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 [off-list ref] wrote:

On Mon, Jun 03, 2019 at 06:55:18PM +0200, Andrey Konovalov wrote:

quoted

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 <redacted>

BTW, you could add

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

since I wrote the malloc() etc. hooks.

Sure!

quoted

+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

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