OS161 execv System Call

Basically, execv do more or less the same thing with runprogram in $OS161_SRC/kern/syscall/runprogram.c. The overall flow of sys_execv are:

1. Copy arguments from user space into kernel buffer
2. Open the executable, create a new address space and load the elf into it
3. Copy the arguments from kernel buffer into user stack
4. Return user mode using enter_new_process

Note that I highlighted step 1 and 3 since they are the trickiest part of execv, step 2 and 4 are just the same with runprogram.

Format of uargs

The first argument is progname (e.g., /testbin/argtest), and the second argument is uargs, it's an array of pointers, each pointer points to a user space string. The last pointer of uargs is NULL.

Since we don't know how many arguments are there in uargs, we need to copy the pointers one by one using copyin until we encounter a NULL.

Copy arguments into kernel buffer

In whichever way to do this, one of step 1 and 3 must be complicated. I choose to carefully pack the arguments into a kernel buffer and then just directly copy this buffer into user stack in bulk. Note that in MIPS, pointers must be aligned by 4. So don't forget to padding when necessary

For convenience, assume that arguments are {foo, os161, execv, NULL}. Then after packing, my kernel buffer looks like this:

Typo: kargv[2] should be 28, instead of 26.

Note that kargv[i] stores the offset of the i'th arguments within the kargv array, since up to now we don't know their real user address yet.

Copy the arguments into user stack

Why user stack, not anywhere else? Because it's the only space we know for sure. We can use as_define_stack to get the value of initial stack pointer (normally 0x80000000, aka USER_SPACE_TOP). So what we do is

1. Fill kargv[i] with actual user space pointer, and
2. Copy kargv array into the stack
3. Minus stackptr by the length of kargv array.

Note that we must modify kargs[i] before we do the actual copy, otherwise some weird bus error or TLB miss will occur.

The steps are shown as follows (here we assume stackptr initial value is 0x80000000):