Lecture 16 – Mar 9th, 2020

Setup

1. Log in to clyde.
2. Create a directory and cd into it.
3. Copy ~steve/ex/arr.c into the directory.

Task

1. Look at the code for arr.c. It currently doesn’t do much more than look at its command line parameters. Compile the program and run it with a positive number, a negative number, 0, and something that isn’t a number but starts with a number like 35foo.
2. Inside main, create a variable-length array, seq, of ints of size n. Initialize this array using a for loop such seq[idx] has the value idx*idx if idx is even and -idx*idx if idx is odd.
3. Write a function

   int sum(size_t len, int arr[len]);
   

   that computes and returns the sum of the elements of arr.

   Inside main, print out the result of sum(n, seq).

   Compile and run the code with a few small values of the command line argument NUM. Make sure you compile with

   $ clang -Wall -std=c11 -o arr arr.c
   

   and fix any warnings that appear.

4. Change the code to print out sum(n+10, seq) instead. Before you compile and run this, make a prediction as to what will happen? Now compile and run. Did it match your expectation?

5. In task 4, the code was changed so that the sum function would read beyond the end of the array. The compiler didn’t complain about this. At runtime, you got strange results, but there was no error or warning.

   This is bad. Nearly every C program in existence has a similar problem.

   Compile your program with

   $ clang -Wall -std=c11 -fsanitize=address,undefined -o arr arr.c
   

   and run it again. This time it should crash with a colorful error message. This additional option tells the compiler (and the linker) to insert additional code in the program to detect these sorts of undefined behavior.

6. You can get a nice backtrace (a list of the functions that were called leading to the error) with a couple of more steps.

   Compile with

   $ clang -Wall -std=c11 -fsanitize=address,undefined -g -o arr arr.c
   

   where the additional -g option adds debugging information to the binary. Run the program with a particular environment variable set.

   $ ASAN_SYMBOLIZER_PATH=/usr/lib/llvm-6.0/bin/llvm-symbolizer ./arr 8
   

   When I do that, I get the output

   ==7410==ERROR: AddressSanitizer: dynamic-stack-buffer-overflow on address 0x7ffd4d3aee80 at pc 0x00000051222d bp 0x7ffd4d3aedd0 sp 0x7ffd4d3aedc8
   READ of size 4 at 0x7ffd4d3aee80 thread T0
    #0 0x51222c in sum /usr/users/noquota/faculty/steve/inclass/arr.c:8:14
    #1 0x5128f8 in main /usr/users/noquota/faculty/steve/inclass/arr.c:25:18
    #2 0x7f7ab6782b96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #3 0x419d59 in _start (/net/storage/zfs/faculty/steve/inclass/arr+0x419d59)
   

   where this is telling me that the error happened in arr.c in the sum function on line 8, column 14. It also shows where sum was called from, namely main at line 25, column 18.

7. Write a program to concatenate all command line arguments into a single string separated by spaces. I.e., you want something like

   int main(int argc, char *argv[argc]) {
     char msg[] = "You ran the following command:";
     char command_line[50];
     /* copy argv[0] to command_line */
     for (int i = 1; i < argc; ++i) {
       /* append a space to command_line */
       /* append argv[i] to command_line */
     }
     puts(msg);
     puts(command_line);
     return 0;
   }
   

   Use strcpy(3) and strcat(3) to perform the copies.

   Run the program with slightly more than 50 characters of command line arguments. What happens? Recompile with -fsanitize=address,undefined and run the program again.

8. Modify the program to use strlcpy(3) and strlcat(3) instead. Recompile with -fsanitize=address,undefined and see what happens.