Lab 4: C Review & GDB

In this lab we will introduce two tools for debugging C code - AddressSanitizer (ASan) and the GNU Debugger (GDB). ASan is useful for catching many common memory bugs. GDB allows you to step through your code one line at a time, with the ability to see values of variables along the way.

In this lab, you a program, sel_sort.c, that contains multiple bugs. Your job is to find these bugs, using the capabilities of GDB and ASan.

ASIDE: Working with Docker + QEMU + GDB

As with other assignments in this course, you should carry out all of your work within the Docker container that is distributed as part of the course infrastructure. The combination of Docker, QEMU, and GDB appears in several real-world applications (for example, kernel debugging), so beyond the standardization it offers for our class assignments, being able to use GDB in this way will turn out to be a useful skill for you.

However, the combination of these three adds some additional complexity to the use of GDB:

• Because it needs to work at the level of the target machine’s ISA (i.e., RISC-V), you can’t just run a compiled program directly with GDB. Instead, you will need to use GDB’s remote-connection facility.

• The remote-connection facility requires that you have two open terminal windows: one for the executable being run under QEMU and the other for GDB to connect to that process. Unfortunately, the fact that we are running QEMU in a Docker container adds even more complication:

  • Because you are running everything in a Docker container, you need to make sure that both terminal windows are invoking the exact same container instance.

Adding Debugging Support To The CS3410 Container

The CS3410 course infrastructure document suggests that you define an alias (or, on Windows, an equivalent PowerShell function):

alias rv='docker run -i --init -e NETID=<YOUR_NET_ID> --rm -v "$PWD":/root ghcr.io/sampsyo/cs3410-infra'

where <YOUR_NET_ID> should be replaced with your actual Cornell NetID.

We’ll use this as the basis for an invocation that adds two additional pieces of functionality, control of the container image’s name and support for core dumps in the current working directory:

alias rv-debug='docker run -it --rm --init -e NETID=<YOUR_NET_ID> --name testing --ulimit core=-1 --mount type=bind,source="$PWD"/,target="$PWD"/ -v "$PWD":/root ghcr.io/sampsyo/cs3410-infra'

To make the alias stick around when you open a new terminal shell, you will need to add it to your shell’s configuration file. You can do this by pasting the alias at the end of your shell’s configuration file or by typing these commands in your terminal but fill in the appropriate file according to your shell.

echo "alias rv-debug='docker run -it --rm --init -e NETID=<YOUR_NET_ID> --name testing --ulimit core=-1 --mount type=bind,source=\"\$PWD\"/,target=\"\$PWD\"/ -v \"\$PWD\":/root ghcr.io/sampsyo/cs3410-infra'" >> ~/.bashrc

As before, you don’t really need to understand the details of Docker to use this in your work, but for the curious:

• --name testing changes the name of the container image to “testing”, but you can choose any other name value, so long as it begins with an upper or lowercase letter. This is useful for situations in which you need to run multiple terminal windows with access to the same container image, as you will in the next section of this assignment.

• --ulimit core=-1 --mount <etc.> enables support for core dumps, which are created when a program crashes. The specific form used here ensures that a core file is always created in the current working directory.

Like rv, you can run rv-debug with zero, one, or more arguments. With zero arguments, you’ll get a bash prompt in the Docker container itself. Any arguments that are supplied are considered to be an execution of an application within the container itself.

As before, there is a similar PowerShell function that you can define if you’re working on a Windows system:

Function rv_debug {
   if (($args.Count) -eq 0) {
      docker run -i --init --rm -e NETID=<YOUR_NET_ID> --name testing --ulimit core=-1 --mount type=bind,source="$PWD"/,target="$PWD"/ -v ${PWD}:/root ghcr.io/sampsyo/cs3410-infra
   }
   else {
      $app_args=""
      foreach ($a in $args[1..($args.count-2)) {
         $app_args = $app_args + $a + " "
      }
      $app_args = $app_args.Substring(0,$app_args.Length-1);
      docker run -i --init --rm -e NETID=<YOUR_NET_ID> --name testing --ulimit core=-1 --mount type=bind,source="$PWD"/,target="$PWD"/ -v ${PWD}:/root ghcr.io/sampsyo/cs3410-infra $args[0] $app_args
   }
}

Try adding this to the function_rv_d file in which you have already defined rv_d. As with the Linux/MacOS version, you should be able to run this just like rv_d, with or without additional arguments.

