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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | NOTES | BUGS | SEE ALSO | COLOPHON |
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modify_ldt(2) System Calls Manual modify_ldt(2)
modify_ldt - get or set a per-process LDT entry
Standard C library (libc, -lc)
#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).
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.
Linux.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-05-02 modify_ldt(2)
Pages that refer to this page: arch_prctl(2), set_thread_area(2), syscalls(2)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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