linux-kernel - Re: [PATCH v15 17/17] selftests, arm64: add a selftest for passing tagged pointers to kernel

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Date:   Wed, 22 May 2019 15:16:12 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        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,
        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>,
        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>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v15 17/17] selftests, arm64: add a selftest for passing
 tagged pointers to kernel

On Mon, May 06, 2019 at 06:31:03PM +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.

That's probably sufficient for a simple example. Something we could add
to Documentation maybe is a small library that can be LD_PRELOAD'ed so
that you can run a lot more tests like LTP.

We could add this to selftests but I think it's too glibc specific.

--------------------8<------------------------------------
#include <stdlib.h>

#define TAG_SHIFT	(56)
#define TAG_MASK	(0xffUL << TAG_SHIFT)

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

static void *untag_ptr(void *ptr)
{
	return (void *)((unsigned long)ptr & ~TAG_MASK);
}

void *malloc(size_t size)
{
	return tag_ptr(__libc_malloc(size));
}

void free(void *ptr)
{
	__libc_free(untag_ptr(ptr));
}

void *realloc(void *ptr, size_t size)
{
	return tag_ptr(__libc_realloc(untag_ptr(ptr), size));
}

void *calloc(size_t nmemb, size_t size)
{
	return tag_ptr(__libc_calloc(nmemb, size));
}