Man page - arch_prctl(2)

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Manual

arch_prctl

NAME
LIBRARY
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
STANDARDS
NOTES
SEE ALSO

NAME

arch_prctl - set architecture-specific thread state

LIBRARY

Standard C library ( libc , -lc )

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 op , unsigned long addr );
int syscall(SYS_arch_prctl, int op , unsigned long * addr );

Note : glibc provides no wrapper for arch_prctl (), necessitating the use of syscall (2).

DESCRIPTION

arch_prctl () sets architecture-specific process or thread state. op selects an operation 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		op is not a valid operation.
	ENODEV		ARCH_SET_CPUID was requested, but the underlying hardware does not support CPUID faulting.
	EPERM		addr is outside the process address space.

STANDARDS

Linux/x86-64.

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.