See the course infrastructure document for details on making this and the rv alias a permanent part of your working environment.

Part 1: Memory Bugs in sel_sort.c

Now that you have the aliases setup for GDB, compile sel_sort.c using the below command:

rv gcc -g -std=c23 -Wall -Werror sel_sort.c -o sel_sort

And run your code:

rv bash # Enter the interactive rv bash shell
# qemu sel_sort
Segmentation fault (core dumped)
# Your code may also hang, in that case press ^C three times in a row to exit.

Now add -fsanitize=address,undefined to the compile command, like so:

rv gcc -g -std=c23 -Wall -fsanitize=address,undefined -Werror sel_sort.c -o sel_sort

Running your code using qemu should give you something similar to this output:

# qemu sel_sort
sel_sort.c:28:10: runtime error: load of misaligned address 0x000000000001 for type 'long int', which requires 8 byte alignment
0x000000000001: note: pointer points here
<memory cannot be printed>
AddressSanitizer:DEADLYSIGNAL
=================================================================
==1==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000001 (pc 0x000000010eec bp 0x001555d569d0 sp 0x001555d56990 T0)
==1==The signal is caused by a READ memory access.
==1==Hint: address points to the zero page.
    #0 0x10eee in swap /root/sel_sort.c:28
    #1 0x11182 in selection_sort /root/sel_sort.c:40
    #2 0x11582 in main /root/sel_sort.c:69
    #3 0x1556ace922 in __libc_start_call_main (/lib/libc.so.6+0x2b922)
    #4 0x1556acea0e in __libc_start_main@GLIBC_2.27 (/lib/libc.so.6+0x2ba0e)
    #5 0x10bda in _start (/root/sel_sort+0x10bda)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /root/sel_sort.c:28 in swap
==1==ABORTING

The important line to focus on here is:

#0 0x10eee in swap /root/sel_sort.c:28

It tells us that line 28 in sel_sort.c caused the segmentation fault. Can you figure out what is wrong on line 28? ASan output can be confusing at times, if you are struggling do not be afraid to ask course staff for help.

After fixing both bugs, you might notice that your code does not print the correct output. Unfortunately, ASan cannot help find logic bugs in your code. For those, GDB is needed.

Part 2: Logic Bugs in Selection Sort

Introduction

The file sel_sort.c contains an implementation of the selection sort algorithm, with a main procedure that tests it on two different arrays. A version that passes its tests will display each array in ascending order. Sadly, it does not pass. In fact, trying to run it results in an unsorted array:

# qemu sel_sort
Test array #1:
[an unsorted array]

Test array #2:
[another unsorted array]

First, lets get GDB set up for your sel_sort.c.

Building Source Files for Debugging

In order to debug a program with GDB, you must first compile its source code with debugging symbols that allow GDB to inspect the resulting executable and display information such as program execution and variable values in terms of the original C code. To do this, compile the source file with the additional -g flag. This flag will add debugging symbols to the executable that will allow GDB to debug much more effectively.

Using GDB’s Remote Debugging

Let’s use GDB to find the bug in selection sort! Assuming you’ve already compiled sel_sort.c, open two terminal tabs or windows (a split view is recommended if your terminal app supports it):

1. In one window, start qemu with the rv-debug alias to create a named docker container:

   # "-g 1234" tells qemu to wait for gdb to connect on port 1234
   rv-debug qemu -g 1234 sel_sort
   

   This will appear to hang, which is what you want.

2. In the other terminal window, run gdb inside the same container with the following docker command:

   # If you changed "--name testing" to something else in the alias, use that name here
   docker exec -it testing gdb -q -ex 'target remote localhost:1234' -ex 'set sysroot /opt/riscv/sysroot' -ex 'set can-use-hw-watchpoints 0' sel_sort
   

   You should see output like this:

   # docker exec -it testing gdb -q -ex 'target remote localhost:1234' -ex 'set sysroot /opt/riscv/sysroot' -ex 'set can-use-hw-watchpoints 0' sel_sort
   Reading symbols from sel_sort...
   Remote debugging using localhost:1234
   Reading symbols from /opt/riscv/sysroot/lib/ld-linux-riscv64-lp64d.so.1...
   (No debugging symbols found in /opt/riscv/sysroot/lib/ld-linux-riscv64-lp64d.so.1)
   0x0000001555d6b9f4 in _start () from /opt/riscv/sysroot/lib/ld-linux-riscv64-lp64d.so.1
   Reading symbols from /opt/riscv/sysroot/lib/ld-linux-riscv64-lp64d.so.1...
   (No debugging symbols found in /opt/riscv/sysroot/lib/ld-linux-riscv64-lp64d.so.1)
   (gdb)
   

   Now you have a gdb prompt and can start executing gdb commands.

