linux-kernel - Re: [PATCH v15 05/17] arms64: untag user pointers passed to memory syscalls

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Message-ID: <20190528145411.GA709@e119886-lin.cambridge.arm.com>
Date:   Tue, 28 May 2019 15:54:11 +0100
From:   Andrew Murray <andrew.murray@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        Mark Rutland <mark.rutland@....com>, kvm@...r.kernel.org,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        Will Deacon <will.deacon@....com>,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org,
        Felix Kuehling <Felix.Kuehling@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Leon Romanovsky <leon@...nel.org>, linux-rdma@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, Dmitry Vyukov <dvyukov@...gle.com>,
        Dave Martin <Dave.Martin@....com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        linux-media@...r.kernel.org, Kevin Brodsky <kevin.brodsky@....com>,
        Kees Cook <keescook@...omium.org>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Kostya Serebryany <kcc@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Yishai Hadas <yishaih@...lanox.com>,
        linux-kernel@...r.kernel.org,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Lee Smith <Lee.Smith@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        Christian Koenig <Christian.Koenig@....com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Subject: Re: [PATCH v15 05/17] arms64: untag user pointers passed to memory
 syscalls

On Mon, May 27, 2019 at 03:37:20PM +0100, Catalin Marinas wrote:
> On Mon, May 06, 2019 at 06:30:51PM +0200, Andrey Konovalov wrote:
> > This patch is a part of a series that extends arm64 kernel ABI to allow to
> > pass tagged user pointers (with the top byte set to something else other
> > than 0x00) as syscall arguments.
> > 
> > This patch allows tagged pointers to be passed to the following memory
> > syscalls: brk, get_mempolicy, madvise, mbind, mincore, mlock, mlock2,
> > mmap, mmap_pgoff, mprotect, mremap, msync, munlock, munmap,
> > remap_file_pages, shmat and shmdt.
> > 
> > This is done by untagging pointers passed to these syscalls in the
> > prologues of their handlers.
> > 
> > Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> 
> Actually, I don't think any of these wrappers get called (have you
> tested this patch?). Following commit 4378a7d4be30 ("arm64: implement
> syscall wrappers"), I think we have other macro names for overriding the
> sys_* ones.

What is the value in adding these wrappers?

The untagged_addr macro is defined for all in linux/mm.h and these patches
already use untagged_addr in generic code. Thus adding a few more
untagged_addr in the generic syscall handlers (which turn to a nop for most)
is surely better than adding wrappers?

Even if other architectures implement untagged_addr in the future it would
be more consistent if they untagged in the same places and thus not adding
these wrappers enforces that.

Thanks,

Andrew Murray

> 
> -- 
> Catalin
> 
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> linux-arm-kernel@...ts.infradead.org
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