NAME

       arch_prctl - set architecture-specific thread state

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);

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.

       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

       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 entries.

       As of version 2.7, glibc provides no prototype for arch_prctl().  You have to  declare  it  yourself  for
       now.  This may be fixed in future glibc versions.

       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

COLOPHON

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