MODIFY_LDT(2)             (2020-02-09)              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 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 Program-
          ming 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, 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 struc-
          ture.

          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

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          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.

     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 pro-
          grams 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 system
          calls.

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

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          segments to be installed, however.

          Even on 64-bit kernels, modify_ldt() cannot be used to cre-
          ate a long mode (i.e., 64-bit) code segment.  The undocu-
          mented 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 kernels will still notice
          the bit even when set in a 32-bit process.

     SEE ALSO
          arch_prctl(2), set_thread_area(2), vm86(2)

     COLOPHON
          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 https://www.kernel.org/doc/man-pages/.

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