arch_prctl(2) Linux Man Page

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

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.

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