modify_ldt(2) System Calls Manual modify_ldt(2)
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 refer-
enced by user code. Linux allows processes to configure a per-process
(actually per-mm) LDT. For more information about the LDT, see the In-
tel 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, mod-
ify_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 byte-
count 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 de-
scriptions 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 set-
ting 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
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) in-
stead, 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 ker-
nels 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 man-pages 6.03 2022-11-10 modify_ldt(2)
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