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
       referenced 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

       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, modify_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 bytecount
       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
       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
       setting 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)
       instead, 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

       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
       kernels will still notice the bit even when set in a 32-bit process.

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

       This page is part of release 5.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2020-02-09                      MODIFY_LDT(2)