Project2: User Programs

Now that you've worked with Pintos and are becoming familiar with its infrastructure and thread package, it's time to start working on the parts of the system that allow running user programs. The base code already supports loading and running user programs, but no I/O or interactivity is possible. In this project, you will enable programs to interact with the OS via system calls. You will be working out of the userprog directory for this assignment, but you will also be interacting with almost every other part of Pintos. We will describe the relevant parts below.

You must build project 2 on top of your project 1 submission. Although no code from project 1 affects code for project 2, you still have to pass test cases for project 1 because this is an incremental project.

Also, please notify there is extra challenge from project2. This is just an optional implementation. Also, for the extra challenge, there isn't any skeleton code for you except the test cases. All designs are up to you. To submit and test your extra requirements, you will need to edit userprog/Make.vars.

Lastly, please note that the code without TODO does not always you don't need to change that code. You can freely modify any source codes in project2, except the testing codes.

Background

Up to now, all of the code you have run under Pintos has been part of the operating system kernel. This means, for example, that all the test code from the last assignment ran as part of the kernel, with full access to privileged parts of the system. Once we start running user programs on top of the operating system, this is no longer true. This project deals with the consequences.

We allow more than one process to run at a time. Each process has one thread (multithreaded processes are not supported). User programs are written under the illusion that they have the entire machine. This means that when you load and run multiple processes at a time, you must manage memory, scheduling, and other state correctly to maintain this illusion.

In the previous project, we compiled our test code directly into your kernel, so we had to require certain specific function interfaces within the kernel. From now on, we will test your operating system by running user programs. This gives you much greater freedom. You must make sure that the user program interface meets the specifications described here, but given that constraint you are free to restructure or rewrite kernel code however you wish. All of your codes should never located in block that enclosed by #ifdef VM. This blocks will be included after enabling the virtual memory subsystem, which you will implement in the project 3. Also, if the codes is enclosed by #ifndef VM, those codes will be omitted from project3.

We strongly recommend you to read synchronization and virtual addrees before you start.

Source Files

The easiest way to get an overview of the programming you will be doing is to simply go over each part you'll be working with. In userprog, you'll find a small number of files, but here is where the bulk of your work will be:

  • process.c, process.h

    Loads ELF binaries and starts processes.

  • syscall.c, syscall.h

    Whenever a user process wants to access some kernel functionality, it invokes a system call. This is a skeleton system call handler. Currently, it just prints a message and terminates the user process. In part 2 of this project you will add code to do everything else needed by system calls.

  • syscall-entry.S

    Small assembly codes that bootstrap the system call handler. You don't need to understand this code.

  • exception.c, exception.h

    When a user process performs a privileged or prohibited operation, it traps into the kernel as an exception or fault. These files handle exceptions. Currently all exceptions simply print a message and terminate the process. Some, but not all, solutions to project 2 require modifying page_fault() in this file.

  • gdt.c, gdt.h

    The x86-64 is a segmented architecture. The Global Descriptor Table (GDT) is a table that describes the segments in use. These files set up the GDT. You should not need to modify these files for any of the projects. You can read the code if you're interested in how the GDT works.

  • tss.c, tss.h

    The Task-State Segment (TSS) was used for x86 architectural task switching. However, in x86-64, task switching is deprecated. Nontheless, TSS is still there to find the stack pointer during the ring switching.

    This means when a user process enters an interrupt handler, the hardware consults to the tss to look up the kernel's stack pointer. You should not need to modify these files for any of the pojects. You can read the code if you're interested in how the TSS works.

Using the File System

You will need to interface to the file system code for this project, because user programs are loaded from the file system and many of the system calls you must implement deal with the file system. However, the focus of this project is not the file system, so we have provided a simple but complete file system in the filesys directory. You will want to look over the filesys.h and file.h interfaces to understand how to use the file system, and especially its many limitations.

There is no need to modify the file system code for this project, and so we recommend that you do not. Working on the file system is likely to distract you from this project's focus.

Proper use of the file system routines now will make life much easier for project 4, when you improve the file system implementation. Until then, you will have to tolerate the following limitations:

  • No internal synchronization. Concurrent accesses will interfere with one another. You should use synchronization to ensure that only one process at a time is executing file system code.
  • File size is fixed at creation time. The root directory is represented as a file, so the number of files that may be created is also limited.
  • File data is allocated as a single extent, that is, data in a single file must occupy a contiguous range of sectors on disk. External fragmentation can therefore become a serious problem as a file system is used over time.
  • No subdirectories
  • File names are limited to 14 characters.
  • A system crash mid-operation may corrupt the disk in a way that cannot be repaired automatically. There is no file system repair tool anyway.

