Manpages - arch_prctl.2

arch_prctl - set architecture-specific thread state

  #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).

*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.

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

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.

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

*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.

*mmap*(2), *modify_ldt*(2), *prctl*(2), *set_thread_area*(2)

AMD X86-64 Programmer’s manual

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