Outline
TTIT61:
Process Programming and
Operating Systems
„ System calls
„ Single user program
„ Lab 2 Assignment
Sergiu Rafiliu
serra@ida.liu.se
phone: 282281, room: B 329:228
alean@ida
Lab 2 Outline
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Four Components of a Computer System
„ Lab 2: “System calls”
„ Description
„ Pintos stack
„ Pintos memory
„ Pintos file system
„ Halt, Exit
„ Create, Open/Close
„ Read/Write
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System Calls
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System Call Execution
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
/* System calls are written in bold italic. Type “man
2 sys_call_name” for info in Linux or “man –s 2
sys_call_name” in Solaris */
int fd, n;
char buf[1024];
fd = open(“datafile.txt”);
n = read(fd, buf, 1024);
printf(“Have read %d bytes: %s\n”, n, buf);
lseek(fd, 0, SEEK_SET);
write(fd, “Some other text”, strlen(“Some other text”)
+ 1);
write(fd, buf, n);
close(fd);
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Systems Calls Description
Systems Calls - Implementation
user_program.c
„ System calls:
„ communication between the user program and the
kernel
„ functions, called from the user program and performed
by the kernel
„ computers often use internal interrupts (traps) to
accomplish that switch from user code to system code
Calling
corresponding
function in
the wrapper
Calling
corresponding
exception in
the kernel
#include <syscal.h>
bool flag;
flag = create(“file.txt”, 1000);
Return result back
to the user program
pintos/src/lib/user/syscall.c
System call wrapper
bool create (const char *file, unsigned initial_size) {
return syscall2 (SYS_CREATE, file, initial_size);
}
Pintos kernel
pintos/src/userprog/syscall.c
User exception handler
// Your code here!
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User Program - Example
User Programs
„ Located in the Pintos directory: examples/
„ Compiled with a compiler that produces ELF executable
„ ELF format is used by Linux, Solaris and other
operating systems for object files, shared libraries,
and executables.
„ Pintos provides a loader for ELF files in
userprog/process.c
„ Modify examples/Makefile in order to add other user
programs
„ Shutdown program :
/* halt.c */
#include <syscall.h>
void main(void) {
halt();
}
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Pintos disk
„ User programs must be loaded on a disk before they can be run.
„ Unix like file system is implemented in Pintos in filesystem
directory.
„ Create a simulated disk using pintos-mkdisk (in userprog/build do
this)
„ make a disk of size 2 MB
pintos-mkdisk diskname.dsk 2
„ format the disk
pintos –-qemu –f –q
„ copy a program on the disk
pintos –-qemu –p programname –a newname -- -q
„ Run a program on the disk
pintos --qemu -- run newname
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Return result back
to the wrapper
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Pintos disk example
„ Adding to a disk and running the program echo, with
parameter ’x’ in Pintos:
pintos-mkdisk fs.dsk 2
pintos –-qemu –f –q
pintos –-qemu –p ../../examples/echo –a echo -- -q
pintos --qemu -- run ’echo x’
„ Alternatively you can do this:
pintos –-qemu –fs-disk=2 –p ../../examples/echo –a
echo -- -f –q run ’echo x’
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Lab 2 : Task
„ You will need to implement:
Useful Files
„ You have to have a look at:
„ create - creates a file.
„ open - opens a file.
„ close - closes a file.
„ read - reads from a file or the console (the keyboard).
„ write - writes to a file or the console (the monitor).
„ halt - halts the processor.
„ exit - Terminates a program and deallocates resources
occupied by the program, for example, closes all files
opened by the program.
„ pintos/src/lib/user/syscall[.h|.c] – the wrapper…
„ userprog/syscall[.h|.c] – Your implementation of system
calls!!!
„ threads/interrupt.[h|c] – important structures!!!
„ lib/syscall-nr.h – system call constants
„filesys/filesys.[h|c] Pintos file system implementation…
„ examples/lab2test.c – the “official” test program for this
lab
„ filesys/file.[h|c] – useful functions for operations with files.
Things which you don’t find in “filesys”, you find here
„userprog/process.c Look up what is here and tell me…
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How To Start?
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Pintos Stack
„ STEP 1
„Understand what the user program is
„Look into “examples” directory
„Look into Makefile in this directory to understand how the user
programs are compiled
„Compile the user programs by “gmake”
„ STEP 2
„Prepare Pintos disk
„Copy one or two user programs to the disk
„ STEP 3
„into userprog/process.c, find setup_stack()
*esp = PHYS_BASE;
change to *esp = PHYS_BASE - 12;
„ STEP 4
„Make sure that you understand the problems which may arise if
you access (in the kernel) data stored in the user memory using
the pointers provided by the user program as system call
arguments.
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Create Example
bool create (const char *file, unsigned initial_size)
„ Example: create(“file.txt”, 1000);
Create Example
„ To implement the system calls, which operate with files,
look at “filesys/filesys.[h|c]” and “filesys/file.[h|c]”.
„ Everything is already done! Just call the functions…
How to get the call name and parameters?
Answer: f->esp
How to return a value?
Answer: f->eax
Hint:
So simple? Not yet…
… note that, in order to get
a string, you will need get a void pointer
from esp and then get a char pointer to which
points that void pointer. The char pointer will point to the first
element of the string …
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Memory issues!!!
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Problem 1: If the pointer above PHYS_BASE,
It points to Kernel memory! UNSAFE!
All the pointers on the variables,
which you get from the user program,
must be validated!
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Memory Issues in Pintos
Kernel VM
Physical Memory
Kernel process
PHYS_BASE
Memory Issues in Pintos
Kernel VM
Physical Memory
Kernel process
PHYS_BASE
Check if the pointer is below
User process
User process
Page directory
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If no entry?
a)Kill user
b) Handle page fault
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Page directory
Check if the pointer is in the page directory
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If no entry?
a)Kill user
b) Handle page fault
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Other System Calls (1)
STRUCT THREAD
„ To implement the system calls, which operate with files,
look at “filesys/filesys.[h|c]” and “filesys/file.[h|c]”.
„ Everything is already done there. Just call the functions!
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Other System Calls (2)
int open (const char *file)
void close (int fd)
Other System Calls (3)
int read (int fd, void *buffer, unsigned size)
„ Sink when you manage file IDs! I suggest using an array
of *file elements and return indexes of this array as file
IDs
„ 0 and 1 IDs must be always reserved for the console!
„ 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() (defined in devices/input.h).
NO SYNCHRONIZATION YET!!!
NO SYNCHRONIZATION YET!!!
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Other System Calls (4)
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Other System Calls (5)
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 or -1 if the file 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 -1 if no bytes could be written at all.
„ When fd=1 then the system call should write to the console. Your
code which writes to the console should write all of buffer in one call
to putbuf() (check lib/kernel/stdio.h and lib/kernel/console.c), 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 the user.
void exit (int status)
„ Terminates the current user program, returning status to
the kernel. Conventionally, a status of 0 indicates success
and nonzero values indicate errors. Remember to free all
the resources will be not needed anymore.
„ This system call will be improved in the following labs.
NO SYNCHRONIZATION YET!!!
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Other System Calls (6)
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CLOSE ALL OPEN FILES CLOSE ALL OPEN FILES
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