| .\" Copyright (C) 2003 Andi Kleen |
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| .TH ARCH_PRCTL 2 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| arch_prctl \- set architecture-specific thread state |
| .SH SYNOPSIS |
| .nf |
| .BR "#include <asm/prctl.h>" " /* Definition of " ARCH_* " constants */" |
| .BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */" |
| .B #include <unistd.h> |
| .PP |
| .BI "int syscall(SYS_arch_prctl, int " code ", unsigned long " addr ); |
| .BI "int syscall(SYS_arch_prctl, int " code ", unsigned long *" addr ); |
| .fi |
| .SH DESCRIPTION |
| .BR arch_prctl () |
| sets architecture-specific process or thread state. |
| .I code |
| selects a subfunction |
| and passes argument |
| .I addr |
| to it; |
| .I addr |
| is interpreted as either an |
| .I "unsigned long" |
| for the "set" operations, or as an |
| .IR "unsigned long\ *" , |
| for the "get" operations. |
| .PP |
| Subfunctions for both x86 and x86-64 are: |
| .TP |
| .BR ARCH_SET_CPUID " (since Linux 4.12)" |
| .\" commit e9ea1e7f53b852147cbd568b0568c7ad97ec21a3 |
| Enable |
| .RI ( "addr != 0" ) |
| or disable |
| .RI ( "addr == 0" ) |
| the |
| .I cpuid |
| instruction for the calling thread. |
| The instruction is enabled by default. |
| If disabled, any execution of a |
| .I cpuid |
| instruction will instead generate a |
| .B SIGSEGV |
| signal. |
| This feature can be used to emulate |
| .I cpuid |
| results that differ from what the underlying |
| hardware would have produced (e.g., in a paravirtualization setting). |
| .IP |
| The |
| .BR ARCH_SET_CPUID |
| setting is preserved across |
| .BR fork (2) |
| and |
| .BR clone (2) |
| but reset to the default (i.e., |
| .I cpuid |
| enabled) on |
| .BR execve (2). |
| .TP |
| .BR ARCH_GET_CPUID " (since Linux 4.12)" |
| Return the setting of the flag manipulated by |
| .B ARCH_SET_CPUID |
| as the result of the system call (1 for enabled, 0 for disabled). |
| .I addr |
| is ignored. |
| .TP |
| Subfunctions for x86-64 only are: |
| .TP |
| .B ARCH_SET_FS |
| Set the 64-bit base for the |
| .I FS |
| register to |
| .IR addr . |
| .TP |
| .B ARCH_GET_FS |
| Return the 64-bit base value for the |
| .I FS |
| register of the calling thread in the |
| .I unsigned long |
| pointed to by |
| .IR addr . |
| .TP |
| .B ARCH_SET_GS |
| Set the 64-bit base for the |
| .I GS |
| register to |
| .IR addr . |
| .TP |
| .B ARCH_GET_GS |
| Return the 64-bit base value for the |
| .I GS |
| register of the calling thread in the |
| .I unsigned long |
| pointed to by |
| .IR addr . |
| .SH RETURN VALUE |
| On success, |
| .BR arch_prctl () |
| returns 0; on error, \-1 is returned, and |
| .I errno |
| is set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EFAULT |
| .I addr |
| points to an unmapped address or is outside the process address space. |
| .TP |
| .B EINVAL |
| .I code |
| is not a valid subcommand. |
| .TP |
| .B EPERM |
| .I addr |
| is outside the process address space. |
| .TP |
| .B ENODEV |
| .B ARCH_SET_CPUID |
| was requested, but the underlying hardware does not support CPUID faulting. |
| .\" .SH AUTHOR |
| .\" Man page written by Andi Kleen. |
| .SH CONFORMING TO |
| .BR arch_prctl () |
| is a Linux/x86-64 extension and should not be used in programs intended |
| to be portable. |
| .SH NOTES |
| .BR arch_prctl () |
| is supported only on Linux/x86-64 for 64-bit programs currently. |
| .PP |
| The 64-bit base changes when a new 32-bit segment selector is loaded. |
| .PP |
| .B ARCH_SET_GS |
| is disabled in some kernels. |
| .PP |
| Context switches for 64-bit segment bases are rather expensive. |
| As an optimization, if a 32-bit TLS base address is used, |
| .BR arch_prctl () |
| may use a real TLS entry as if |
| .BR 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 |
| .BR mmap (2) |
| with the |
| .B MAP_32BIT |
| flag. |
| .PP |
| Because of the aforementioned optimization, using |
| .BR arch_prctl () |
| and |
| .BR set_thread_area (2) |
| in the same thread is dangerous, as they may overwrite each other's |
| TLS entries. |
| .PP |
| .I FS |
| may be already used by the threading library. |
| Programs that use |
| .B ARCH_SET_FS |
| directly are very likely to crash. |
| .SH SEE ALSO |
| .BR mmap (2), |
| .BR modify_ldt (2), |
| .BR prctl (2), |
| .BR set_thread_area (2) |
| .PP |
| AMD X86-64 Programmer's manual |