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CSCI 247 Computer Systems I Lab 3 : assembly, gdb In lecture we have briefly discussed assembly language, and how you might reverse engineer (disassemble) an executable to see the assembly-level instructions. The Objectives of this lab are the following: • Inspect some of the architecture elements of the CS linux computers • Become familiar with GDB, which is the standard (plain text) debugger available on most linuxish computers • Use the disassemble feature of gdb to inspect the assembly code equivalent of a program As was the case for labs 1 and 2, ask the TAs lots of questions. They are there to help. And, you CAN work with peers in completing the labs. This is a bit tricky when everybody is remote; you may use slack, email, discord, etc. Up to you. But, in no circumstances should you SHARE code, which involves emailing code and then submitting it as your own. Submitting your work Submit the asked-for file as the Lab 3 Submission item on Canvas. You must submit your file by the due date/time specified on Canvas. Logging into your CS account Use ssh to connect to one of the CS linux computers. All instructions in this lab – referring to gdb, gcc, etc. – assume the use of the CS machines. I. Architecture specifics In this part of the lab, you’ll explore some of the architecture elements of the computer that you are logged into. First, let us inspect if the CS machines are 32-, or 64-bit machines. There are many ways to find this out; only a few are shown here. The uname linux command prints system information. Issue uname, with the -a(ll) falg: $uname -a You’ll see a list of details, including the name of the machine, the processor type, the operating system, etc. Page 2 CSCI 247, Lab 3 Filip Jagodzinski, Summer 2020 Next, let us inspect the hardware and memory attributes of the computer that you’ve logged onto. One such command to get that information is the free command, which can be issued with a variety of flags, including m(egabytes), h(uman readable), g(igabytes), etc. Issue each of the following (they provide the same info, but different formatting) to determine how much memory is available on the linux machine where you are logged on. $free -g $free -m $free -h The program who and htop inform you who else is logged onto the computer, and which processes are currently running – and what resources they are using. Issue who, to see who is logged on. Your username should be among those listed. $who Although not the focus of this class (instead, you’ll learn more about this when you take CSCI 447, Operating Systems), there are a variety of command line programs that you can use to inspect what is running – and what memory resources each program is using – on your computer. Issue htop, to see who is logged on, what the load is on each of the CPUs, how much memory each process is using, etc. The htop program includes help (F1), and a variety of other options. Page 3 CSCI 247, Lab 3 Filip Jagodzinski, Summer 2020 II. The C-program Now that we have a rudimentary understanding what type of computer you are logged into – which is important, if ever you needed to look at the instruction set to get a better understanding of the assembly opcodes that are available – let us start by writing and compiling a simple C program. Save the C program into the file sample.c, shown right. This seems harmless enough … and it is. As mentioned in lecture, there are multiple ways that you can inspect the assembly code that would be generated from the source code file. III. Using gcc to generate the assembly code As was shown in the lecture slides, issue the following, to generate the .s file for the program sample.c: $gcc -S sample.c Let’s inspect the assembly code, which is in the file sample.s. pushq %rbp movq %rsp, %rbp movl $10, -8(%rbp) movl $15, -4(%rbp) movl -4(%rbp), %eax subl -8(%rbp), %eax movl %eax, -8(%rbp) movl -8(%rbp), %eax addl %eax, -4(%rbp) movl $0, %eax popq %rbp ret All lines that begin with a period are not being shown here, because they are annotations for the linker. Become familiar with the syntax and registers. Do a web search and/or ask your TA for the questions to the following: • What does a % refer to? • What does a $ designate • What do rbp, eax, and rsp refer to? • What does a “-8” refer to, as for example -8(%rbp) • What do the opcodes pushq, mov, sub, add, pop, ret specify • What do the single letter suffixes l, q (and are there others?) specify, as for example movq versus movl. int main() { int x, y; x = 10; y = 15; x = y-x; y = x+y; } Page 4 CSCI 247, Lab 3 Filip Jagodzinski, Summer 2020 IV. First use of gdb. GDB is the GNU Debugger, which is a text-only portable debugger that is included with most Linux-ish operating systems. It provides all of the features that you’d find in any IDE debugger, such as setting breakpoints, stepping through functions, stepping into functions, etc. Available in the files section of Canvas is a GDB debugger document. Although the gdb commands needed for this lab are all presented here, you will need to refer to the documentation for (probably) homework 2, and definitely project 2. To use a program with gdb, you must compile the program with the -g flag, to inform the compiler to insert debugger symbols so that the executable can be intercepted by gdb to permit inspecting various elements of the program. $gcc -g -o sample sample.c Now that you have created an executable suitable for use in gdb, start gdb, and “load” the sample executable into gdb: $gdb ./sample You’ll see the (not too stylish – hey I did say text-only!) gdb screen, similar to what is shown below. That is “the” gdb prompt. At the (gdb) prompt, set a breakpoint at the start of the main() function of the program by using the b command. This will make the debugger pause when it starts executing the main() function. (gdb) b main When issued, gdb will inform you that a breakpoint has been set. Page 5 CSCI 247, Lab 3 Filip Jagodzinski, Summer 2020 At the (gdb) prompt, run the program by using the r command: (gdb) r You will see the output similar to what is shown in the following figure: At this point, gdb has reached a breakpoint, and is now waiting for your command. There are a variety of commands (refer to the gdb tutorial). One of these commands is next, which you can use to proceed to the next line of code. Two useful commands are continue, and display, where display allows you to inspect the value of any variable that is in scope. For example, assume you want to inspect the value of the variable x: (gdb) disp x V. Disassembling GDB includes a disassemble option to convert the machine language of the program back to assembly language. Instruct gdb to disassemble the program (machine language) into assembly language, by issuing the disassemble command: (gdb) disassemble The output should be similar to what is shown in the figure on the right. What similarities and differences are there between this disassembled code, and the assembly code that you generated using the gcc -S flag? For starters, notice that gdb is telling you, via the =>, where the program is currently executing. Neat! Page 6 CSCI 247, Lab 3 Filip Jagodzinski, Summer 2020 Note that %rbp (the base pointer) is set to the top of the stack. The local variables x and y are stored on the stack at positions beyond the base pointer. Since x and y are both int, each requires 4 bytes (that’s the C default) so their values are stored at addresses -0x8(%rbp) and -0x4(%rbp), respectively, where the 8 and 4 refer to addresses BEYOND the start of the memory location referred to by rbp. Here’s the assembly instructions again, with annotations: push %rbp # save the old value of %rbp mov %rsp,%rbp # set %rbp to the value of %rsp movl $0xa,-0x8(%rbp) # x = 10 movl $0xf,-0x4(%rbp) # y = 15 mov -0x4(%rbp),%eax # load y into %eax sub -0x8(%rbp),%eax # y - x is now in %eax mov %eax,-0x8(%rbp) # x = y - x mov -0x8(%rbp),%eax # load x into %eax add %eax,-0x4(%rbp) # y = x + y mov $0x0,%eax pop %rbp # restore the old value of %rbp retq # return from main() VI. Disassembling aProgram For this part of the lab, you will retrieve an executable from my (Filip’s) home directory, and disassemble it using gdb. Retrieve the aProgram executable file from the following directory, on the linux CS machines: /home/jagodzf/public_html/teaching/csci247/labs/lab3 Refer to lab 1 on how to copy a file from one directory to another. Your task: Use gdb to set a breakpoint, and disassemble the program. Then, compose the file aProgram.c, which you will upload to Canvas. Rubric and submission Please upload your aProgram.c file to Canvas. The aProgram.c file contains the plain text .c source code that would generate the assembly code for the executable aProgram. 10 points 10 points