💾 Archived View for gmi.noulin.net › man › man2 › modify_ldt.2.gmi captured on 2024-08-31 at 13:39:22. Gemini links have been rewritten to link to archived content

View Raw

More Information

⬅️ Previous capture (2022-06-12)

-=-=-=-=-=-=-

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

NAME
       modify_ldt - get or set a per-process LDT entry

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,
                   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.

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

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)

Linux                                                                          2021-03-22                                                                  MODIFY_LDT(2)