200 строки
7.0 KiB
ReStructuredText
200 строки
7.0 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
Using FS and GS segments in user space applications
|
|
===================================================
|
|
|
|
The x86 architecture supports segmentation. Instructions which access
|
|
memory can use segment register based addressing mode. The following
|
|
notation is used to address a byte within a segment:
|
|
|
|
Segment-register:Byte-address
|
|
|
|
The segment base address is added to the Byte-address to compute the
|
|
resulting virtual address which is accessed. This allows to access multiple
|
|
instances of data with the identical Byte-address, i.e. the same code. The
|
|
selection of a particular instance is purely based on the base-address in
|
|
the segment register.
|
|
|
|
In 32-bit mode the CPU provides 6 segments, which also support segment
|
|
limits. The limits can be used to enforce address space protections.
|
|
|
|
In 64-bit mode the CS/SS/DS/ES segments are ignored and the base address is
|
|
always 0 to provide a full 64bit address space. The FS and GS segments are
|
|
still functional in 64-bit mode.
|
|
|
|
Common FS and GS usage
|
|
------------------------------
|
|
|
|
The FS segment is commonly used to address Thread Local Storage (TLS). FS
|
|
is usually managed by runtime code or a threading library. Variables
|
|
declared with the '__thread' storage class specifier are instantiated per
|
|
thread and the compiler emits the FS: address prefix for accesses to these
|
|
variables. Each thread has its own FS base address so common code can be
|
|
used without complex address offset calculations to access the per thread
|
|
instances. Applications should not use FS for other purposes when they use
|
|
runtimes or threading libraries which manage the per thread FS.
|
|
|
|
The GS segment has no common use and can be used freely by
|
|
applications. GCC and Clang support GS based addressing via address space
|
|
identifiers.
|
|
|
|
Reading and writing the FS/GS base address
|
|
------------------------------------------
|
|
|
|
There exist two mechanisms to read and write the FS/GS base address:
|
|
|
|
- the arch_prctl() system call
|
|
|
|
- the FSGSBASE instruction family
|
|
|
|
Accessing FS/GS base with arch_prctl()
|
|
--------------------------------------
|
|
|
|
The arch_prctl(2) based mechanism is available on all 64-bit CPUs and all
|
|
kernel versions.
|
|
|
|
Reading the base:
|
|
|
|
arch_prctl(ARCH_GET_FS, &fsbase);
|
|
arch_prctl(ARCH_GET_GS, &gsbase);
|
|
|
|
Writing the base:
|
|
|
|
arch_prctl(ARCH_SET_FS, fsbase);
|
|
arch_prctl(ARCH_SET_GS, gsbase);
|
|
|
|
The ARCH_SET_GS prctl may be disabled depending on kernel configuration
|
|
and security settings.
|
|
|
|
Accessing FS/GS base with the FSGSBASE instructions
|
|
---------------------------------------------------
|
|
|
|
With the Ivy Bridge CPU generation Intel introduced a new set of
|
|
instructions to access the FS and GS base registers directly from user
|
|
space. These instructions are also supported on AMD Family 17H CPUs. The
|
|
following instructions are available:
|
|
|
|
=============== ===========================
|
|
RDFSBASE %reg Read the FS base register
|
|
RDGSBASE %reg Read the GS base register
|
|
WRFSBASE %reg Write the FS base register
|
|
WRGSBASE %reg Write the GS base register
|
|
=============== ===========================
|
|
|
|
The instructions avoid the overhead of the arch_prctl() syscall and allow
|
|
more flexible usage of the FS/GS addressing modes in user space
|
|
applications. This does not prevent conflicts between threading libraries
|
|
and runtimes which utilize FS and applications which want to use it for
|
|
their own purpose.
|
|
|
|
FSGSBASE instructions enablement
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
The instructions are enumerated in CPUID leaf 7, bit 0 of EBX. If
|
|
available /proc/cpuinfo shows 'fsgsbase' in the flag entry of the CPUs.
