modify_ldt(2) - Linux manual page (original) (raw)
modifyldt(2) System Calls Manual modifyldt(2)
NAME top
modify_ldt - get or set a per-process LDT entryLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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)](../man2/syscall.2.html).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->entrynumber_. _ptr_ points to a _userdesc_
structure and _bytecount_ must equal the size of this structure.
The _userdesc_ 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
_modifyldtldts_.
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 _userdesc_ is considered "empty" if _readexeconly_ and
_segnotpresent_ are set to 1 and all of the other fields are 0.
An LDT entry can be cleared by setting it to an "empty" _userdesc_
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 _segnotpresent_ 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](../man3/errno.3.html)_ 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 _userdesc_, or _func_ is 1 or 0x11 and
the new LDT entry has invalid values.
**ENOSYS** _func_ is neither 0, 1, 2, nor 0x11.STANDARDS top
Linux.NOTES top
**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)](../man2/set%5Fthread%5Farea.2.html) or
[arch_prctl(2)](../man2/arch%5Fprctl.2.html) 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 _userdesc_ 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
_userdesc_ 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)](../man2/arch%5Fprctl.2.html), [set_thread_area(2)](../man2/set%5Fthread%5Farea.2.html), [vm86(2)](../man2/vm86.2.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 modifyldt(2)
Pages that refer to this page:arch_prctl(2), set_thread_area(2), syscalls(2)