On Mon Jun 25, 2007 at 20:36:26 +1300, Valery V. Sedletski wrote:
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.
This is defined per thread, so it's not privileged.
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).
The kernel makes sure that the PL is 3 and that the present bit is set. In particular it doesn't care about base or limits.
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?
Yes, just take lldt as a sycall for user land.
(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?)
Yes, the current interface was made in a way to make it work smoothly with Linux. We didn't have another user yet.
Adam