arch_prctl(2) System Calls Manual arch_prctl(2)

arch_prctl - set architecture-specific thread state

Standard C library (libc, -lc)

#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).

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:

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).
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.
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.

addr points to an unmapped address or is outside the process address space.
op is not a valid operation.
ARCH_SET_CPUID was requested, but the underlying hardware does not support CPUID faulting.
addr is outside the process address space.

Linux/x86-64.

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

2024-05-02 Linux man-pages 6.9.1