mirror of https://github.com/mkerrisk/man-pages
195 lines
4.8 KiB
Groff
195 lines
4.8 KiB
Groff
.\" Copyright (C) 2003 Andi Kleen
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH ARCH_PRCTL 2 2020-04-11 "Linux" "Linux Programmer's Manual"
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.SH NAME
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arch_prctl \- set architecture-specific thread state
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.SH SYNOPSIS
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.nf
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.B #include <asm/prctl.h>
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.B #include <sys/prctl.h>
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.PP
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.BI "int arch_prctl(int " code ", unsigned long " addr );
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.BI "int arch_prctl(int " code ", unsigned long *" addr );
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.fi
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.PP
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.IR Note :
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There is no glibc wrapper for this system call; see NOTES.
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.SH DESCRIPTION
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.BR arch_prctl ()
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sets architecture-specific process or thread state.
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.I code
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selects a subfunction
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and passes argument
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.I addr
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to it;
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.I addr
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is interpreted as either an
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.I "unsigned long"
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for the "set" operations, or as an
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.IR "unsigned long\ *" ,
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for the "get" operations.
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.PP
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Subfunctions for both x86 and x86-64 are:
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.TP
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.BR ARCH_SET_CPUID " (since Linux 4.12)"
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.\" commit e9ea1e7f53b852147cbd568b0568c7ad97ec21a3
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Enable
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.RI ( "addr != 0" )
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or disable
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.RI ( "addr == 0" )
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the
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.I cpuid
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instruction for the calling thread.
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The instruction is enabled by default.
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If disabled, any execution of a
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.I cpuid
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instruction will instead generate a
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.B SIGSEGV
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signal.
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This feature can be used to emulate
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.I cpuid
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results that differ from what the underlying
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hardware would have produced (e.g., in a paravirtualization setting).
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.IP
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The
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.BR ARCH_SET_CPUID
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setting is preserved across
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.BR fork (2)
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and
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.BR clone (2)
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but reset to the default (i.e.,
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.I cpuid
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enabled) on
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.BR execve (2).
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.TP
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.BR ARCH_GET_CPUID " (since Linux 4.12)"
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Return the setting of the flag manipulated by
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.B ARCH_SET_CPUID
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as the result of the system call (1 for enabled, 0 for disabled).
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.I addr
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is ignored.
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.TP
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Subfunctions for x86-64 only are:
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.TP
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.B ARCH_SET_FS
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Set the 64-bit base for the
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.I FS
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register to
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.IR addr .
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.TP
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.B ARCH_GET_FS
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Return the 64-bit base value for the
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.I FS
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register of the calling thread in the
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.I unsigned long
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pointed to by
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.IR addr .
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.TP
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.B ARCH_SET_GS
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Set the 64-bit base for the
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.I GS
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register to
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.IR addr .
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.TP
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.B ARCH_GET_GS
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Return the 64-bit base value for the
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.I GS
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register of the calling thread in the
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.I unsigned long
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pointed to by
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.IR addr .
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.SH RETURN VALUE
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On success,
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.BR arch_prctl ()
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returns 0; on error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EFAULT
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.I addr
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points to an unmapped address or is outside the process address space.
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.TP
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.B EINVAL
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.I code
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is not a valid subcommand.
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.TP
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.B EPERM
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.I addr
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is outside the process address space.
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.TP
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.B ENODEV
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.B ARCH_SET_CPUID
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was requested, but the underlying hardware does not support CPUID faulting.
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.\" .SH AUTHOR
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.\" Man page written by Andi Kleen.
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.SH CONFORMING TO
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.BR arch_prctl ()
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is a Linux/x86-64 extension and should not be used in programs intended
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to be portable.
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.SH NOTES
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.BR arch_prctl ()
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is supported only on Linux/x86-64 for 64-bit programs currently.
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.PP
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The 64-bit base changes when a new 32-bit segment selector is loaded.
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.PP
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.B ARCH_SET_GS
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is disabled in some kernels.
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.PP
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Context switches for 64-bit segment bases are rather expensive.
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As an optimization, if a 32-bit TLS base address is used,
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.BR arch_prctl ()
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may use a real TLS entry as if
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.BR set_thread_area (2)
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had been called, instead of manipulating the segment base register directly.
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Memory in the first 2\ GB of address space can be allocated by using
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.BR mmap (2)
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with the
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.B MAP_32BIT
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flag.
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.PP
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Because of the aforementioned optimization, using
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.BR arch_prctl ()
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and
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.BR set_thread_area (2)
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in the same thread is dangerous, as they may overwrite each other's
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TLS entries.
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.PP
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Glibc does not provide a wrapper for this system call; call it using
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.BR syscall (2).
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.PP
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.I FS
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may be already used by the threading library.
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Programs that use
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.B ARCH_SET_FS
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directly are very likely to crash.
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.SH SEE ALSO
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.BR mmap (2),
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.BR modify_ldt (2),
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.BR prctl (2),
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.BR set_thread_area (2)
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.PP
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AMD X86-64 Programmer's manual
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