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