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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 can build project 2 on top of your project 1 submission or you can start fresh. No code from project 1 is required for this assignment. The "alarm clock" functionality may be useful in projects 3 and 4, but it is not strictly required.
You might find it useful to go back and reread how to run the tests (see section 1.2.1 Testing).
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.
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
pagedir.c
pagedir.h
syscall.c
syscall.h
exception.c
exception.h
page_fault() in this file.
gdt.c
gdt.h
tss.c
tss.h
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:
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.
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 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 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.
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.
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.
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.
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.
We suggest first implementing the following, which can happen in parallel:
For now, you may simply wish to change
*esp = PHYS_BASE; |
*esp = PHYS_BASE - 12; |
setup_stack(). That will work for any test program that doesn't
examine its arguments, although its name will be printed as
(null).
Until you implement argument passing, you should only run programs without passing command-line arguments. Attempting to pass arguments to a program will include those arguments in the name of the program, which will probably fail.
exit system call. Every user program that finishes in the
normal way calls exit. Even a program that returns from
main() calls exit indirectly (see _start() in
lib/user/entry.c).
write system call for writing to fd 1, the system console.
All of our test programs write to the console (the user process version
of printf() is implemented this way), so they will all malfunction
until write is available.
process_wait() to an infinite loop (one that waits
forever). The provided implementation returns immediately, so Pintos
will power off before any processes actually get to run. You will
eventually need to provide a correct implementation.
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.
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.
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.
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.
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
:
power_off() (declared in
threads/init.h). This should be seldom used, because you lose some information about possible deadlock situations, etc.
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.
exec until it
knows whether the child process successfully loaded its executable.
You must use appropriate synchronization to ensure this.
If pid is still alive, waits until it terminates. Then, returns
the status that pid passed to exit. If pid did not
call exit(), but was terminated by the kernel (e.g. killed
due to an exception), wait(pid) must return -1. It is perfectly
legal for a parent process to wait for child processes that have already
terminated by the time the parent calls wait, but the kernel must
still allow the parent to retrieve its child's exit status, or learn
that the child was terminated by the kernel.
wait must fail and return -1 immediately if any of the
following conditions is true:
exec.
Note that children are not inherited: if A spawns child B
and B spawns child process C, then A cannot wait for
C, even if B is dead. A call to wait(C) by process
A must fail. Similarly, orphaned processes are not assigned to
a new parent if their parent process exits before they do.
wait has already called wait on
pid. That is, a process may wait for any given child at most
once.
Processes may spawn any number of children, wait for them in any order,
and may even exit without having waited for some or all of their children.
Your design should consider all the ways in which waits can occur.
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.
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().
Implementing this system call requires considerably more work than any of the rest.
open system call.
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.
input_getc().
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.
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.
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.
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.
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(-) |
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.
pintos -p ../file --, fileisn'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.
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.
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).
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).
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.
Modify src/examples/Makefile
, then run make.
Yes, with some limitations. See section E.5 GDB.
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.
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.
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.
struct file * to get a file descriptor?
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.
It is better not to set an arbitrary limit. You may impose a limit of 128 open files per process, if necessary.
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.
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.
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.
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:
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
.
CALL, does both.
EAX.
RET
instruction.
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 |
+----------------+
|
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:
void
_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.| |
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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