16-bit code and L4

Valery V. Sedletski _valerius at mail.ru
Mon Jun 25 09:36:26 CEST 2007

On Mon, 25 Jun 2007 18:43:55 +1300 (MSD), Valery V. Sedletski wrote:

>Yes, I see:
>L4_INLINE void
>fiasco_gdt_set(void *desc, unsigned int size,
>               unsigned int entry_number_start, l4_threadid_t tid);
>It is privileged architecture-specific syscall, as I understood. And what means
>"sanity checking" by the kernel? Does the kernel simply check that the descriptor
>defines a valid  segment, or, there are some additional chacks. For example, must
>segments to be a non-overlapping, or not? (For 32-bit<-->16-bit thunking it is important
>to have a possibility for the segments to be overlapping -- the code can be at the same 
>time to be in 16-bit segment and 32-bit segment).

In  segment.h header, there is an implementation of this function, through using LLDT instruction. (and
not through using a special syscall). So, LLDT instruction is allowed for usermode programs, and the kernel
intercept lldt instruction. -- Probably when this instruction is used in usermode program, the TRAP occurs,
and the kernel handles this trap, modifying the LDT or GDT. Is this right? (I searched an fiasco_gdt_set() 
function in the source code and I found that L4Linux uses this function. So, it is, probably, was created
for demands of L4Linux development?)


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