I386_GET_LDT(2) BSD System Calls Manual I386_GET_LDT(2)
i386_get_ldt, i386_set_ldt -- manage i386 per-process Local Descriptor Table entries
Standard C Library (libc, -lc)
i386_get_ldt(int start_sel, union ldt_entry *descs, int num_sels);
i386_set_ldt(int start_sel, union ldt_entry *descs, int num_sels);
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 code_desc_t, data_desc_t or call_gate_t and are defined in <architecture/i386/desc.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 reallo-
cated 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.
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.
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.
i386 Microprocessor Programmer's Reference Manual, Intel
You can really hose your process using this.
September 20, 1993 BSD