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

       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.

       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.

       Standard C Library (libc, -lc)

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

       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.

       i386 Microprocessor Programmer's Reference Manual, Intel

#include<machine/segments.h>#include<machine/sysarch.h>inti386_get_ldt(intstart_sel, uniondescriptor*descs, intnum_sels);

       inti386_set_ldt(intstart_sel, uniondescriptor*descs, intnum_sels);

       You can really hose your process using this.

Debian                                          October 14, 2006                                 I386_GET_LDT(2)