|
|
|
|
The availability of the instructions does not enable them
|
|
automatically. The kernel has to enable them explicitly in CR4. The
|
|
reason for this is that older kernels make assumptions about the values in
|
|
the GS register and enforce them when GS base is set via
|
|
arch_prctl(). Allowing user space to write arbitrary values to GS base
|
|
would violate these assumptions and cause malfunction.
|
|
|
|
On kernels which do not enable FSGSBASE the execution of the FSGSBASE
|
|
instructions will fault with a #UD exception.
|
|
|
|
The kernel provides reliable information about the enabled state in the
|
|
ELF AUX vector. If the HWCAP2_FSGSBASE bit is set in the AUX vector, the
|
|
kernel has FSGSBASE instructions enabled and applications can use them.
|
|
The following code example shows how this detection works::
|
|
|
|
#include <sys/auxv.h>
|
|
#include <elf.h>
|
|
|
|
/* Will be eventually in asm/hwcap.h */
|
|
#ifndef HWCAP2_FSGSBASE
|
|
#define HWCAP2_FSGSBASE (1 << 1)
|
|
#endif
|
|
|
|
....
|
|
|
|
unsigned val = getauxval(AT_HWCAP2);
|
|
|
|
if (val & HWCAP2_FSGSBASE)
|
|
printf("FSGSBASE enabled\n");
|
|
|
|
FSGSBASE instructions compiler support
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
GCC version 4.6.4 and newer provide instrinsics for the FSGSBASE
|
|
instructions. Clang 5 supports them as well.
|
|
|
|
=================== ===========================
|
|
_readfsbase_u64() Read the FS base register
|
|
_readfsbase_u64() Read the GS base register
|
|
_writefsbase_u64() Write the FS base register
|
|
_writegsbase_u64() Write the GS base register
|
|
=================== ===========================
|
|
|
|
To utilize these instrinsics <immintrin.h> must be included in the source
|
|
code and the compiler option -mfsgsbase has to be added.
|
|
|
|
Compiler support for FS/GS based addressing
|
|
-------------------------------------------
|
|
|
|
GCC version 6 and newer provide support for FS/GS based addressing via
|
|
Named Address Spaces. GCC implements the following address space
|
|
identifiers for x86:
|
|
|
|
========= ====================================
|
|
__seg_fs Variable is addressed relative to FS
|
|
__seg_gs Variable is addressed relative to GS
|
|
========= ====================================
|
|
|
|
The preprocessor symbols __SEG_FS and __SEG_GS are defined when these
|
|
address spaces are supported. Code which implements fallback modes should
|
|
check whether these symbols are defined. Usage example::
|
|
|
|
#ifdef __SEG_GS
|
|
|
|
long data0 = 0;
|
|
long data1 = 1;
|
|
|
|
long __seg_gs *ptr;
|
|
|
|
/* Check whether FSGSBASE is enabled by the kernel (HWCAP2_FSGSBASE) */
|
|
....
|
|
|
|
/* Set GS base to point to data0 */
|
|
_writegsbase_u64(&data0);
|
|
|
|
/* Access offset 0 of GS */
|
|
ptr = 0;
|
|
printf("data0 = %ld\n", *ptr);
|
|
|
|
/* Set GS base to point to data1 */
|
|
_writegsbase_u64(&data1);
|
|
/* ptr still addresses offset 0! */
|
|
printf("data1 = %ld\n", *ptr);
|
|
|
|
|
|
Clang does not provide the GCC address space identifiers, but it provides
|
|
address spaces via an attribute based mechanism in Clang 2.6 and newer
|
|
versions:
|
|
|
|
==================================== =====================================
|
|
__attribute__((address_space(256)) Variable is addressed relative to GS
|
|
__attribute__((address_space(257)) Variable is addressed relative to FS
|
|
==================================== =====================================
|
|
|
|
FS/GS based addressing with inline assembly
|
|
-------------------------------------------
|
|
|
|
In case the compiler does not support address spaces, inline assembly can
|
|
be used for FS/GS based addressing mode::
|
|
|
|
mov %fs:offset, %reg
|
|
mov %gs:offset, %reg
|
|
|
|
mov %reg, %fs:offset
|
|
mov %reg, %gs:offset
|