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       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:

              Set the 64-bit base for the FS register to addr.

              Return  the  64-bit  base  value  for  the FS register of the calling thread in the
              unsigned long pointed to by addr.

              Set the 64-bit base for the GS register to addr.

              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

       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


       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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