==========================
AArch64 TAGGED ADDRESS ABI
==========================

Authors: Vincenzo Frascino <vincenzo.frascino@arm.com>
         Catalin Marinas <catalin.marinas@arm.com>

Date: 21 August 2019

This document describes the usage and semantics of the Tagged Address
ABI on AArch64 Linux.

1. Introduction
---------------

On AArch64 the ``TCR_EL1.TBI0`` bit is set by default, allowing
userspace (EL0) to perform memory accesses through 64-bit pointers with
a non-zero top byte. This document describes the relaxation of the
syscall ABI that allows userspace to pass certain tagged pointers to
kernel syscalls.

2. AArch64 Tagged Address ABI
-----------------------------

From the kernel syscall interface perspective and for the purposes of
this document, a "valid tagged pointer" is a pointer with a potentially
non-zero top-byte that references an address in the user process address
space obtained in one of the following ways:

- ``mmap()`` syscall where either:

  - flags have the ``MAP_ANONYMOUS`` bit set or
  - the file descriptor refers to a regular file (including those
    returned by ``memfd_create()``) or ``/dev/zero``

- ``brk()`` syscall (i.e. the heap area between the initial location of
  the program break at process creation and its current location).

- any memory mapped by the kernel in the address space of the process
  during creation and with the same restrictions as for ``mmap()`` above
  (e.g. data, bss, stack).

The AArch64 Tagged Address ABI has two stages of relaxation depending on
how the user addresses are used by the kernel:

1. User addresses not accessed by the kernel but used for address space
   management (e.g. ``mprotect()``, ``madvise()``). The use of valid
   tagged pointers in this context is allowed with the exception of
   ``brk()``, ``mmap()`` and the ``new_address`` argument to
   ``mremap()`` as these have the potential to alias with existing
   user addresses.

   NOTE: This behaviour changed in v5.6 and so some earlier kernels may
   incorrectly accept valid tagged pointers for the ``brk()``,
   ``mmap()`` and ``mremap()`` system calls.

2. User addresses accessed by the kernel (e.g. ``write()``). This ABI
   relaxation is disabled by default and the application thread needs to
   explicitly enable it via ``prctl()`` as follows:

   - ``PR_SET_TAGGED_ADDR_CTRL``: enable or disable the AArch64 Tagged
     Address ABI for the calling thread.

     The ``(unsigned int) arg2`` argument is a bit mask describing the
     control mode used:

     - ``PR_TAGGED_ADDR_ENABLE``: enable AArch64 Tagged Address ABI.
       Default status is disabled.

     Arguments ``arg3``, ``arg4``, and ``arg5`` must be 0.

   - ``PR_GET_TAGGED_ADDR_CTRL``: get the status of the AArch64 Tagged
     Address ABI for the calling thread.

     Arguments ``arg2``, ``arg3``, ``arg4``, and ``arg5`` must be 0.

   The ABI properties described above are thread-scoped, inherited on
   clone() and fork() and cleared on exec().

   Calling ``prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0)``
   returns ``-EINVAL`` if the AArch64 Tagged Address ABI is globally
   disabled by ``sysctl abi.tagged_addr_disabled=1``. The default
   ``sysctl abi.tagged_addr_disabled`` configuration is 0.

When the AArch64 Tagged Address ABI is enabled for a thread, the
following behaviours are guaranteed:

- All syscalls except the cases mentioned in section 3 can accept any
  valid tagged pointer.

- The syscall behaviour is undefined for invalid tagged pointers: it may
  result in an error code being returned, a (fatal) signal being raised,
  or other modes of failure.

- The syscall behaviour for a valid tagged pointer is the same as for
  the corresponding untagged pointer.


A definition of the meaning of tagged pointers on AArch64 can be found
in Documentation/arm64/tagged-pointers.rst.

3. AArch64 Tagged Address ABI Exceptions
-----------------------------------------

The following system call parameters must be untagged regardless of the
ABI relaxation:

- ``prctl()`` other than pointers to user data either passed directly or
  indirectly as arguments to be accessed by the kernel.

- ``ioctl()`` other than pointers to user data either passed directly or
  indirectly as arguments to be accessed by the kernel.

- ``shmat()`` and ``shmdt()``.

- ``brk()`` (since kernel v5.6).

- ``mmap()`` (since kernel v5.6).

- ``mremap()``, the ``new_address`` argument (since kernel v5.6).

Any attempt to use non-zero tagged pointers may result in an error code
being returned, a (fatal) signal being raised, or other modes of
failure.

4. Example of correct usage
---------------------------
.. code-block:: c

   #include <stdlib.h>
   #include <string.h>
   #include <unistd.h>
   #include <sys/mman.h>
   #include <sys/prctl.h>
   
   #define PR_SET_TAGGED_ADDR_CTRL	55
   #define PR_TAGGED_ADDR_ENABLE	(1UL << 0)
   
   #define TAG_SHIFT		56
   
   int main(void)
   {
   	int tbi_enabled = 0;
   	unsigned long tag = 0;
   	char *ptr;
   
   	/* check/enable the tagged address ABI */
   	if (!prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0))
   		tbi_enabled = 1;
   
   	/* memory allocation */
   	ptr = mmap(NULL, sysconf(_SC_PAGE_SIZE), PROT_READ | PROT_WRITE,
   		   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
   	if (ptr == MAP_FAILED)
   		return 1;
   
   	/* set a non-zero tag if the ABI is available */
   	if (tbi_enabled)
   		tag = rand() & 0xff;
   	ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
   
   	/* memory access to a tagged address */
   	strcpy(ptr, "tagged pointer\n");
   
   	/* syscall with a tagged pointer */
   	write(1, ptr, strlen(ptr));
   
   	return 0;
   }