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
- flags have the
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:
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 ofbrk()
,mmap()
and thenew_address
argument tomremap()
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()
andmremap()
system calls.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 viaprctl()
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
, andarg5
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
, andarg5
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 bysysctl abi.tagged_addr_disabled=1
. The defaultsysctl 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()
andshmdt()
.
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;
}