Revision 10717f45639f6c1bc27b56405252c3a027406d92 authored by Trond Myklebust on 27 January 2020, 14:58:19 UTC, committed by Anna Schumaker on 03 February 2020, 21:35:07 UTC
Delegations can be expensive to return, and can cause scalability issues for the server. Let's therefore try to limit the number of inactive delegations we hold. Once the number of delegations is above a certain threshold, start to return them on close. Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com> Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
1 parent d2269ea
tagged-address-abi.rst
==========================
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
---------------------------
.. 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;
}
![swh spinner](/static/img/swh-spinner.gif)
Computing file changes ...