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3. Project 2: User Programs

For Lab2, you will work on a system call Pintos. We have provided you the basic Pintos code and you will work on 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 might find it useful to read how to run the tests (see section 1.2.1 Testing).

3.1 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.

3.1.1 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:

Loads ELF binaries and starts processes.

A simple manager for 80x86 hardware page tables. Although you probably won't want to modify this code for this project, you may want to call some of its functions. See section Page Tables, for more information.

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.

When a user process performs a privileged or prohibited operation, it traps into the kernel as an "exception" or "fault."(2) 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.

The 80x86 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.

The Task-State Segment (TSS) is used for 80x86 architectural task switching. Pintos uses the TSS only for switching stacks when a user process enters an interrupt handler, as does Linux. 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 TSS works.

3.1.2 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:

One important feature is included:

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 --filesys-size=2. This command creates a simulated disk named filesys.dsk that contains a 2 MB Pintos file system partition. Then format the file system partition by passing -f -q on the kernel's command line: pintos -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. (The -- is needed because -p is for the pintos script, not for the simulated kernel.) To copy it to the Pintos file system under the name newname, add -a newname: pintos -p file -a 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 get 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 echo program into the new disk, and then run echo, passing argument x. (Argument passing won't work until you implemented it.) It assumes that you've already built the examples in examples and that the current directory is userprog/build:

pintos-mkdisk filesys.dsk --filesys-size=2
pintos -f -q
pintos -p ../../examples/echo -a echo -- -q
pintos -q run 'echo x'

The three final steps can actually be combined into a single command:

pintos-mkdisk filesys.dsk --filesys-size=2
pintos -p ../../examples/echo -a echo -- -f -q run 'echo x'

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 --filesys-size=2 -p ../../examples/echo -a echo -- -f -q run 'echo x'

You can delete a file from the Pintos file system using the rm file kernel action, e.g. pintos -q rm file. Also, ls lists the files in the file system and cat file prints a file's contents to the display.

3.1.3 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.

The src/examples directory contains a few sample user programs. The Makefile in this directory compiles the provided examples, and you can edit it compile your own programs as well. Some of the example programs will only work once projects 3 or 4 have been implemented.

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 80x86 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.

3.1.4 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 PHYS_BASE, which is defined in threads/vaddr.h and defaults to 0xc0000000 (3 GB). Kernel virtual memory occupies the rest of the virtual address space, from PHYS_BASE up to 4 GB.

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 pagedir_activate() in userprog/pagedir.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 PHYS_BASE. That is, virtual address PHYS_BASE accesses physical address 0, virtual address PHYS_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:

   PHYS_BASE +----------------------------------+
             |            user stack            |
             |                 |                |
             |                 |                |
             |                 V                |
             |          grows downward          |
             |                                  |
             |                                  |
             |                                  |
             |                                  |
             |           grows upward           |
             |                 ^                |
             |                 |                |
             |                 |                |
             | uninitialized data segment (BSS) |
             |     initialized data segment     |
             |           code segment           |
  0x08048000 +----------------------------------+
             |                                  |
             |                                  |
             |                                  |
             |                                  |
             |                                  |
           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 0x08084000, approximately 128 MB from the bottom of the address space. This value is specified in [ SysV-i386] 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 (80x86) or i386-elf-objdump (SPARC) with the -p option.

3.1.5 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 PHYS_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 userprog/pagedir.c and in 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 PHYS_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 PHYS_BASE.
   Returns the byte value if successful, -1 if a segfault
   occurred. */
static int
get_user (const uint8_t *uaddr)
  int result;
  asm ("movl $1f, %0; movzbl %1, %0; 1:"
       : "=&a" (result) : "m" (*uaddr));
  return result;
/* Writes BYTE to user address UDST.
   UDST must be below PHYS_BASE.
   Returns true if successful, false if a segfault occurred. */
static bool
put_user (uint8_t *udst, uint8_t byte)
  int error_code;
  asm ("movl $1f, %0; movb %b2, %1; 1:"
       : "=&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 PHYS_BASE. They also assume that you've modified page_fault() so that a page fault in the kernel merely sets eax to 0xffffffff and copies its former value into eip.

