Task: Access Counter

Navigate to the labs/lab-03/tasks/access-counter/support directory.

Your goal is to update the src/access_counter.c source code file to capture memory access exceptions (i.e. the SIGSEGV signal) and to update page permissions in order for the access to eventually succeed. Use mprotect to update the protection of the pages in stages: read, write and then exec. Each time an update is made, the counter variable is increased; this is used for testing.

The signal handler is already in place as the access_handler() function. It is called any time a SIGSEGV signal is being sent out to the current process. You will update the handler by following the TODO comments and instructions here.

The pages array stores information about accessed pages. Assume the MAX_PAGES size of the array is enough to store information. When an existing page is accessed and causes a memory exception, the permission is update, in the stages mentioned above: read, write, and then exec. When a new page is accessed, a new entry is filled in the pages array, initialized with read protection. Use mmap() to reserve virtual pages. Use anonymous mapping (i.e. the MAP_ANONYMOUS) flag. Use any permissions required.

To test it, enter the tests/ directory and run:

make check

In case of a correct solution, you will get an output such as:

./run_all_tests.sh
test_access_read                  ........................ passed ...   9
test_access_write                 ........................ passed ...   9
test_access_exec                  ........................ passed ...  10
test_access_read_write            ........................ passed ...  12
test_access_read_exec             ........................ passed ...  12
test_access_write_exec            ........................ passed ...  12
test_access_exec_read             ........................ passed ...  12
test_access_exec_write            ........................ passed ...  12
test_access_write_read            ........................ passed ...  12

Total:                                                           100/100

If you’re having difficulties solving this exercise, go through this reading material.