linux-kernel - Re: [PATCH 16/18] arm64: ptrace: handle ptrace_request differently for aarch32 and ilp32

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Message-ID: <20161208131232.GA15141@e104818-lin.cambridge.arm.com>
Date:   Thu, 8 Dec 2016 13:12:32 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-doc@...r.kernel.org, szabolcs.nagy@....com,
        heiko.carstens@...ibm.com, cmetcalf@...hip.com,
        Yury Norov <ynorov@...iumnetworks.com>,
        philipp.tomsich@...obroma-systems.com, joseph@...esourcery.com,
        linux-arch@...r.kernel.org, zhouchengming1@...wei.com,
        Prasun.Kapoor@...iumnetworks.com, agraf@...e.de,
        geert@...ux-m68k.org, kilobyte@...band.pl,
        manuel.montezelo@...il.com, pinskia@...il.com,
        linyongting@...wei.com, klimov.linux@...il.com, broonie@...nel.org,
        bamvor.zhangjian@...wei.com,
        Bamvor Zhang Jian <bamvor.zhangjian@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, maxim.kuvyrkov@...aro.org,
        Nathan_Lynch@...tor.com, linux-kernel@...r.kernel.org,
        schwidefsky@...ibm.com, davem@...emloft.net,
        christoph.muellner@...obroma-systems.com
Subject: Re: [PATCH 16/18] arm64: ptrace: handle ptrace_request differently
 for aarch32 and ilp32

On Wed, Dec 07, 2016 at 09:40:13PM +0100, Arnd Bergmann wrote:
> On Wednesday, December 7, 2016 4:59:13 PM CET Catalin Marinas wrote:
> > On Tue, Dec 06, 2016 at 11:55:08AM +0530, Yury Norov wrote:
> > > On Mon, Dec 05, 2016 at 04:34:23PM +0000, Catalin Marinas wrote:
> > > > On Fri, Oct 21, 2016 at 11:33:15PM +0300, Yury Norov wrote:
> > > > > New aarch32 ptrace syscall handler is introduced to avoid run-time
> > > > > detection of the task type.
> > > > 
> > > > What's wrong with the run-time detection? If it's just to avoid a
> > > > negligible overhead, I would rather keep the code simpler by avoiding
> > > > duplicating the generic compat_sys_ptrace().
> > > 
> > > Nothing wrong. This is how Arnd asked me to do. You already asked this
> > > question: http://lkml.iu.edu/hypermail/linux/kernel/1604.3/00930.html
> > 
> > Hmm, I completely forgot about this ;). There is still an advantage to
> > doing run-time checking if we avoid touching core code (less acks to
> > gather and less code duplication).
> > 
> > Let's see what Arnd says but the initial patch looked simpler.
> 
> I don't currently have either version of the patch in my inbox
> (the archive is on a different machine), but in general I'd still
> think it's best to avoid the runtime check for aarch64-ilp32
> altogether. I'd have to look at the overall kernel source to
> see if it's worth avoiding one or two instances though, or
> if there are an overwhelming number of other checks that we
> can't avoid at all.

Just in case you haven't found them already, current version:

https://marc.info/?l=linux-arm-kernel&m=147708276818318&w=2

Original version:

https://patchwork.kernel.org/patch/7980521/

The old one looks more readable and given that ptrace is not really a
fast path, I'm not two worried about run-time checks

> Regarding ptrace, I notice that arch/tile doesn't even use
> the compat entry point for its ilp32 user space on 64-bit
> kernels, it just calls the regular 64-bit one. Would that
> help here?

I don't know whether it would work, we have incompatible siginfo_t on
AArch64/ILP32.

-- 
Catalin