mirror of https://github.com/mkerrisk/man-pages
231 lines
5.7 KiB
Groff
231 lines
5.7 KiB
Groff
.\" Copyright (C) 2003 Free Software Foundation, Inc.
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.\" Copyright (C) 2015 Andrew Lutomirski
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.\" Author: Kent Yoder
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.\"
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.\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
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.\" This file is distributed according to the GNU General Public License.
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.\" %%%LICENSE_END
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.\"
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.TH SET_THREAD_AREA 2 2021-03-22 "Linux" "Linux Programmer's Manual"
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.SH NAME
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get_thread_area, set_thread_area \- manipulate thread-local storage information
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.SH SYNOPSIS
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.nf
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.BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */"
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.B #include <unistd.h>
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.PP
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.B #if defined __i386__ || defined __x86_64__
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.BR "# include <asm/ldt.h>" " /* Definition of " "struct user_desc" " */"
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.PP
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.BI "int syscall(SYS_get_thread_area, struct user_desc *" u_info );
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.BI "int syscall(SYS_set_thread_area, struct user_desc *" u_info );
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.PP
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.B #elif defined __m68k__
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.PP
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.B "int syscall(SYS_get_thread_area);"
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.BI "int syscall(SYS_set_thread_area, unsigned long " tp );
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.PP
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.B #elif defined __mips__
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.PP
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.BI "int syscall(SYS_set_thread_area, unsigned long " addr );
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.PP
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.B #endif
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.fi
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.PP
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.IR Note :
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glibc provides no wrappers for these system calls,
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necessitating the use of
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.BR syscall (2).
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.SH DESCRIPTION
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These calls provide architecture-specific support for a thread-local storage
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implementation.
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At the moment,
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.BR set_thread_area ()
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is available on m68k, MIPS, and x86 (both 32-bit and 64-bit variants);
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.BR get_thread_area ()
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is available on m68k and x86.
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.PP
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On m68k and MIPS,
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.BR set_thread_area ()
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allows storing an arbitrary pointer (provided in the
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.B tp
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argument on m68k and in the
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.B addr
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argument on MIPS)
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in the kernel data structure associated with the calling thread;
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this pointer can later be retrieved using
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.BR get_thread_area ()
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(see also NOTES
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for information regarding obtaining the thread pointer on MIPS).
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.PP
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On x86, Linux dedicates three global descriptor table (GDT) entries for
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thread-local storage.
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For more information about the GDT, see the
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Intel Software Developer's Manual or the AMD Architecture Programming Manual.
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.PP
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Both of these system calls take an argument that is a pointer
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to a structure of the following type:
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.PP
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.in +4n
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.EX
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struct user_desc {
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unsigned int entry_number;
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unsigned int base_addr;
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unsigned int limit;
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unsigned int seg_32bit:1;
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unsigned int contents:2;
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unsigned int read_exec_only:1;
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unsigned int limit_in_pages:1;
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unsigned int seg_not_present:1;
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unsigned int useable:1;
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#ifdef __x86_64__
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unsigned int lm:1;
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#endif
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};
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.EE
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.in
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.PP
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.BR get_thread_area ()
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reads the GDT entry indicated by
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.I u_info\->entry_number
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and fills in the rest of the fields in
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.IR u_info .
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.PP
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.BR set_thread_area ()
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sets a TLS entry in the GDT.
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.PP
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The TLS array entry set by
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.BR set_thread_area ()
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corresponds to the value of
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.I u_info\->entry_number
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passed in by the user.
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If this value is in bounds,
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.BR set_thread_area ()
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writes the TLS descriptor pointed to by
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.I u_info
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into the thread's TLS array.
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.PP
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When
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.BR set_thread_area ()
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is passed an
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.I entry_number
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of \-1, it searches for a free TLS entry.
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If
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.BR set_thread_area ()
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finds a free TLS entry, the value of
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.I u_info\->entry_number
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is set upon return to show which entry was changed.
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.PP
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A
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.I user_desc
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is considered "empty" if
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.I read_exec_only
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and
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.I seg_not_present
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are set to 1 and all of the other fields are 0.
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If an "empty" descriptor is passed to
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.BR set_thread_area (),
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the corresponding TLS entry will be cleared.
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See BUGS for additional details.
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.PP
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Since Linux 3.19,
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.BR set_thread_area ()
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cannot be used to write non-present segments, 16-bit segments, or code
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segments, although clearing a segment is still acceptable.
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.SH RETURN VALUE
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On x86, these system calls
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return 0 on success, and \-1 on failure, with
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.I errno
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set to indicate the error.
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.PP
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On MIPS and m68k,
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.BR set_thread_area ()
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always returns 0.
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On m68k,
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.BR get_thread_area ()
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returns the thread area pointer value
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(previously set via
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.BR set_thread_area ()).
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.SH ERRORS
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.TP
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.B EFAULT
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\fIu_info\fP is an invalid pointer.
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.TP
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.B EINVAL
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\fIu_info\->entry_number\fP is out of bounds.
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.TP
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.B ENOSYS
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.BR get_thread_area ()
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or
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.BR set_thread_area ()
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was invoked as a 64-bit system call.
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.TP
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.B ESRCH
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.RB ( set_thread_area ())
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A free TLS entry could not be located.
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.SH VERSIONS
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.BR set_thread_area ()
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first appeared in Linux 2.5.29.
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.BR get_thread_area ()
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first appeared in Linux 2.5.32.
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.SH CONFORMING TO
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.BR set_thread_area ()
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and
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.BR get_thread_area ()
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are Linux-specific and should not be used in programs that are intended
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to be portable.
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.SH NOTES
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These system calls are generally intended for use only by threading libraries.
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.PP
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.BR arch_prctl (2)
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can interfere with
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.BR set_thread_area ()
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on x86.
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See
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.BR arch_prctl (2)
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for more details.
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This is not normally a problem, as
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.BR arch_prctl (2)
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is normally used only by 64-bit programs.
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.PP
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On MIPS, the current value of the thread area pointer can be obtained
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using the instruction:
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.PP
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.in +4n
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.EX
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rdhwr dest, $29
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.EE
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.in
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.PP
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This instruction traps and is handled by kernel.
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.SH BUGS
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On 64-bit kernels before Linux 3.19,
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.\" commit e30ab185c490e9a9381385529e0fd32f0a399495
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one of the padding bits in
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.IR user_desc ,
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if set, would prevent the descriptor from being considered empty (see
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.BR modify_ldt (2)).
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As a result, the only reliable way to clear a TLS entry is to use
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.BR memset (3)
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to zero the entire
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.I user_desc
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structure, including padding bits, and then to set the
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.I read_exec_only
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and
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.I seg_not_present
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bits.
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On Linux 3.19, a
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.I user_desc
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consisting entirely of zeros except for
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.I entry_number
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will also be interpreted as a request to clear a TLS entry, but this
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behaved differently on older kernels.
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.PP
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Prior to Linux 3.19, the DS and ES segment registers must not reference
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TLS entries.
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.SH SEE ALSO
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.BR arch_prctl (2),
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.BR modify_ldt (2),
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.BR ptrace (2)
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.RB ( PTRACE_GET_THREAD_AREA " and " PTRACE_SET_THREAD_AREA )
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