GDB Basics

After you entered GDB, there are different commands you can use to help you narrow down the problems. We introduce some of them briefly in the following. With the exception of run, all of these commands should work the same way, whether you’re using GDB in our CS3410 container or natively.

Run

In the remote debugging you’ll use for this assignment and other assignments in the class, you won’t ever use this command (the qemu -g 1234 <etc.> is already running the program you’re debugging). In other settings, however, run is a fundamental part of the basic GDB toolbox. The command runs your program until a breakpoint or crash is encountered. If you are not using GDB remotely, run is the command you would type to begin execution of your program. You can also pause your program by pressing Control-C (useful for finding infinite loops). When one of these is encountered, you will be able to inspect the state of your program with any of the commands below.

Breakpoints, next, step, continue, finish

If we want to stop and see what is going on at a particular point in our program, we can use breakpoints. To do this in GDB, type break, followed by the line number of the source code file where you want to stop. For example, break 64 will set a breakpoint at the beginning of the main in sel_sort.c (i.e. on Line 64). If you want to set a breakpoint at the entry to a procedure, without reference to a line number, you can type break <procedure name> instead.

If the program is already running but paused, continue will resume execution. It will stop at the next breakpoint if there is one, and run to the end, otherwise. If you only want to run to the end of the current procedure, you can use the finish command instead.

After the program stops at a breakpoint, you can use either next or step to execute the program line by line.

(gdb) break main
Breakpoint 1 at 0x10860: file sel_sort.c, line 60.
(gdb) continue
Continuing.
warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?

Breakpoint 1, main (argc=1, argv=0x1555d56d18) at sel_sort.c:60
60          long test_array[5] = {1,4,2,0,3};
(gdb) continue
Continuing.
[Inferior 1 (process 9) exited normally]

Disable/delete breakpoints

Use the delete <N> command to delete breakpoint N, or disable <N> if you only want to disable it. It reverse, enable N , is used to enable breakpoint N. Typing either delete or disable with no arguments will delete/disable all breakpoints at once.

Backtrace

When GDB reaches an error or a breakpoint it will only tell you the line of code that it occurred on. In order to see the whole backtrace, the whole set of stack frames associated with the file at the time, type backtrace. Use this to find the function that called the function. sel_sort.c:<line number> tells you the file and line number of the instruction that was running when the breakpoint was triggered.

(gdb) break swap
Breakpoint 1 at 0x106b8: file sel_sort.c, line 28.
(gdb) continue
Continuing.
Breakpoint 1, swap (a=0x1555d56b58, b=0x1555d56b70) at sel_sort.c:28
28          long tmp = *a;
(gdb) backtrace
#0  swap (a=0x1555d56b58, b=0x1555d56b70) at sel_sort.c:28
#1  0x000000000001077c in selection_sort (arr=0x1555d56b58, len=5) at sel_sort.c:40
#2  0x00000000000108c4 in main (argc=1, argv=0x1555d56d18) at sel_sort.c:69

This gives the state of the call stack and program execution point at the moment that the breakpoint was triggered. This output tells us that the last instruction to run was line 28 of a call to swap, which itself was called on line 42 of selection_sort, and so on.

Print

While having this much information about the call stack is helpful, we will often want to have a more detailed view of what’s going on in the program. We can see the value of any variable that is in scope in the current stack frame by using the commands print and display. These instructions print the value of any expression that is semantically valid at the current line of execution; in particular, they are useful for seeing the current values of declared variables. The difference between them is that display will show the value of its expresion argument after every instruction step, while print displays it just once.

