mirror of https://github.com/mkerrisk/man-pages
modify_ldt.2: Overhaul the documentation
This clarifies the behavior and documents all four functions. Signed-off-by: Andy Lutomirski <luto@amacapital.net> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Andy Lutomirski
Michael Kerrisk
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commit
38f7c37983
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@ -1,4 +1,5 @@
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.\" Copyright (c) 1995 Michael Chastain (mec@duracef.shout.net), 22 July 1995.
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.\" Copyright (c) 2015 Andrew Lutomirski
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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@ -23,37 +24,43 @@
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.\"
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.TH MODIFY_LDT 2 2012-07-13 "Linux" "Linux Programmer's Manual"
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.SH NAME
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modify_ldt \- get or set ldt
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modify_ldt \- get or set a per-process LDT entry
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.SH SYNOPSIS
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.nf
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.B #include <sys/types.h>
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.sp
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.BI "int modify_ldt(int " "func" ", void *" "ptr" ", unsigned long " "bytecount" );
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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 modify_ldt ()
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reads or writes the local descriptor table (ldt) for a process.
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The ldt is a per-process memory management table used by the i386 processor.
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For more information on this table, see an Intel 386 processor handbook.
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reads or writes the local descriptor table (LDT) for a process. The LDT
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is an array of segment descriptors that can be referenced by user code.
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Linux allows processes to configure a per-process (actually per-mm) LDT.
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For more information about the LDT, see the Intel Software Developer's
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Manual or the AMD Architecture Programming Manual.
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.PP
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When
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.I func
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is 0,
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.BR modify_ldt ()
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reads the ldt into the memory pointed to by
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reads the LDT into the memory pointed to by
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.IR ptr .
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The number of bytes read is the smaller of
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.I bytecount
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and the actual size of the ldt.
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and the actual size of the LDT, although the kernel may act as though
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the LDT is padded with additional trailing zero bytes. On success,
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.BR modify_ldt ()
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will return the number of bytes read.
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.PP
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When
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.I func
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is 1,
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is 1 or 0x11,
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.BR modify_ldt ()
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modifies one ldt entry.
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modifies the LDT entry indicated by
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.I ptr\->entry_number.
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.I ptr
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points to a
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.I user_desc
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@ -61,11 +68,6 @@ structure
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and
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.I bytecount
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must equal the size of this structure.
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.\"
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.\" FIXME Should this page say something about func == 2 and func == 0x11?
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.\" In Linux 2.4, func == 2 returned "the default ldt"
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.\" In Linux 2.6, func == 2 is a nop, returning a zeroed out structure.
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.\" Linux 2.4 and 2.6 implement an operation for func == 0x11
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The
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.I user_desc
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@ -89,12 +91,44 @@ struct user_desc {
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.PP
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In Linux 2.4 and earlier, this structure was named
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.IR modify_ldt_ldt_s .
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.\" .PP
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.\" The ldt is specific for the calling process. Any attempts to change
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.\" the ldt to include the address space of another process or the kernel
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.\" will result in a segmentation violation when trying to access the memory
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.\" outside of the process address space. The memory protection is enforced
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.\" at the paging layer.
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.PP
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The
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.I contents
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field is the segment type (data, expand-down data, non-conforming code, or
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conforming code). The other fields match their descriptions in the
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CPU manual, although
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.BR modify_ldt ()
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cannot set the accessed bit.
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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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An LDT entry can be cleared by setting it to an "empty"
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.I user_desc
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or, if
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.I func
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is 1, by setting both
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.I base
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and
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.I limit
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to 0.
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.PP
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A conforming code segment will be rejected if
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.I
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func
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is 1 or if
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.I seg_not_present
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is 0.
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.PP
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When
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.I func
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is 2,
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.BR modify_ldt ()
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will read zeros. This appears to be a leftover from Linux 2.4.
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.SH RETURN VALUE
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On success,
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.BR modify_ldt ()
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@ -119,19 +153,40 @@ or
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is 1 and
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.I bytecount
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is not equal to the size of the structure
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.IR modify_ldt_ldt_s ,
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.IR user_desc ,
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or
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.I func
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is 1 and the new ldt entry has invalid values.
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is 1 or 0x11 and the new ldt entry has invalid values.
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.TP
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.B ENOSYS
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.I func
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is neither 0 nor 1.
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is neither 0, 1, 2, nor 0x11.
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.SH CONFORMING TO
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This call is Linux-specific and should not be used in programs intended
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to be portable.
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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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if set, would prevent the descriptor from being considered empty.
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.SH NOTES
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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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.BR modify_ldt ()
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should not be used for thread-local storage, as it slows down context
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switches and only supports a limited number of threads. Threading libraries
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should use
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.BR set_thread_area (2)
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or
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.BR arch_prctl (2)
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instead, except on extremely old kernels that do not support those system
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calls.
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.PP
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The normal use for
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.BR modify_ldt (2)
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is to run legacy 16-bit or segmented 32-bit code. Not all kernels allow
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16-bit segments to be installed, however.
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.SH SEE ALSO
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.BR set_thread_area (2),
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.BR arch_prctl (2),
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.BR vm86 (2)
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