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MODIFY_LDT(2) Linux Programmer's Manual MODIFY_LDT(2)

NAME

     modify_ldt - get or set a per-process LDT entry

SYNOPSIS

     #include <sys/types.h>

     int modify_ldt(int func, void *ptr, unsigned long bytecount);

     Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

     modify_ldt()  reads  or  writes  the local descriptor table (LDT) for a
     process.  The LDT is an array of segment descriptors that can be refer-
     enced  by user code.  Linux allows processes to configure a per-process
     (actually per-mm) LDT.  For more information about  the  LDT,  see  the
     Intel  Software  Developer's Manual or the AMD Architecture Programming
     Manual.

     When func is 0, modify_ldt() reads the LDT into the memory  pointed  to
     by  ptr.   The number of bytes read is the smaller of bytecount and the
     actual size of the LDT, although the kernel may act as though  the  LDT
     is  padded  with  additional  trailing  zero  bytes.   On success, mod-
     ify_ldt() will return the number of bytes read.

     When func is 1 or 0x11, modify_ldt() modifies the LDT  entry  indicated
     by  ptr->entry_number.   ptr  points to a user_desc structure and byte-
     count must equal the size of this structure.

     The user_desc structure is defined in <asm/ldt.h> as:

         struct user_desc {
             unsigned int  entry_number;
             unsigned long base_addr;
             unsigned int  limit;
             unsigned int  seg_32bit:1;
             unsigned int  contents:2;
             unsigned int  read_exec_only:1;
             unsigned int  limit_in_pages:1;
             unsigned int  seg_not_present:1;
             unsigned int  useable:1; };

     In Linux 2.4 and earlier, this structure was named modify_ldt_ldt_s.

     The contents field is the segment type (data,  expand-down  data,  non-
     conforming  code,  or  conforming  code).  The other fields match their
     descriptions in the CPU manual, although modify_ldt()  cannot  set  the
     hardware-defined "accessed" bit described in the CPU manual.

     A user_desc is considered "empty" if read_exec_only and seg_not_present
     are set to 1 and all of the other fields are 0.  An LDT  entry  can  be
     cleared by setting it to an "empty" user_desc or, if func is 1, by set-
     ting both base and limit to 0.

     A conforming code segment (i.e., one with contents==3) will be rejected
     if func is 1 or if seg_not_present is 0.

     When  func  is  2,  modify_ldt() will read zeros.  This appears to be a
     leftover from Linux 2.4.

RETURN VALUE

     On success, modify_ldt() returns either the actual number of bytes read
     (for  reading) or 0 (for writing).  On failure, modify_ldt() returns -1
     and sets errno to indicate the error.

ERRORS

     EFAULT ptr points outside the address space.

     EINVAL ptr is 0, or func is 1 and bytecount is not equal to the size of
            the  structure  user_desc,  or func is 1 or 0x11 and the new LDT
            entry has invalid values.

     ENOSYS func is neither 0, 1, 2, nor 0x11.

CONFORMING TO

     This call is Linux-specific and should not be used in programs intended
     to be portable.

NOTES

     Glibc  does  not  provide a wrapper for this system call; call it using
     syscall(2).

     modify_ldt() should not be used for thread-local storage, as  it  slows
     down  context  switches  and only supports a limited number of threads.
     Threading libraries  should  use  set_thread_area(2)  or  arch_prctl(2)
     instead, except on extremely old kernels that do not support those sys-
     tem calls.

     The normal use for modify_ldt() is to run legacy  16-bit  or  segmented
     32-bit  code.   Not  all kernels allow 16-bit segments to be installed,
     however.

     Even on 64-bit kernels, modify_ldt() cannot be used to  create  a  long
     mode  (i.e.,  64-bit)  code  segment.   The  undocumented field "lm" in
     user_desc is not useful, and, despite its name, does not  result  in  a
     long mode segment.

BUGS

     On  64-bit kernels before Linux 3.19, setting the "lm" bit in user_desc
     prevents the descriptor from being considered empty.  Keep in mind that
     the "lm" bit does not exist in the 32-bit headers, but these buggy ker-
     nels will still notice the bit even when set in a 32-bit process.