Jos-lab5 (上)

Exercise 1.i386_init identifies the file system environment bypassing the typeENV_TYPE_FS to your environment creationfunction,env_create.Modifyenv_create in env.c,so that it gives the file system environment I/O privilege,but never gives that privilege to any other environment.

在env_create()中加入如下的代码

417         if (type == ENV_TYPE_FS) {
418                 e->env_tf.tf_eflags = e->env_tf.tf_eflags | FL_IOPL_3;
419         }   
我原本直接write_eflags(),其实这只是改变了当前在kernel下的访问权限，并没有对这个enviroment造成影响。

Exercise 2.Implement the bc_pgfault functions infs/bc.c.bc_pgfault is a page fault handler, just like the oneyour wrote in the previous lab for copy-on-write fork, except thatits job is to load pages in from the disk in response to a pagefault. When writing this, keep in mind that (1)addrmay not be aligned to a block boundary and (2) ide_readoperates in sectors, not blocks.

 36         addr = ROUNDDOWN(addr, PGSIZE);
 37         if ((r = sys_page_alloc(0, addr, PTE_W|PTE_U|PTE_P))<0)
 38                 panic("%s, sys_page_alloc error: %e",__func__,r);
 39         ide_read(blockno*BLKSECTS,addr,BLKSECTS);

原本用了page_alloc() & page_insert(),可是这是kernel里的API，在用户空间必须透过包装过sys call: sys_page_alloc()来调用。

Exercise 3.spawn relies on the new syscallsys_env_set_trapframe to initialize the state of thenewly created environment. Implementsys_env_set_trapframe. Test your code by running theuser/spawnhello programfrom kern/init.c, which will attempt tospawn /hello from the file system.

145         struct Env *e;
146         int r = envid2env(envid, &e, 1);
147         if (r < 0){
148                 cprintf("%s(),%d: return %d!\n",__func__,__LINE__,r);
149                 return r;
150         }
152         user_mem_check(e, tf, sizeof(struct Trapframe), PTE_U);
153         e->env_tf = *tf;
154         e->env_tf.tf_cs = GD_UT | 3;
155         e->env_tf.tf_eflags |= FL_IF;
156         return 0;

不过在测试的时候一直出现了open()->ipv_send()->sys_page_map()->envid2env()这个函数的权限检查错误。发现这个函数里限定了只有父子关系的进程才能发生ipc 囧。。。

因此我不得不dirty hack了envid2env()中代码，以便让所有进程都能和fs_fs这个进程进行ipc通讯：

102         if (checkperm && e != curenv && e->env_parent_id != curenv->env_id)     {
103                 if ((e->env_type != ENV_TYPE_FS) && (curenv->env_type != EN    V_TYPE_FS)) {  // dirty hack to help fsipc between fs and othe    r user env.
104                         *env_store = 0;
105                         return -E_BAD_ENV;
106                 }
107         }