arch_prctl - Online Linux Manual Page

Section : 2
Updated : 2022-10-30
Source : Linux man-pages 6.03

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

code

, unsigned long

addr

);

int syscall(SYS_arch_prctl, int

code

, 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. 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.

STANDARDS

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.