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

[Catalin Marinas <catalin.marinas@xxxxxxx>]

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@xxxxxxxxxx>

BTW, you could add

Co-developed-by: Catalin Marinas <catalin.marinas@xxxxxxx>

since I wrote the malloc() etc. hooks.


> +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);

	if (!ptr)
		return ptr;
	if (!tagged_addr_err)
		tag = rand() & 0xff;

	return (void *)((unsigned long)ptr | (tag << TAG_SHIFT));
}

-- 
Catalin