 startup and thread creation (v15)

----- On Mar 20, 2020, at 9:44 AM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:
[...]
> Actually, here is an important clarification: the Linux kernel validates
> the struct rseq alignment on registration:
> 
>        if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
>            rseq_len != sizeof(*rseq))
>                return -EINVAL;
> 
> So removing the aligned attribute from struct rseq is actually an
> ABI-breaking change, because it would be incompatible with older
> kernels which perform the IS_ALIGNED check expecting at least at
> 32 bytes alignment.

So I plan to add the following to glibc's sys/rseq.h:

#include <sys/cdefs.h>

[...]

/* 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");

/* Allocations of struct rseq and struct rseq_cs on the heap need to
   be aligned on 32 bytes. Therefore, use of malloc is discouraged
   because it does not guarantee alignment. posix_memalign should be
   used instead.  */

Does it help mitigating your concerns ?

Thanks,

Mathieu

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