3.2 Suggested Order of Implementation

We suggest first implementing the following, which can happen in parallel:

After the above are implemented, user processes should work minimally. At the very least, they can write to the console and exit correctly. You can then refine your implementation so that some of the tests start to pass.

3.3 Requirements

3.3.1 Design Document

Before you turn in your project, you must copy the project 2 design document template into your source tree under the name pintos/src/userprog/DESIGNDOC and fill it in. We recommend that you read the design document template before you start working on the project. See section D. Project Documentation, for a sample design document that goes along with a fictitious project.

3.3.2 Process Termination Messages

Whenever a user process terminates, because it called exit or for any other reason, print the process's name and exit code, formatted as if printed by printf ("%s: exit(%d)\n", ...);. The name printed should be the full name passed to process_execute(), omitting command-line arguments. Do not print these messages when a kernel thread that is not a user process terminates, or when the halt system call is invoked. The message is optional when a process fails to load.

Aside from this, don't print any other messages that Pintos as provided doesn't already print. You may find extra messages useful during debugging, but they will confuse the grading scripts and thus lower your score.

3.3.3 Argument Passing

Currently, process_execute() does not support passing arguments to new processes. Implement this functionality, by extending process_execute() so that instead of simply taking a program file name as its argument, it divides it into words at spaces. The first word is the program name, the second word is the first argument, and so on. That is, process_execute("grep foo bar") should run grep passing two arguments foo and bar.

Within a command line, multiple spaces are equivalent to a single space, so that process_execute("grep foo bar") is equivalent to our original example. You can impose a reasonable limit on the length of the command line arguments. For example, you could limit the arguments to those that will fit in a single page (4 kB). (There is an unrelated limit of 128 bytes on command-line arguments that the pintos utility can pass to the kernel.)

You can parse argument strings any way you like. If you're lost, look at strtok_r(), prototyped in lib/string.h and implemented with thorough comments in lib/string.c. You can find more about it by looking at the man page (run man strtok_r at the prompt).

See section 3.5.1 Program Startup Details, for information on exactly how you need to set up the stack.

3.3.4 System Calls

Implement the system call handler in userprog/syscall.c. The skeleton implementation we provide "handles" system calls by terminating the process. It will need to retrieve the system call number, then any system call arguments, and carry out appropriate actions.

Implement the following system calls. The prototypes listed are those seen by a user program that includes lib/user/syscall.h. (This header, and all others in lib/user, are for use by user programs only.) System call numbers for each system call are defined in lib/syscall-nr.h:

System Call: void halt (void)
Terminates Pintos by calling power_off() (declared in threads/init.h). This should be seldom used, because you lose some information about possible deadlock situations, etc.

System Call: void exit (int status)
Terminates the current user program, returning status to the kernel. If the process's parent waits for it (see below), this is the status that will be returned. Conventionally, a status of 0 indicates success and nonzero values indicate errors.

System Call: pid_t exec (const char *cmd_line)
Runs the executable whose name is given in cmd_line, passing any given arguments, and returns the new process's program id (pid). Must return pid -1, which otherwise should not be a valid pid, if the program cannot load or run for any reason. Thus, the parent process cannot return from the exec until it knows whether the child process successfully loaded its executable. You must use appropriate synchronization to ensure this.

System Call: int wait (pid_t pid)
If process pid is still alive, waits until it dies. Then, returns the status that pid passed to exit, or -1 if pid was terminated by the kernel (e.g. killed due to an exception). If pid does not refer to a child of the calling thread, or if wait has already been successfully called for the given pid, returns -1 immediately, without waiting.

You must ensure that Pintos does not terminate until the initial process exits. The supplied Pintos code tries to do this by calling process_wait() (in userprog/process.c) from main() (in threads/init.c). We suggest that you implement process_wait() according to the comment at the top of the function and then implement the wait system call in terms of process_wait().

All of a process's resources, including its struct thread, must be freed whether its parent ever waits for it or not, and regardless of whether the child exits before or after its parent.

Children are not inherited: if A has child B and B has child C, then wait(C) always returns immediately when called from A, even if B is dead.

