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I386_GET_LDT(2) System Calls Manual I386_GET_LDT(2)


i386_get_ldt, i386_set_ldtmanage i386 per-process Local Descriptor Table entries


Standard C Library (libc, -lc)


#include <machine/segments.h>
#include <machine/sysarch.h>
i386_get_ldt(int start_sel, union descriptor *descs, int num_sels);
i386_set_ldt(int start_sel, union descriptor *descs, int num_sels);


The i386_get_ldt() system call returns a list of the i386 descriptors in the current process' LDT. The i386_set_ldt() system call sets a list of i386 descriptors in the current process' LDT. For both routines, start_sel specifies the index of the selector in the LDT at which to begin and descs points to an array of num_sels descriptors to be set or returned.
Each entry in the descs array can be either a 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.


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 on success. If the kernel allocated a descriptor in the LDT, the allocated index is 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:
An inappropriate value was used for start_sel or num_sels.
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.
October 14, 2006 Debian