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 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.
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
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.
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)