Consider all the ways a wait can occur: nested waits (A waits for B, then B waits for C), multiple waits (A waits for B, then A waits for C), and so on.

Implementing this system call requires considerably more work than any of the rest.

System Call: bool create (const char *file, unsigned initial_size)
Creates a new file called file initially initial_size bytes in size. Returns true if successful, false otherwise. Creating a new file does not open it: opening the new file is a separate operation which would require a open system call.

System Call: bool remove (const char *file)
Deletes the file called file. Returns true if successful, false otherwise. A file may be removed regardless of whether it is open or closed, and removing an open file does not close it. See Removing an Open File, for details.

System Call: int open (const char *file)
Opens the file called file. Returns a nonnegative integer handle called a "file descriptor" (fd), or -1 if the file could not be opened.

File descriptors numbered 0 and 1 are reserved for the console: fd 0 (STDIN_FILENO) is standard input, fd 1 (STDOUT_FILENO) is standard output. The open system call will never return either of these file descriptors, which are valid as system call arguments only as explicitly described below.

Each process has an independent set of file descriptors. File descriptors are not inherited by child processes.

When a single file is opened more than once, whether by a single process or different processes, each open returns a new file descriptor. Different file descriptors for a single file are closed independently in separate calls to close and they do not share a file position.

System Call: int filesize (int fd)
Returns the size, in bytes, of the file open as fd.

System Call: int read (int fd, void *buffer, unsigned size)
Reads size bytes from the file open as fd into buffer. Returns the number of bytes actually read (0 at end of file), or -1 if the file could not be read (due to a condition other than end of file). Fd 0 reads from the keyboard using input_getc().

System Call: int write (int fd, const void *buffer, unsigned size)
Writes size bytes from buffer to the open file fd. Returns the number of bytes actually written, which may be less than size if some bytes could not be written.

Writing past end-of-file would normally extend the file, but file growth is not implemented by the basic file system. The expected behavior is to write as many bytes as possible up to end-of-file and return the actual number written, or 0 if no bytes could be written at all.

Fd 1 writes to the console. Your code to write to the console should write all of buffer in one call to putbuf(), at least as long as size is not bigger than a few hundred bytes. (It is reasonable to break up larger buffers.) Otherwise, lines of text output by different processes may end up interleaved on the console, confusing both human readers and our grading scripts.

System Call: void seek (int fd, unsigned position)
Changes the next byte to be read or written in open file fd to position, expressed in bytes from the beginning of the file. (Thus, a position of 0 is the file's start.)

A seek past the current end of a file is not an error. A later read obtains 0 bytes, indicating end of file. A later write extends the file, filling any unwritten gap with zeros. (However, in Pintos files have a fixed length until project 4 is complete, so writes past end of file will return an error.) These semantics are implemented in the file system and do not require any special effort in system call implementation.

System Call: unsigned tell (int fd)
Returns the position of the next byte to be read or written in open file fd, expressed in bytes from the beginning of the file.

System Call: void close (int fd)
Closes file descriptor fd. Exiting or terminating a process implicitly closes all its open file descriptors, as if by calling this function for each one.

The file defines other syscalls. Ignore them for now. You will implement some of them in project 3 and the rest in project 4, so be sure to design your system with extensibility in mind.

To implement syscalls, you need to provide ways to read and write data in user virtual address space. You need this ability before you can even obtain the system call number, because the system call number is on the user's stack in the user's virtual address space. This can be a bit tricky: what if the user provides an invalid pointer, a pointer into kernel memory, or a block partially in one of those regions? You should handle these cases by terminating the user process. We recommend writing and testing this code before implementing any other system call functionality. See section 3.1.5 Accessing User Memory, for more information.

You must synchronize system calls so that any number of user processes can make them at once. In particular, it is not safe to call into the file system code provided in the filesys directory from multiple threads at once. Your system call implementation must treat the file system code as a critical section. Don't forget that process_execute() also accesses files. For now, we recommend against modifying code in the filesys directory.

