arch_prctl(2) - Linux manual page (original) (raw)

archprctl(2) System Calls Manual archprctl(2)

NAME top

   arch_prctl - set architecture-specific thread state

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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)](../man2/syscall.2.html).

DESCRIPTION top

   **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)](../man2/fork.2.html) and
          [clone(2)](../man2/clone.2.html) but reset to the default (i.e., _cpuid_ enabled) on
          [execve(2)](../man2/execve.2.html).

   **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 top

   On success, **arch_prctl**() returns 0; on error, -1 is returned, and
   _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   **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 top

   Linux/x86-64.

NOTES top

   **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)](../man2/set%5Fthread%5Farea.2.html) 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)](../man2/mmap.2.html) with the **MAP_32BIT** flag.

   Because of the aforementioned optimization, using **arch_prctl**() and
   [set_thread_area(2)](../man2/set%5Fthread%5Farea.2.html) 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.

SEE ALSO top

   [mmap(2)](../man2/mmap.2.html), [modify_ldt(2)](../man2/modify%5Fldt.2.html), [prctl(2)](../man2/prctl.2.html), [set_thread_area(2)](../man2/set%5Fthread%5Farea.2.html)

   AMD X86-64 Programmer's manual

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Linux man-pages 6.10 2024-07-23 archprctl(2)

Pages that refer to this page:clone(2), modify_ldt(2), set_thread_area(2), syscalls(2)