modify_ldt(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       modify_ldt - get or set a per-process LDT entry

SYNOPSIS         top

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

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

DESCRIPTION         top

       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         top

       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         top

       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         top

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

NOTES         top

       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 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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.11 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/.

Linux                          2021-03-22                  MODIFY_LDT(2)

Pages that refer to this page: arch_prctl(2)set_thread_area(2)syscalls(2)