MODIFY_LDT
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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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 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 long 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
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
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 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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