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+==========================
+AArch64 TAGGED ADDRESS ABI
+==========================
+
+Authors: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
+ Catalin Marinas <catalin.marinas@xxxxxxx>
+
+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
+---------------------------
+.. 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;
+ }
