NAME

       modify_ldt - get or set a per-process LDT entry

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <asm/ldt.h>         /* Definition of struct user_desc */
       #include <sys/syscall.h>     /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_modify_ldt, int func, void ptr[.bytecount],
                   unsigned long bytecount);

       Note: glibc provides no wrapper for modify_ldt(), necessitating the use of syscall(2).

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 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, 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.

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.

STANDARDS

       Linux.

NOTES

       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 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
       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)