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

Catalin Marinas catalin.marinas at arm.com
Wed May 22 14:16:12 UTC 2019

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.

#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)

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));

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