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

       modify_ldt - get or set a per-process LDT entry

       #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.

       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 In-
       tel  Software  Developer's  Manual  or the AMD Architecture Programming

       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 int  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 de-
       scriptions 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.

       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.

       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.

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

       Glibc  does  not  provide a wrapper for this system call; call it using

       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)  in-
       stead, except on extremely old kernels that do not support those system

       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,

       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.

       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.

       arch_prctl(2), set_thread_area(2), vm86(2)

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Linux                             2020-02-09                     MODIFY_LDT(2)
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