[QUESTION] why redhat and others disable lcall7/lcall27?

[Kirill Korotaev]

All redhat kernels contain the patch given below which disables 
lcall7/lcall27. Why? I've heared from some people that these calls are 
insecure or something like that. But what is the real problem with it?
Why mainstream kernel still keeps these calls then?

Kirill

diff -urNp linux-1130/arch/i386/kernel/traps.c 
linux-10000/arch/i386/kernel/traps.c
--- linux-1130/arch/i386/kernel/traps.c
+++ linux-10000/arch/i386/kernel/traps.c
@@ -1021,9 +1021,10 @@ void __init trap_init(void)
         * default LDT is a single-entry callgate to lcall7 for iBCS
         * and a callgate to lcall27 for Solaris/x86 binaries
         */
+#if 0
        set_call_gate(&default_ldt[0],lcall7);
        set_call_gate(&default_ldt[4],lcall27);
-
+#endif
        /*
         * Should be a barrier for any external CPU state.
         */

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