Ubuntu Manpage: arch_prctl - set architecture-specific thread state

NAME

       arch_prctl - set architecture-specific thread state

LIBRARY

       Standard C library (libc, -lc)

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 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.

       ENODEV ARCH_SET_CPUID was requested, but the underlying hardware does  not  support  CPUID
              faulting.

       EPERM  addr is outside the process address space.

STANDARDS

       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.

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