Breakpoint 1, selection_sort (arr=0x1555d56b58, len=5) at sel_sort.c:38
38          for (int i = 0; i < len; i++)
(gdb) print (i < len)
$1 = 1
(gdb) print a
No symbol "a" in current context.
(gdb) display i
1: i = 0
(gdb) step
39              int swap_idx = smallest_idx(&arr[i], len - i);
1: i = 0
(gdb) display (i < len)
2: (i < len) = 1
(gdb) s
smallest_idx (arr=0x1555d56b58, len=5) at sel_sort.c:10
10          int smallest_i = 0;

Notice how the displays fof both i and (i < len) cease when execution steps into the body of smallest_idx. Once smallest_idx returns, the display of these expressions will resume. You can cancel an ongoing fdisplay with undisplay.

(gdb) finish
Run till exit from #0  smallest_idx (arr=0x1555d56b58, len=5) at sel_sort.c:13
0x0000000000010748 in selection_sort (arr=0x1555d56b58, len=5) at sel_sort.c:39
39              int swap_idx = smallest_idx(&arr[i], len - i);
1: i = 0
2: (i < len) = 1
Value returned is $3 = 3
(gdb) undisplay 2
(gdb) s
42              swap((long *)arr[i], (long *)arr[swap_idx]);
1: i = 0
(gdb)

Finally, a related command, x, gives a more low-level version of this same feature by showing the contents of memory at a given address. See https://visualgdb.com/gdbreference/commands/x, among other resources, for a detailed explanation.

Info

The info command provides brief summaries of important program information:

• info locals—displays the values of every local variable in the current stack frame
• info args—displays the values of every parameter in the current stack frame
• info stack—displays the current call stack
• info break—displays all currently-defined breakpoints, whether they are enabled or not.

Some Advanced GDB Features

Watchpoints

Watchpoints break the program execution whenever the value of an expression changes, and the value changes will be displayed. To set a new watchpoint, you need to invoke watch with either an expression or a raw memory address. If you watch an expression, it must be semantically valid for the current execution point (i.e. all variables in scope, etc.); the watchpoint will be deleted when execution leaves the block in which the expression is meaningful. To watch the contents of a memory address regardless of the program’s block structure, use the -location (or -l) flag. For example, you could set a watchpoint on index 0 of the array test_array.

Breakpoint 1, main (argc=1, argv=0x1555d56d18) at buggy_sel_sort.c:64
64          long test_array[5] = {1,4,2,0,3};
(gdb) watch test_array[0]
Watchpoint 2: test_array[0]
(gdb) watch -location test_array[0]
Watchpoint 3: -location test_array[0]
(gdb) continue
Continuing.

Watchpoint 2: test_array[0]

Old value = 0
New value = 1

Watchpoint 3: -location test_array[0]

Old value = 0
New value = 1
0x000000000001088c in main (argc=1, argv=0x1555d56d18) at buggy_sel_sort.c:64
64          long test_array[5] = {1,4,2,0,3};
(gdb) continue
Continuing.

Watchpoint 2 deleted because the program has left the block in
which its expression is valid.
(gdb) info break
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x000000000001086c in main at buggy_sel_sort.c:64
        breakpoint already hit 1 time
3       watchpoint     keep y                      -location test_array[0]
        breakpoint already hit 1 time
4       breakpoint     keep y   0x0000000000010710 in selection_sort at buggy_sel_sort.c:38

The command info break will show watchpoints as well as breakpoints. To disable a watchpoint, type disable <watchpoint_num>.

Conditional Breakpoints

Conditional breakpoints enable you to break execution on a line of code when an expression evaluates to true. To set a new conditional breakpoint, type break <line number> if <expression>. For example, to break from execution when smallest_idx is not equal to arr[0] on line 17, you can type break 17 if smallest != arr[0]. Conditional breakpoints allow you to debug specific scenarios and limit the messages that you would collect otherwise when debugging without specific conditions.

(gdb) break 17 if smallest != arr[0]
Breakpoint 1 at 0x1065c: file buggy_sel_sort.c, line 17.
(gdb) continue
Continuing.
warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?

Breakpoint 1, smallest_idx (arr=0x1555d56b58, len=5) at buggy_sel_sort.c:17
17                  smallest_i = i;
(gdb) print smallest_idx
$1 = {int (long *, int)} 0x105f0 <smallest_idx>

Initialization Args & Files

Having to type target remote localhost:1234 and break 17 and continue or similar every time you want to debug a program is quite tedious. Fortunately, there are several ways to automate gdb to run commands immediately when the debugger starts: the gdb source command, -x and -ex arguments, and .gdbinit files. We actually used the -ex flag earlier!

Fix the Sorting

Now, use GDB to see what is causing your selection sort to fail.