.. 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