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+ARM64 TAGGED ADDRESS ABI
+========================
+
+This document describes the usage and semantics of the Tagged Address
+ABI on arm64.
+
+1. Introduction
+---------------
+
+On arm64 the TCR_EL1.TBI0 bit has been always enabled on the kernel, hence
+the userspace (EL0) is entitled to perform a user memory access through a
+64-bit pointer with a non-zero top byte but the resulting pointers are not
+allowed at the user-kernel syscall ABI boundary.
+
+This document describes a relaxation of the ABI that makes it possible to
+to pass tagged pointers to the syscalls, when these pointers are in memory
+ranges obtained as described in section 2.
+
+Since it is not desirable to relax the ABI to allow tagged user addresses
+into the kernel indiscriminately, arm64 provides a new sysctl interface
+(/proc/sys/abi/tagged_addr) that is used to prevent the applications from
+enabling the relaxed ABI and a new prctl() interface that can be used to
+enable or disable the relaxed ABI.
+A detailed description of the newly introduced mechanisms will be provided
+in section 2.
+
+2. ARM64 Tagged Address ABI
+---------------------------
+
+From the kernel syscall interface perspective, we define, for the purposes
+of this document, a "valid tagged pointer" as a pointer that either has a
+zero value set in the top byte or has a non-zero value, it is in memory
+ranges privately owned by a userspace process and it is obtained in one of
+the following ways:
+  - mmap() done by the process itself, where either:
+    * flags have MAP_PRIVATE and MAP_ANONYMOUS
+    * flags have MAP_PRIVATE and the file descriptor refers to a regular
+      file 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 process's address space during
+    creation and following the restrictions presented above (i.e. data, bss,
+    stack).
+
+The ARM64 Tagged Address ABI is an opt-in feature, and an application can
+control it using the following:
+ - /proc/sys/abi/tagged_addr: a new sysctl interface that can be used to
+        prevent the applications from enabling the relaxed ABI.
+        The sysctl is meant also for testing purposes in order to provide a
+        simple way for the userspace to verify the return error checking of
+        the prctl() commands without having to reconfigure the kernel.
+        The sysctl supports the following configuration options:
+         - 0: Disable ARM64 Tagged Address ABI for all the applications.
+         - 1 (Default): Enable ARM64 Tagged Address ABI for all the
+                        applications.
+        If the ARM64 Tagged Address ABI is disabled at a certain point in
+        time, all the applications that were using tagging before this event
+        occurs, will continue to use tagging.
+
+ - prctl()s:
+  - PR_SET_TAGGED_ADDR_CTRL: can be used to enable or disable the Tagged
+        Address ABI.
+        The (unsigned int) arg2 argument is a bit mask describing the
+        control mode used:
+          - PR_TAGGED_ADDR_ENABLE: Enable ARM64 Tagged Address ABI.
+        The arguments arg3, arg4, and arg5 are ignored.
+
+  - PR_GET_TAGGED_ADDR_CTRL: can be used to check the status of the Tagged
+        Address ABI.
+        The arguments arg2, arg3, arg4, and arg5 are ignored.
+
+The ABI properties set by the mechanisms described above are inherited by threads
+of the same application and fork()'ed children but cleared by execve().
+
+As a consequence of invoking PR_SET_TAGGED_ADDR_CTRL prctl() by an applications,
+the ABI guarantees the following behaviours:
+
+  - Every current or newly introduced syscall can accept any valid tagged
+    pointers.
+
+  - If a non valid tagged pointer is passed to a syscall then the behaviour
+    is undefined.
+
+  - Every valid tagged pointer is expected to work as an untagged one.
+
+  - The kernel preserves any valid tagged pointers and returns them to the
+    userspace unchanged (i.e. on syscall return) in all the cases except the
+    ones documented in the "Preserving tags" section of tagged-pointers.txt.
+
+A definition of the meaning of tagged pointers on arm64 can be found in:
+Documentation/arm64/tagged-pointers.txt.
+
+3. ARM64 Tagged Address ABI Exceptions
+--------------------------------------
+
+The behaviours described in section 2, with particular reference to the
+acceptance by the syscalls of any valid tagged pointer are not applicable
+to the following cases:
+  - mmap() addr parameter.
+  - mremap() new_address parameter.
+  - prctl_set_mm() struct prctl_map fields.
+  - prctl_set_mm_map() struct prctl_map fields.
+
+Any attempt to use non-zero tagged pointers will lead to undefined behaviour.
+
+4. Example of correct usage
+---------------------------
+
+void main(void)
+{
+	static int tbi_enabled = 0;
+	unsigned long tag = 0;
+
+	char *ptr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
+			 MAP_ANONYMOUS, -1, 0);
+
+	if (prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE,
+		  0, 0, 0) == 0)
+		tbi_enabled = 1;
+
+	if (ptr == (void *)-1) /* MAP_FAILED */
+		return -1;
+
+	if (tbi_enabled)
+		tag = rand() & 0xff;
+
+	ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
+
+	*ptr = 'a';
+
+	...
+}
+