One important feature is included:

Unix-like semantics for filesys_remove() are implemented. That is, if a file is open when it is removed, its blocks are not deallocated and it may still be accessed by any threads that have it open, until the last one closes it. See Removing an Open File for more information.

Unlike project 1 where all test programs were already present in the kernel image, you have to put test programs (which runs in the user space) into the Pintos virtual machine. To make your life much easier, our testing scripts (i.e. make check) automatically handle this, so in most cases, you don't need to understand this. However, knowing this would significantly help you to run individual test cases.

To put a file into the Pintos virtual machine, firstly you need to be able to create a simulated disk with a file system partition. The pintos-mkdisk program provides this functionality. From the userprog/build directory, execute pintos-mkdisk filesys.dsk 2. This command creates a simulated disk named filesys.dsk that contains a 2 MB Pintos file system partition. Then, specify the disk by passing --fs-disk filesys.dsk after pintos but before -- (i.e. pintos --fs-disk filesys.disk -- KERNEL_COMMANDS...). The -- is needed because --fs-disk is for the pintos script, not for the simulated kernel. Afterward, format the file system partition by passing -f -q on the kernel's command line: pintos SCRIPT_COMMANDS -- -f -q. The -f option causes the file system to be formatted, and -q causes Pintos to exit as soon as the format is done.

You'll need a way to copy files in and out of the simulated file system. The pintos -p ("put") and -g ("get") options do this. To copy 'file' into the Pintos file system, use the command pintos -p file -- -q. To copy it to the Pintos file system under the name newname, add :newname after the original filename: pintos -p file:newname -- -q. The commands for copying files out of a VM are similar, but substitute -g for -p.

Incidentally, these commands work by passing special commands extract and append on the kernel's command line and copying to and from a special simulated "scratch" partition. If you're very curious, you can look at the pintos script as well as filesys/fsutil.c to learn the implementation details.

Here's a summary of how to create a disk with a file system partition, format the file system, copy the args-single program—which is the second test case for this project—into the new disk, and then run it, passing argument 'onearg'. (Argument passing won't work until you implemented it.) It assumes that you've already built the test cases and that the current directory is userprog/build:

pintos-mkdisk filesys.dsk 10
pintos --fs-disk filesys.dsk -p tests/userprog/args-single:args-single -- -q -f run 'args-single onearg'

If you don't want to keep the file system disk around for later use or inspection, you can even combine all four steps into a single command. The --filesys-size=n option creates a temporary file system partition approximately n megabytes in size just for the duration of the pintos run. The Pintos automatic test suite makes extensive use of this syntax:

pintos --fs-disk=10 -p tests/userprog/args-single:args-single -- -q -f run 'args-single onearg'

How User Programs Work

Pintos can run normal C programs, as long as they fit into memory and use only the system calls you implement. Notably, malloc() cannot be implemented because none of the system calls required for this project allow for memory allocation. Pintos also can't run programs that use floating point operations, since the kernel doesn't save and restore the processor's floating-point unit when switching threads.

Pintos can load ELF executables with the loader provided for you in userprog/process.c. ELF is a file format used by Linux, Solaris, and many other operating systems for object files, shared libraries, and executables.

You can actually use any compiler and linker that output x86-64 ELF executables to produce programs for Pintos. (We've provided compilers and linkers that should do just fine.) You should realize immediately that, until you copy a test program to the simulated file system, Pintos will be unable to do useful work. You won't be able to do interesting things until you copy a variety of programs to the file system. You might want to create a clean reference file system disk and copy that over whenever you trash your filesys.dsk beyond a useful state, which may happen occasionally while debugging.

Virtual Memory Layout

Virtual memory in Pintos is divided into two regions: user virtual memory and kernel virtual memory. User virtual memory ranges from virtual address 0 up to KERN_BASE, which is defined in include/threads/vaddr.h and defaults to 0x8004000000 Kernel virtual memory occupies the rest of the virtual address space.

User virtual memory is per-process. When the kernel switches from one process to another, it also switches user virtual address spaces by changing the processor's page directory base register (see pml4_activate() in thread/mmu.c). struct thread contains a pointer to a process's page table.

