linux-kernel - Re: [PATCH v16 16/16] selftests, arm64: add a selftest for passing tagged pointers to kernel

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Date:   Tue, 11 Jun 2019 19:18:04 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-rdma@...r.kernel.org, linux-media@...r.kernel.org,
        kvm@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        Yishai Hadas <yishaih@...lanox.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
        Christian Koenig <Christian.Koenig@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Khalid Aziz <khalid.aziz@...cle.com>, enh <enh@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christoph Hellwig <hch@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v16 16/16] selftests, arm64: add a selftest for passing
 tagged pointers to kernel

On Tue, Jun 11, 2019 at 5:01 PM Catalin Marinas <catalin.marinas@....com> wrote:
>
> On Mon, Jun 03, 2019 at 06:55:18PM +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 adds a simple test, that calls the uname syscall with a
> > tagged user pointer as an argument. Without the kernel accepting tagged
> > user pointers the test fails with EFAULT.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
>
> BTW, you could add
>
> Co-developed-by: Catalin Marinas <catalin.marinas@....com>
>
> since I wrote the malloc() etc. hooks.

Sure!

>
>
> > +static void *tag_ptr(void *ptr)
> > +{
> > +     unsigned long tag = rand() & 0xff;
> > +     if (!ptr)
> > +             return ptr;
> > +     return (void *)((unsigned long)ptr | (tag << TAG_SHIFT));
> > +}
>
> With the prctl() option, this function becomes (if you have a better
> idea, fine by me):
>
> ----------8<---------------
> #include <stdlib.h>
> #include <sys/prctl.h>
>
> #define TAG_SHIFT       (56)
> #define TAG_MASK        (0xffUL << TAG_SHIFT)
>
> #define PR_SET_TAGGED_ADDR_CTRL         55
> #define PR_GET_TAGGED_ADDR_CTRL         56
> # define PR_TAGGED_ADDR_ENABLE          (1UL << 0)
>
> void *__libc_malloc(size_t size);
> void __libc_free(void *ptr);
> void *__libc_realloc(void *ptr, size_t size);
> void *__libc_calloc(size_t nmemb, size_t size);
>
> static void *tag_ptr(void *ptr)
> {
>         static int tagged_addr_err = 1;
>         unsigned long tag = 0;
>
>         if (tagged_addr_err == 1)
>                 tagged_addr_err = prctl(PR_SET_TAGGED_ADDR_CTRL,
>                                         PR_TAGGED_ADDR_ENABLE, 0, 0, 0);

I think this requires atomics. malloc() can be called from multiple threads.

>
>         if (!ptr)
>                 return ptr;
>         if (!tagged_addr_err)
>                 tag = rand() & 0xff;
>
>         return (void *)((unsigned long)ptr | (tag << TAG_SHIFT));
> }
>
> --
> Catalin