ARCH_PRCTL

NAME

SYNOPSIS

#include <asm/prctl.h>
#include <sys/prctl.h>

int arch_prctl(int code, unsigned long addr);
int arch_prctl(int code, unsigned long *addr);

Note: There is no glibc wrapper for this system call; see NOTES.  

DESCRIPTION

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

ERRORS

EFAULT

addr points to an unmapped address or is outside the process address space.

EINVAL

code is not a valid subcommand.

EPERM

addr is outside the process address space.

ENODEV

ARCH_SET_CPUID was requested, but the underlying hardware does not support CPUID faulting.

CONFORMING TO

NOTES

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.

Glibc does not provide a wrapper for this system call; call it using syscall(2).

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

SEE ALSO

AMD X86-64 Programmer's manual  

COLOPHON

Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

CONFORMING TO

NOTES

SEE ALSO

COLOPHON