Kernel virtual memory is global. It is always mapped the same way, regardless of what user process or kernel thread is running. In Pintos, kernel virtual memory is mapped one-to-one to physical memory, starting at KERN_BASE. That is, virtual address KERN_BASE accesses physical address 0, virtual address KERN_BASE + 0x1234 accesses physical address 0x1234, and so on up to the size of the machine's physical memory.

A user program can only access its own user virtual memory. An attempt to access kernel virtual memory causes a page fault, handled by page_fault() in userprog/exception.c, and the process will be terminated. Kernel threads can access both kernel virtual memory and, if a user process is running, the user virtual memory of the running process. However, even in the kernel, an attempt to access memory at an unmapped user virtual address will cause a page fault.

Typical Memory Layout

Conceptually, each process is free to lay out its own user virtual memory however it chooses. In practice, user virtual memory is laid out like this:

USER_STACK +----------------------------------+
           |             user stack           |
           |                 |                |
           |                 |                |
           |                 V                |
           |           grows downward         |
           |                                  |
           |                                  |
           |                                  |
           |                                  |
           |           grows upward           |
           |                 ^                |
           |                 |                |
           |                 |                |
           +----------------------------------+
           | uninitialized data segment (BSS) |
           +----------------------------------+
           |     initialized data segment     |
           +----------------------------------+
           |            code segment          |
 0x400000  +----------------------------------+
           |                                  |
           |                                  |
           |                                  |
           |                                  |
           |                                  |
       0   +----------------------------------+

In this project, the user stack is fixed in size, but in project 3 it will be allowed to grow. Traditionally, the size of the uninitialized data segment can be adjusted with a system call, but you will not have to implement this.

The code segment in Pintos starts at user virtual address 0x400000, approximately 128 MB from the bottom of the address space. This value was typical value in ubuntu and has no deep significance.

The linker sets the layout of a user program in memory, as directed by a "linker script" that tells it the names and locations of the various program segments. You can learn more about linker scripts by reading the "Scripts" chapter in the linker manual, accessible via info ld.

To view the layout of a particular executable, run objdump with the -p option.

Accessing User Memory

As part of a system call, the kernel must often access memory through pointers provided by a user program. The kernel must be very careful about doing so, because the user can pass a null pointer, a pointer to unmapped virtual memory, or a pointer to kernel virtual address space (above KERN_BASE). All of these types of invalid pointers must be rejected without harm to the kernel or other running processes, by terminating the offending process and freeing its resources.

There are at least two reasonable ways to do this correctly. The first method is to verify the validity of a user-provided pointer, then dereference it. If you choose this route, you'll want to look at the functions in thread/mmu.c and in include/threads/vaddr.h. This is the simplest way to handle user memory access.

The second method is to check only that a user pointer points below KERN_BASE, then dereference it. An invalid user pointer will cause a "page fault" that you can handle by modifying the code for page_fault() in userprog/exception.c. This technique is normally faster because it takes advantage of the processor's MMU, so it tends to be used in real kernels (including Linux).

In either case, you need to make sure not to "leak" resources. For example, suppose that your system call has acquired a lock or allocated memory with malloc(). If you encounter an invalid user pointer afterward, you must still be sure to release the lock or free the page of memory. If you choose to verify user pointers before dereferencing them, this should be straightforward. It's more difficult to handle if an invalid pointer causes a page fault, because there's no way to return an error code from a memory access. Therefore, for those who want to try the latter technique, we'll provide a little bit of helpful code:

/* Reads a byte at user virtual address UADDR.
 * UADDR must be below KERN_BASE.
 * Returns the byte value if successful, -1 if a segfault
 * occurred. */
static int64_t
get_user (const uint8_t *uaddr) {
    int64_t result;
    __asm __volatile (
    "movabsq $done_get, %0\n"
    "movzbq %1, %0\n"
    "done_get:\n"
    : "=&a" (result) : "m" (*uaddr));
    return result;
}

/* Writes BYTE to user address UDST.
 * UDST must be below KERN_BASE.
 * Returns true if successful, false if a segfault occurred. */
static bool
put_user (uint8_t *udst, uint8_t byte) {
    int64_t error_code;
    __asm __volatile (
    "movabsq $done_put, %0\n"
    "movb %b2, %1\n"
    "done_put:\n"
    : "=&a" (error_code), "=m" (*udst) : "q" (byte));
    return error_code != -1;
}

Each of these functions assumes that the user address has already been verified to be below KERN_BASE. They also assume that you've modified page_fault() so that a page fault in the kernel merely sets rax to -1 and copies its former value into %rip.

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