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+==========================
+AArch64 TAGGED ADDRESS ABI
+==========================
+
+Authors: Vincenzo Frascino <vincenzo.frascino@arm.com>
+         Catalin Marinas <catalin.marinas@arm.com>
+
+Date: 15 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() done by the process itself (or its parent), where either:
+
+  - flags have the **MAP_ANONYMOUS** bit set
+  - the file descriptor refers to a regular file (including those
+    returned by memfd_create()) or **/dev/zero**
+
+- brk() system call done by the process itself (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.
+
+- A valid tagged pointer has the same semantics as 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 arguments pointing to user structures to be
+  accessed by the kernel.
+
+- ioctl() other than arguments pointing to user structures 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;
+   }
