arch_prctl(2) — Linux manual pages

Linux manual pages	Section 2

Name

arch_prctl — set architecture-specific thread state

Synopsis

#include <asm/prctl.h>         /* Definition of  ARCH_*  constants */
#include <sys/syscall.h>       /* Definition of  SYS_*  constants */
#include <unistd.h>

`int syscall(`	`SYS_arch_prctl`,
	int `code`,
	unsigned long `addr)`;

`int syscall(`	`SYS_arch_prctl`,
	int `code`,
	unsigned long *`addr)`;

	Note
	glibc provides no wrapper for

.BR arch_prctl (),
necessitating the use of

DESCRIPTION

arch_prctl() sets architecture-specific process or thread state. code selects a subfunction and passes argument addr to it; addr is interpreted as either an unsigned long for the "set" operations, or as an unsigned long *, for the "get" operations.

Subfunctions for both x86 and x86-64 are:

ARCH_SET_CPUID (since Linux 4.12)

Enable (addr != 0) or disable (addr == 0) the cpuid instruction for the calling thread. The instruction is enabled by default. If disabled, any execution of a cpuid instruction will instead generate a SIGSEGV signal. This feature can be used to emulate cpuid results that differ from what the underlying hardware would have produced (e.g., in a paravirtualization setting).

The ARCH_SET_CPUID setting is preserved across fork(2) and clone(2) but reset to the default (i.e., cpuid enabled) on execve(2).

ARCH_GET_CPUID (since Linux 4.12)

Return the setting of the flag manipulated by ARCH_SET_CPUID as the result of the system call (1 for enabled, 0 for disabled). addr is ignored.

Subfunctions for x86-64 only are:

ARCH_SET_FS

Set the 64-bit base for the FS register to addr.

ARCH_GET_FS

Return the 64-bit base value for the FS register of the calling thread in the unsigned long pointed to by addr.

ARCH_SET_GS

Set the 64-bit base for the GS register to addr.

ARCH_GET_GS

Return the 64-bit base value for the GS register of the calling thread in the unsigned long pointed to by addr.

RETURN VALUE

On success, arch_prctl() returns 0; on error, −1 is returned, and errno is set to indicate the error.

ERRORS

EFAULT: addr points to an unmapped address or is outside the process address space.
EINVAL: code is not a valid subcommand.
ENODEV: ARCH_SET_CPUID was requested, but the underlying hardware does not support CPUID faulting.
EPERM: addr is outside the process address space.

CONFORMING TO

arch_prctl() is a Linux/x86-64 extension and should not be used in programs intended to be portable.

NOTES

arch_prctl() is supported only on Linux/x86-64 for 64-bit programs currently.

The 64-bit base changes when a new 32-bit segment selector is loaded.

ARCH_SET_GS is disabled in some kernels.

Context switches for 64-bit segment bases are rather expensive. As an optimization, if a 32-bit TLS base address is used, arch_prctl() may use a real TLS entry as if set_thread_area(2) had been called, instead of manipulating the segment base register directly. Memory in the first 2 GB of address space can be allocated by using mmap(2) with the MAP_32BIT flag.

Because of the aforementioned optimization, using arch_prctl() and set_thread_area(2) in the same thread is dangerous, as they may overwrite each other's TLS entries.

FS may be already used by the threading library. Programs that use ARCH_SET_FS directly are very likely to crash.

COLOPHON

This page is part of release 5.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.

  Copyright (C) 2003 Andi Kleen

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