arch_prctl(2)                 System Calls Manual                arch_prctl(2)

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 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).

DESCRIPTION
       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) 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 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
       Linux/x86-64.

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.

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

       AMD X86-64 Programmer's manual

Linux man-pages 6.7               2024-03-03                     arch_prctl(2)