mirror of https://github.com/mkerrisk/man-pages
set_thread_area.2: Various fixes for Andy Lutomirski's patch
Notably, various info about get_thread_area() got dropped during the merge of the two pages. Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk
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@ -10,15 +10,26 @@
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.SH NAME
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set_thread_area \- set a GDT entry for thread-local storage
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.SH SYNOPSIS
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.nf
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.B #include <linux/unistd.h>
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.br
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.B #include <asm/ldt.h>
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.sp
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.BI "int get_thread_area(struct user_desc *" u_info );
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.BI "int set_thread_area(struct user_desc *" u_info );
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.in +4n
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.nf
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.fi
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.IR Note :
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There are no glibc wrappers for these system calls; see NOTES.
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.SH DESCRIPTION
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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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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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.in +4n
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struct user_desc {
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unsigned int entry_number;
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unsigned long base_addr;
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@ -30,21 +41,13 @@ struct user_desc {
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unsigned int seg_not_present:1;
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unsigned int useable:1;
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};
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.fi
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.in
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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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Linux dedicates three global descriptor table (GDT) entries for
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thread-local storage. For more information about the LDT, see the
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Intel Software Developer's Manual or the AMD Architecture Programming Manual.
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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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.I u_info.
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.IR u_info .
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.BR set_thread_area ()
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sets a TLS entry in the GDT.
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@ -77,56 +80,76 @@ 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. If an "empty" descriptor
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is passed to
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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. See BUGS for additional
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details.
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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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On Linux 3.19 and newer,
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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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.BR set_thread_area ()
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returns 0 on success, and \-1 on failure, with
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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 appropriately.
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.SH ERRORS
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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 EFAULT
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\fIu_info\fP is an invalid pointer.
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.TP
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.B ESRCH
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A free TLS entry could not be located.
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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 (2)
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or
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.BR set_thread_area (2)
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was invoked as a 64-bit syscall.
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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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A version of
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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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is 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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Glibc does not provide wrappers for these system calls,
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since they are generally intended for use only by threading libraries.
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In the unlikely event that you want to call them directly, use
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.BR syscall (2).
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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 (2).
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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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.SH BUGS
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On 64-bit kernels before Linux 3.19, one of the padding bits in
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.I user_desc,
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.IR user_desc ,
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if set, would prevent the descriptor from being considered empty.
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As a result, the only reliable way to clear a TLS entry is to use
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memset to zero the entire
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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. On Linux 3.19, a
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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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@ -135,20 +158,6 @@ 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 NOTES
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Glibc does not provide a wrapper for this system call,
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since it is generally intended only for use by threading libraries.
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In the unlikely event that you want to call it directly, use
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.BR syscall (2).
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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 (2).
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See
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.BR arch_prctl (2)
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for more details. This is not normally a problem, as
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.BR arch_prctl (2)
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is normally only used by 64-bit programs.
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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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