linux-kernel - Re: [PATCH] compat: Fix endian issue in union sigval

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Message-ID: <20150225103750.GC22206@localhost>
Date:	Wed, 25 Feb 2015 10:37:51 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	linux-arch@...r.kernel.org, benh@...nel.crashing.org,
	davem@...emloft.net, mpe@...erman.id.au, deller@....de,
	hpa@...or.com, heiko.carstens@...ibm.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>, jejb@...isc-linux.org,
	"lizefan@...wei.com" <lizefan@...wei.com>,
	Bamvor Jian Zhang <bamvor.zhangjian@...wei.com>,
	ralf@...ux-mips.org,
	"dingtianhong@...wei.com" <dingtianhong@...wei.com>,
	schwidefsky@...ibm.com, paulus@...ba.org, tglx@...utronix.de,
	mingo@...hat.com,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] compat: Fix endian issue in union sigval

On Tue, Feb 24, 2015 at 04:54:17PM -0500, Chris Metcalf wrote:
> On 2/14/2015 6:22 AM, Catalin Marinas wrote:
> >1. user populates sival_int compat_sigevent and invokes
> >    compat_sys_mq_notify()
> >2. kernel get_compat_sigevent() copies compat_sigevent into the native
> >    sigevent. compat and native sival_int are the same, no problem so
> >    far. The other half of 64-bit sival_ptr is zeroed by a memset in this
> >    function (this other half can be top or bottom, depending on
> >    endianness)
> >3. signal is about to be delivered to user via arch code. The
> >    compat_ptr(from->si_ptr) conversion always takes the least
> >    significant part of the native si_ptr. On big endian 64-bit, this is
> >    zero because get_compat_sigevent() populated the top part of si_ptr
> >    with si_int.
> >
> >So delivering such signals to compat user always sets si_int to 0.
> >Little endian is fine.
> 
> I looked at this again as I was getting ready to do a tile patch, and realized
> why tile and arm64 are different here: tile does a field-by-field copy in
> copy_siginfo_from_user32(), like parisc and s390.  As a result, we initialize
> the 64-bit kernel si_ptr value by cast from the 32-bit user si_ptr value, rather
> than blindly writing into the lower-addressed half of the 64-bit sigval.

It's not about copy_siginfo_from_user32() but the generic
get_compat_sigevent() which always uses sival_int (called from e.g.
compat_sys_timer_create()).

-- 
Catalin
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