i386_get_ldt(2)

NAME

i386_get_ldt, i386_set_ldt – manage i386 per-process Local Descriptor Table entries

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <machine/segments.h>

#include <machine/sysarch.h>

int

i386_get_ldt(int start_sel, union descriptor *descs, int num_sels);

int

i386_set_ldt(int start_sel, union descriptor *descs, int num_sels);

DESCRIPTION

The i386_get_ldt() system call will return the list of i386 descriptors that the process has in its LDT. The i386_set_ldt() system call will set a list of i386 descriptors for the current process in its LDT. Both routines accept a starting selector number start_sel, an array of memory that will contain the descriptors to be set or returned descs, and the number of entries to set or return num_sels.

The argument descs can be either segment_descriptor or gate_descriptor and are defined in ⟨i386/segments.h⟩. These structures are defined by the architecture as disjoint bit-fields, so care must be taken in constructing them.

If start_sel is LDT_AUTO_ALLOC, num_sels is 1 and the descriptor pointed to by descs is legal, then i386_set_ldt() will allocate a descriptor and return its selector number.

If num_descs is 1, start_sels is valid, and descs is NULL, then i386_set_ldt() will free that descriptor (making it available to be reallocated again later).

If num_descs is 0, start_sels is 0 and descs is NULL then, as a special case, i386_set_ldt() will free all descriptors.

RETURN VALUES

Upon successful completion, i386_get_ldt() returns the number of descriptors currently in the LDT. The i386_set_ldt() system call returns the first selector set. In the case when a descriptor is allocated by the kernel, its number will be returned. Otherwise, a value of -1 is returned and the global variable errno is set to indicate the error.

ERRORS

The i386_get_ldt() and i386_set_ldt() system calls will fail if:

[EINVAL]: An inappropriate value was used for start_sel or num_sels.
[EACCES]: The caller attempted to use a descriptor that would circumvent protection or cause a failure.

WARNING

You can really hose your process using this.