|ARCH_PRCTL(2)||Linux Programmer's Manual||ARCH_PRCTL(2)|
#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).
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.
- addr points to an unmapped address or is outside the process address space.
- code is not a valid subcommand.
- ARCH_SET_CPUID was requested, but the underlying hardware does not support CPUID faulting.
- addr is outside the process address space.
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.
AMD X86-64 Programmer's manual