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 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. mmap(), mprotect(), madvise()). The use of valid tagged pointers in this context is always allowed.

  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

#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;
}