linux-kernel - Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v8)

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Date:   Wed, 17 Apr 2019 16:17:17 +0000
From:   Joseph Myers <joseph@...esourcery.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     carlos <carlos@...hat.com>, Will Deacon <will.deacon@....com>,
        Florian Weimer <fweimer@...hat.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha <libc-alpha@...rceware.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Rich Felker <dalias@...c.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup
 and thread creation (v8)

On Wed, 17 Apr 2019, Mathieu Desnoyers wrote:

> > +/* RSEQ_SIG is a signature required before each abort handler code.
> > +
> > +   It is a 32-bit value that maps to actual architecture code compiled
> > +   into applications and libraries. It needs to be defined for each
> > +   architecture. When choosing this value, it needs to be taken into
> > +   account that generating invalid instructions may have ill effects on
> > +   tools like objdump, and may also have impact on the CPU speculative
> > +   execution efficiency in some cases.  */
> > +
> > +#define RSEQ_SIG 0xd428bc00	/* BRK #0x45E0.  */
> 
> After further investigation, we should probably do the following
> to handle compiling with -mbig-endian on aarch64, which generates
> binaries with mixed code vs data endianness (little endian code,
> big endian data):

First, the comment on RSEQ_SIG should specify whether it is to be 
interpreted in the code or the data endianness.

> For ARM32, the situation is a bit more complex. Only armv6+
> generates mixed-endianness code vs data with -mbig-endian.
> Prior to armv6, the code and data endianness matches. Therefore,
> I plan to #ifdef the reversed endianness handling with:
> 
> #if __ARM_ARCH >= 6 && __ARM_BIG_ENDIAN
> 
> on arm32.

That doesn't work well because BE code (.o files) can be built for v5te 
(for example) and used on a range of different architecture variants with 
both BE32 and BE8 - the choice between BE32 and BE8 is a link-time choice, 
not a compile-time choice.  So if the value for Arm is a compile-time 
constant, it should also work for both BE32 and BE8.

In turn, that suggests to me that RSEQ_SIG should be defined to be a value 
that is always in the code endianness (and whatever corresponding kernel 
code handles RSEQ_SIG values should act accordingly on architectures where 
the two endiannesses can differ).  If the kernel ABI is already fixed in a 
way that prevents such a definition of RSEQ_SIG semantics as using code 
endianness, a value should be chosen for Arm that works for both 
endiannesses.

(Also, installed glibc headers are supposed to work with older compilers, 
and support for __ARM_ARCH was only added in GCC 4.8.  Before that you 
need to test lots of separate macros for different architecture variants 
to determine a version number.)

-- 
Joseph S. Myers
joseph@...esourcery.com