arch_prctl - set architecture-specific thread state

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 unsignedlong for the "set" operations, or as an unsignedlong*, 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 unsignedlong 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 unsignedlong pointed to by addr.

EFAULTaddr points to an unmapped address or is outside the process address space. EINVALop is not a valid operation. ENODEVARCH_SET_CPUID was requested, but the underlying hardware does not support CPUID faulting. EPERMaddr is outside the process address space.

Standard C library (libc, -lc)

arch_prctl - set architecture-specific thread state

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.

On success, arch_prctl() returns 0; on error, -1 is returned, and errno is set to indicate the error.

mmap(2), modify_ldt(2), prctl(2), set_thread_area(2) AMD X86-64 Programmer's manual Linux man-pages 6.9.1 2024-05-02 arch_prctl(2)

Linux/x86-64.

#include<asm/prctl.h> /* Definition of ARCH_* constants */ #include<sys/syscall.h> /* Definition of SYS_* constants */ #include<unistd.h>intsyscall(SYS_arch_prctl,intop,unsignedlongaddr);intsyscall(SYS_arch_prctl,intop,unsignedlong*addr);Note: glibc provides no wrapper for arch_prctl(), necessitating the use of syscall(2).