We have provided you a user-level function for each system call in lib/user/syscall.c. These provide a way for user processes to invoke each system call from a C program. Each uses a little inline assembly code to invoke the system call and (if appropriate) returns the system call's return value.

When you're done with this part, and forevermore, Pintos should be bulletproof. Nothing that a user program can do should ever cause the OS to crash, panic, fail an assertion, or otherwise malfunction. It is important to emphasize this point: our tests will try to break your system calls in many, many ways. You need to think of all the corner cases and handle them. The sole way a user program should be able to cause the OS to halt is by invoking the halt system call.

If a system call is passed an invalid argument, acceptable options include returning an error value (for those calls that return a value), returning an undefined value, or terminating the process.

See section 3.5.2 System Call Details, for details on how system calls work.

3.3.5 Denying Writes to Executables

Add code to deny writes to files in use as executables. Many OSes do this because of the unpredictable results if a process tried to run code that was in the midst of being changed on disk. This is especially important once virtual memory is implemented in project 3, but it can't hurt even now.

You can use file_deny_write() to prevent writes to an open file. Calling file_allow_write() on the file will re-enable them (unless the file is denied writes by another opener). Closing a file will also re-enable writes. Thus, to deny writes to a process's executable, you must keep it open as long as the process is still running.

3.4 FAQ

How much code will I need to write?

Here's a summary of our reference solution, produced by the diffstat program. The final row gives total lines inserted and deleted; a changed line counts as both an insertion and a deletion.

The reference solution represents just one possible solution. Many other solutions are also possible and many of those differ greatly from the reference solution. Some excellent solutions may not modify all the files modified by the reference solution, and some may modify files not modified by the reference solution.

 threads/thread.c     |   13 
 threads/thread.h     |   26 +
 userprog/exception.c |    8 
 userprog/process.c   |  247 ++++++++++++++--
 userprog/syscall.c   |  468 ++++++++++++++++++++++++++++++-
 userprog/syscall.h   |    1 
 6 files changed, 725 insertions(+), 38 deletions(-)

The kernel always panics when I run pintos -p file -- -q.

Did you format the file system (with pintos -f)?

Is your file name too long? The file system limits file names to 14 characters. A command like pintos -p ../../examples/echo -- -q will exceed the limit. Use pintos -p ../../examples/echo -a echo -- -q to put the file under the name echo instead.

Is the file system full?

Does the file system already contain 16 files? The base Pintos file system has a 16-file limit.

The file system may be so fragmented that there's not enough contiguous space for your file.

When I run pintos -p ../file --, file isn't copied.

Files are written under the name you refer to them, by default, so in this case the file copied in would be named ../file. You probably want to run pintos -p ../file -a file -- instead.

You can list the files in your file system with pintos -q ls.

All my user programs die with page faults.

This will happen if you haven't implemented argument passing (or haven't done so correctly). The basic C library for user programs tries to read argc and argv off the stack. If the stack isn't properly set up, this causes a page fault.

All my user programs die with system call!

You'll have to implement system calls before you see anything else. Every reasonable program tries to make at least one system call (exit()) and most programs make more than that. Notably, printf() invokes the write system call. The default system call handler just prints system call! and terminates the program. Until then, you can use hex_dump() to convince yourself that argument passing is implemented correctly (see section 3.5.1 Program Startup Details).

How can I disassemble user programs?

The objdump (80x86) or i386-elf-objdump (SPARC) utility can disassemble entire user programs or object files. Invoke it as objdump -d file. You can use GDB's disassemble command to disassemble individual functions (see section E.5 GDB).

Why do many C include files not work in Pintos programs?
Can I use libfoo in my Pintos programs?

The C library we provide is very limited. It does not include many of the features that are expected of a real operating system's C library. The C library must be built specifically for the operating system (and architecture), since it must make system calls for I/O and memory allocation. (Not all functions do, of course, but usually the library is compiled as a unit.)

The chances are good that the library you want uses parts of the C library that Pintos doesn't implement. It will probably take at least some porting effort to make it work under Pintos. Notably, the Pintos user program C library does not have a malloc() implementation.

How do I compile new user programs?

Modify src/examples/Makefile, then run make.

Can I run user programs under a debugger?

