Description
System Calls
Prologue
You work for a top-secret shadow government organization dedicated to the rise of the Silurian overlords. You, as a faithful member of the Lizard Legion, are part of the team charged with improving data storage and handling, particularly tracking metadata – that is, data about data – within the organization’s computer systems. You have been tasked to build a coded message subsystem under the guise of process logging for kernels running in “Sky Skink”, the cloud computing system. Naturally, the Legion uses the superior Reptilian operating system distribution.
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| Table 1. Kernel Log Levels and Corresponding Process Log Levels | |||
| Kernel Level Name | Description | # | Process Level Name |
| KERN_EMERG | Emergency / Crash Imminent (no process logging) | 0 | PROC_OVERRIDE |
| KERN_ALERT | Immediate Action Required | 1 | PROC_ALERT |
| KERN_CRIT | Critical/Serious Failure Occurred | 2 | PROC_CRITICAL |
| KERN_ERR | Error Condition Occurred | 3 | PROC_ERROR |
| KERN_WARNING | Warning; recoverable, but may indicate problems | 4 | PROC_WARNING |
| KERN_NOTICE | Notable, but not serious (e.g., security events) | 5 | PROC_NOTICE |
| KERN_INFO | Informational (e.g. initialization / shutdown) | 6 | PROC_INFO |
| KERN_DEBUG | Debug messages | 7 | PROC_DEBUG |
NOTE: Take snapshots of your VM! You will probably brick your machine at some point during this or other projects, and you will not want to start from scratch. No, seriously – take snapshots!
Structure
The project is broken into four main parts:
- Create a kernel-wide process log level
- Create system calls that allow a process to get or set the process log level of the system.
- Create system call that allows a process to add a process log message at a defined log level.
- Create static library functions that allow the system calls to be invoked via a C API.
| User Program | → | Library | → | System Call |
Figure 1: A system call invoked from a user program
While exact implementation may vary, the library functions must match the signatures laid out in this document, and the system calls must apply the security model properly. Logged messages have format “$log_level_name [$executable, $pid]: $message”, e.g.:
PROC_ERR [bacon_pancakes, 21]: Life is scary & dark. That is why we must find the light.
System Call
The system will have a single, kernel-wide process log level which should initialize on boot in the kernel and must be stored persistently (until shutdown / reboot). The rules for logging are as follows:
- Any process can read (get) the process log level.
- Any process may send a process log to the kernel.
- Only a process running as the superuser may write (set) the process log level.
- If a message’s log level is higher than the process log level, the message is ignored.
- If a message’s log level is lower than or equal to the process log level, the message will be logged. 6) The system-wide a process log level should be initialized to zero (0) – i.e., override logging only.
- Log levels can have values between 0-7 (3-bit unsigned integer). Invalid level results in call failure.
- Any successfully logged message should be logged with the corresponding kernel log level.
System calls are called via syscall(call_num, param1, param2). To log a message, the call should be syscall(PROC_LOG_CALL, msg, level). Call parameters are limited to no more than two!
Static Library
You will create a static library to invoke the system calls in a directory named process_log. This include a header, process_log.h (prototypes and level symbols), and static library file named libprocess_log.a. You will also need to provide a Makefile for the library. All sources must be contained within the process_log directory. Please note, these filenames must match exactly!
You will create a tarred gzip file of the process_log directory with name process_log.tar.gz. When testing, we will decompress the archive, enter the process_log directory, and build. All functions enumerated below must be made available by including ” process_log.h”. See Submission for details.
Library Functions
int get_proc_log_level()
Invokes system call which reads system-wide process log level. Returns the process log level on success, and -1 otherwise.
int set_proc_log_level(int new_level)
Invokes system call which attempts to change the system-wide process log level to new_level. Returns new_level on success, and -1 otherwise. On failure, log level should be unchanged.
int proc_log_message(int level, char *message)
Invokes system call to log a message for this process. If logged, the message should appear in dmesg logs at the corresponding kernel level. Returns -1 for invalid log level, and level otherwise.
Harness Functions
In addition to the standard library functions, you will implement testing harness functions. The testing harness functions are used to verify security of the system calls from the system library (and are required for full credit on this assignment). We will call these functions to retrieve the information needed to make a system call. We will then call the system call within our own program. This ensures that no security checks are being done in the user-level library.
System call parameter retrieval data should be returned as a pointer to an int array of 2-4 values that can be used to make the system call (which can be cast from other types). It has this format:
{ call_number, num_parameters [, parameter1] [, parameter2] }
e.g.: { 42, 2, 867, 5309 } à syscall(42, 867, 5309)
Figure 2: Harness functions can directly invoke system calls without the library functions.
These test harness elements must be implemented to test your security model:
#define PROC_LOG_CALL <number>
Definition for the system call number for proc_log_message (see System Call); should be in header.
int* retrieve_set_level_params(int new_level)
Returns an int array of 2-4 values that can be used to make the set-process-log-level system call.
int* retrieve_get_level_params()
Returns an int array of 2-4 values that can be used to make the get-process-log-level system call.
int interpret_set_level_result(int ret_value)
After making the system call, we will pass the syscall return value to this function call. It should return set_proc_log_level’s interpretation of the system call completing with return value ret_value.
int interpret_get_level_result(int ret_value)
After making the system call, we will pass the syscall return value to this function call. It should return get_proc_log_level’s interpretation of the system call completing with return value ret_value.
int interpret_log_message_result(int ret_value)
After making the system call, we will pass the syscall return value to this function call. It should return proc_log_message’s interpretation of the system call completing with return value ret_value.
Note that there is no retrieve function for log_message as its system call format is defined above.
The patch file will include all changes to all files in a single patch. Applying the patches and remaking the necessary parts of Reptilian, then rebooting and then building the test code (which we will also copy over) should compile the test program.
Your project will be tested by applying the patch while in /usr/rep/src/reptilian-kernel:
$ git apply p1.diff
$ make && sudo make install && sudo make modules_install
Compressed Archive (process_log.tar.gz)
Your compressed tar file should have the following directory/file structure:
process_log.tar.gz
process_log.tar process_log (directory)
process_log.h
Makefile
(Other source files)
To build the library, we will execute these commands (from a non-kernel-source directory):
$ tar zxvf process_log.tar.gz
$ cd process_log
$ make $ cd ..
To link against the library, we will execute this command:
$ cc -o program_name sourcefile.c -L ./process_log -lprocess_log
Please test your library build and linking before submission! If your library does not compile it will result in zero credit (0, none, goose-egg) for the library portion of the project.
