Jos-lab3 Exercise 7&9

Exercise 7.Add a handler in the kernelfor interrupt vectorT_SYSCALL.You will have to edit kern/trapentry.S and kern/trap.c'strap_init(). You also need to change trap_dispatch() to handle thesystem call interrupt by callingsyscall()(defined in kern/syscall.c)with the appropriate arguments,and then arranging for the return value to be passed back to the user processin%eax.Finally, you need to implement syscall() inkern/syscall.c.Make suresyscall() returns -E_INVAL if the system call number is invalid.You should read and understandlib/syscall.c(especially the inline assembly routine) in order to confirm your understanding of the system call interface.You may also find it helpful to readinc/syscall.h.

A: Need to be very careful when dealing kernel code.

At first, when I finished this exercise, running hello will cause a vicious cycle in printing "hello world" endless. After tracing the code, I found that the tf->tf_eip always point to the "__start" in user code, which means the curenv->tf wasn't set well.

The root cause is when I set the SYSCALL handler, it should be TRAPHANDLER_NOEC(), but I used the TRAPHANDLER(), so it pushed one more byte into the tf->tf_eip.

Exercise 9.Change kern/trap.c to panic if a page fault happens in kernel mode.

Hint: to determine whether a fault happened in user mode orin kernel mode, check the low bits of thetf_cs.

Read user_mem_assert in kern/pmap.c and implement user_mem_check in that same file.

Change kern/syscall.c to sanity check argumentsto system calls.

Boot your kernel, running user/buggyhello.The environment should be destroyed,and the kernel shouldnot panic.You should see:

[00001000] user_mem_check assertion failure for va 00000001[00001000] free env 00001000Destroyed the only environment - nothing more to do!

Finally, change debuginfo_eip inkern/kdebug.c to call user_mem_check onusd, stabs, andstabstr. If you now runuser/breakpoint, you should be able to runbacktrace from the kernel monitor and see thebacktrace traverse intolib/libmain.c before thekernel panics with a page fault. What causes this page fault?You don't need to fix it, but you should understand why ithappens.

 583 int
 584 user_mem_check(struct Env *env, const void *va, size_t len, int perm)
 585 {
 586         // LAB 3: Jacky 140125.
 587         uint32_t iter;
 588         pte_t *pte;
 589         if ((uint32_t)(va + len) >= ULIM) {
 590                 user_mem_check_addr = (uintptr_t)(va + len);
 591                 return -E_FAULT;
 592         }       
 593         for (iter = (uint32_t)va; iter < (uint32_t)ROUNDUP((va + len), PGSIZE)     ; iter += PGSIZE) {
 594                 pte = pgdir_walk(curenv->env_pgdir, (void *)iter, 0);
 595                 if ((*pte & perm) != perm) {
 596                         cprintf("Need perm = %x, but *pte is %x!\n", perm, *pt     e);                     
 597                         user_mem_check_addr = iter;
 598                         return -E_FAULT;
 599                 }
 600         }       
 601         
 602         return 0;
 603 }