Yes, with some limitations. See section E.5.3 Debugging User Programs.

What's the difference between tid_t and pid_t?

A tid_t identifies a kernel thread, which may have a user process running in it (if created with process_execute()) or not (if created with thread_create()). It is a data type used only in the kernel.

A pid_t identifies a user process. It is used by user processes and the kernel in the exec and wait system calls.

You can choose whatever suitable types you like for tid_t and pid_t. By default, they're both int. You can make them a one-to-one mapping, so that the same values in both identify the same process, or you can use a more complex mapping. It's up to you.

3.4.1 Argument Passing FAQ

Isn't the top of stack in kernel virtual memory?

The top of stack is at PHYS_BASE, typically 0xc0000000, which is also where kernel virtual memory starts. But before the processor pushes data on the stack, it decrements the stack pointer. Thus, the first (4-byte) value pushed on the stack will be at address 0xbffffffc.

Is PHYS_BASE fixed?

No. You should be able to support PHYS_BASE values that are any multiple of 0x10000000 from 0x80000000 to 0xf0000000, simply via recompilation.

3.4.2 System Calls FAQ

Can I just cast a struct file * to get a file descriptor?
Can I just cast a struct thread * to a pid_t?

You will have to make these design decisions yourself. Most operating systems do distinguish between file descriptors (or pids) and the addresses of their kernel data structures. You might want to give some thought as to why they do so before committing yourself.

Can I set a maximum number of open files per process?

It is better not to set an arbitrary limit. You may impose a limit of 128 open files per process, if necessary.

What happens when an open file is removed?

You should implement the standard Unix semantics for files. That is, when a file is removed any process which has a file descriptor for that file may continue to use that descriptor. This means that they can read and write from the file. The file will not have a name, and no other processes will be able to open it, but it will continue to exist until all file descriptors referring to the file are closed or the machine shuts down.

How can I run user programs that need more than 4 kB stack space?

You may modify the stack setup code to allocate more than one page of stack space for each process. In the next project, you will implement a better solution.

What should happen if an exec fails midway through loading?

exec should return -1 if the child process fails to load for any reason. This includes the case where the load fails part of the way through the process (e.g. where it runs out of memory in the multi-oom test). Therefore, the parent process cannot return from the exec system call until it is established whether the load was successful or not. The child must communicate this information to its parent using appropriate synchronization, such as a semaphore (see section A.3.2 Semaphores), to ensure that the information is communicated without race conditions.

3.5 80x86 Calling Convention

This section summarizes important points of the convention used for normal function calls on 32-bit 80x86 implementations of Unix. Some details are omitted for brevity. If you do want all the details, refer to [ SysV-i386].

The calling convention works like this:

  1. The caller pushes each of the function's arguments on the stack one by one, normally using the PUSH assembly language instruction. Arguments are pushed in right-to-left order.

    The stack grows downward: each push decrements the stack pointer, then stores into the location it now points to, like the C expression *--sp = value.

  2. The caller pushes the address of its next instruction (the return address) on the stack and jumps to the first instruction of the callee. A single 80x86 instruction, CALL, does both.

  3. The callee executes. When it takes control, the stack pointer points to the return address, the first argument is just above it, the second argument is just above the first argument, and so on.

  4. If the callee has a return value, it stores it into register EAX.

  5. The callee returns by popping the return address from the stack and jumping to the location it specifies, using the 80x86 RET instruction.

  6. The caller pops the arguments off the stack.

Consider a function f() that takes three int arguments. This diagram shows a sample stack frame as seen by the callee at the beginning of step 3 above, supposing that f() is invoked as f(1, 2, 3). The initial stack address is arbitrary:

                  0xbffffe7c |        3       |
                  0xbffffe78 |        2       |
                  0xbffffe74 |        1       |
stack pointer --> 0xbffffe70 | return address |

3.5.1 Program Startup Details

The Pintos C library for user programs designates _start(), in lib/user/entry.c, as the entry point for user programs. This function is a wrapper around main() that calls exit() if main() returns:

_start (int argc, char *argv[]) 
  exit (main (argc, argv));

The kernel must put the arguments for the initial function on the stack before it allows the user program to begin executing. The arguments are passed in the same way as the normal calling convention (see section 3.5 80x86 Calling Convention).

