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==========================
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()``.
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;
}
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