OS161 Execv Part 2

Continuing from part 1, what exactly do we have left to do and what do we have accomplished?

First of all we have added the syscall to the kernel but it is currently empty. So we have a call to execv(const char *progname, char **args); from the user side which means that the arguments we get on the kernel side are as above.

All we really need to do now is recreate runprogram() with some very small tweaks.

Let us go over what runprogram() does and then we will see what tweaks need to be made.

runprogram

The first thing it does is look for the file name provided. Next we create a new address space and activate it in the TLB (Translation Lookaside Buffer) which is how the hardware caches memory access and is a part of the MIPS ISA (Instruction Set Architechture).

Now that we have our address space set up we can call load_elf which handles the semantics of the ELF format and loading into the correct segment of our address space for us.

Now that we have the binary in memory we can close that file and define our usermode stack.

runprogram() now calls md_usermode which warps it into usermode to start executing at entrypoint which is the location of (most likely) the start symbol in the binary. entrypoint is returned from load_elf, yet another thing we don't have to trouble ourselves with.

So runprogram is done, what is different for execv?

execv

For execv we need to pass the arguments to the program through the exception handler, into the kernel syscall, do some work, pass them to md_usermode.

So getting them through the exception handler is done for you and is detailed in part 1, and passing the to md_usermode is quite obviously trivial.

The question is what work has to be done on them before you pass them to md_usermode?

All you really have to do is count how many arguments you have received in the char **args array and ensure that the last argument in the list is NULL. You can also check that the first argument matches the filename but this is a convention so you shouldn't be strict about it. Once you have counted the number of arguments, you may want to check that each is NULL terminated as well since they are strings, just pass argv and argc into md_usermode.

Remember that you are going to need to copy the arguments into kernel space from userspace and then back out to userspace. You will probably want to put them on the heap, using kmalloc, since the only thing you need to ensure is on the stack when going to usermode are the arguments which are just pointers. By having the memory on the heap we can still access it from usermode.

-FlounderingZ