Consider how to handle arguments for the following example command: /bin/ls -l foo bar. First, break the command into words: /bin/ls, -l, foo, bar. Place the words at the top of the stack. Order doesn't matter, because they will be referenced through pointers.

Then, push the address of each string plus a null pointer sentinel, on the stack, in right-to-left order. These are the elements of argv. The null pointer sentinel ensures that argv[argc] is a null pointer, as required by the C standard. The order ensures that argv[0] is at the lowest virtual address. Word-aligned accesses are faster than unaligned accesses, so for best performance round the stack pointer down to a multiple of 4 before the first push.

Then, push argv (the address of argv[0]) and argc, in that order. Finally, push a fake "return address": although the entry function will never return, its stack frame must have the same structure as any other.

The table below shows the state of the stack and the relevant registers right before the beginning of the user program, assuming PHYS_BASE is 0xc0000000:

Address Name Data Type
0xbffffffc argv[3][...] bar\0 char[4]
0xbffffff8 argv[2][...] foo\0 char[4]
0xbffffff5 argv[1][...] -l\0 char[3]
0xbfffffed argv[0][...] /bin/ls\0 char[8]
0xbfffffec word-align 0 uint8_t
0xbfffffe8 argv[4] 0 char *
0xbfffffe4 argv[3] 0xbffffffc char *
0xbfffffe0 argv[2] 0xbffffff8 char *
0xbfffffdc argv[1] 0xbffffff5 char *
0xbfffffd8 argv[0] 0xbfffffed char *
0xbfffffd4 argv 0xbfffffd8 char **
0xbfffffd0 argc 4 int
0xbfffffcc return address 0 void (*) ()

In this example, the stack pointer would be initialized to 0xbfffffcc.

As shown above, your code should start the stack at the very top of the user virtual address space, in the page just below virtual address PHYS_BASE (defined in threads/vaddr.h).

You may find the non-standard hex_dump() function, declared in <stdio.h>, useful for debugging your argument passing code. Here's what it would show in the above example:

bfffffc0                                      00 00 00 00 |            ....|
bfffffd0  04 00 00 00 d8 ff ff bf-ed ff ff bf f5 ff ff bf |................|
bfffffe0  f8 ff ff bf fc ff ff bf-00 00 00 00 00 2f 62 69 |............./bi|
bffffff0  6e 2f 6c 73 00 2d 6c 00-66 6f 6f 00 62 61 72 00 |n/ls.-l.foo.bar.|

3.5.2 System Call Details

The first project already dealt with one way that the operating system can regain control from a user program: interrupts from timers and I/O devices. These are "external" interrupts, because they are caused by entities outside the CPU (see section A.4.3 External Interrupt Handling).

The operating system also deals with software exceptions, which are events that occur in program code (see section A.4.2 Internal Interrupt Handling). These can be errors such as a page fault or division by zero. Exceptions are also the means by which a user program can request services ("system calls") from the operating system.

In the 80x86 architecture, the int instruction is the most commonly used means for invoking system calls. This instruction is handled in the same way as other software exceptions. In Pintos, user programs invoke int $0x30 to make a system call. The system call number and any additional arguments are expected to be pushed on the stack in the normal fashion before invoking the interrupt (see section 3.5 80x86 Calling Convention).

Thus, when the system call handler syscall_handler() gets control, the system call number is in the 32-bit word at the caller's stack pointer, the first argument is in the 32-bit word at the next higher address, and so on. The caller's stack pointer is accessible to syscall_handler() as the esp member of the struct intr_frame passed to it. (struct intr_frame is on the kernel stack.)

The 80x86 convention for function return values is to place them in the EAX register. System calls that return a value can do so by modifying the eax member of struct intr_frame.

You should try to avoid writing large amounts of repetitive code for implementing system calls. Each system call argument, whether an integer or a pointer, takes up 4 bytes on the stack. You should be able to take advantage of this to avoid writing much near-identical code for retrieving each system call's arguments from the stack.

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