ARCH_PRCTL(2)                                                           Linux Programmer's Manual                                                          ARCH_PRCTL(2)

NAME
       arch_prctl - set architecture-specific thread state

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 un‐
       signed 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  execu‐
              tion  of a cpuid instruction will instead generate a SIGSEGV signal.  This feature can be used to emulate cpuid results that differ from what the underly‐
              ing 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.

CONFORMING TO
       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 en‐
       tries.

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

SEE ALSO
       mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)

       AMD X86-64 Programmer's manual

Linux                                                                          2021-08-27                                                                  ARCH_PRCTL(2)