WEBVTT 00:00.380 --> 00:01.370 Hello, my name is Tayfun. 00:01.370 --> 00:04.970 And in this lecture we will step through the program with the debugger. 00:04.970 --> 00:08.900 Now, as we did in previous lecture, we will look at our program again. 00:08.900 --> 00:14.720 So actually, let's run it our Hello program that we will assemble a program, assembler language. 00:14.720 --> 00:17.300 And here this is our source code for it. 00:17.300 --> 00:21.560 So we will gdb hello again and that's it. 00:21.560 --> 00:26.300 So here what we're going to do is we will firstly. 00:27.150 --> 00:27.360 It. 00:28.530 --> 00:35.400 We will we will put a break in the program, which is pausing the execution and allowing us to examine 00:35.400 --> 00:36.540 a number of things. 00:36.540 --> 00:39.180 So here we will do break main. 00:39.330 --> 00:46.470 So here, as you can see here, it gave us output a breakpoint at, uh, 40 1110 file. 00:46.470 --> 00:46.740 Hello. 00:47.460 --> 00:49.950 And in line eight. 00:49.950 --> 00:53.070 And now in our case, this is the output here. 00:53.070 --> 00:53.340 So. 00:53.340 --> 00:57.900 But this memory numbers will be different at your PC. 00:58.380 --> 01:04.590 And now what we're going to do is we will run the program now run. 01:04.590 --> 01:07.590 And as you can see here, we also have the breakpoint here. 01:07.590 --> 01:13.980 So the debugger stops at the break and throws the next instruction that will be executed. 01:13.980 --> 01:20.880 So that is, uh, move here, move rax one is not executed yet. 01:21.460 --> 01:26.170 And now we will type this info registers here. 01:26.170 --> 01:31.400 And as you can see, we this is the gdb returns the output here. 01:31.420 --> 01:32.170 So. 01:32.920 --> 01:41.230 The content of these registers is not that important now, except for this recipe. 01:41.260 --> 01:44.680 Here we will, as you can see here, our IP. 01:45.100 --> 01:47.950 So the instructor instruction pointer. 01:47.950 --> 01:57.960 So the register our IP has the value of 40 1110, which is the memory address of the next instructions 01:57.970 --> 01:59.320 to execute. 01:59.350 --> 02:08.800 So remember, we put the breakpoint at the four 1110, and this is the same address for 11 1040 1110, 02:09.310 --> 02:09.880 same address. 02:09.880 --> 02:10.430 Right. 02:10.450 --> 02:15.100 So here, this points to the first instruction. 02:15.830 --> 02:19.040 And we also have mov rax one gdb. 02:19.820 --> 02:24.530 Let's actually put this somewhere move rax one gdb here. 02:25.190 --> 02:32.360 This stops and gdb stops just before the instructions and waits for your command. 02:33.180 --> 02:34.410 And it is not important. 02:34.410 --> 02:35.940 And it's sorry. 02:35.940 --> 02:42.390 It's important to remember that the instruction pointed to by RIP here. 02:42.420 --> 02:44.910 This rip is not executed. 02:45.520 --> 02:46.920 Not yet executed here. 02:46.920 --> 02:54.410 So in your case gdb may show something different from the 40 1110 that and that's okay. 02:54.420 --> 03:01.260 It is the address of that particular line in memory and which may be different depending on depending 03:01.260 --> 03:02.700 on your computer configuration. 03:02.700 --> 03:05.520 But in most cases it will be different than this. 03:05.520 --> 03:08.580 And now we will type the step. 03:09.850 --> 03:10.330 Yep. 03:10.540 --> 03:15.940 And as you can see here, we have this here now, which is the now we will also. 03:17.480 --> 03:19.900 Type I and are here. 03:19.910 --> 03:22.820 So this is the abbreviation for info registers. 03:22.820 --> 03:28.820 As you can see here I if stands for EHR info and r for registers. 03:30.460 --> 03:33.400 And after that we will get this output here. 03:33.490 --> 03:41.800 Now indeed Rax contains now zero here, 00X1 here actually. 03:42.700 --> 03:44.620 And here. 03:45.360 --> 03:47.100 0X11. 03:48.030 --> 03:50.850 And and the rip here. 03:50.880 --> 03:55.940 The rip here contains the address of the next instruction to execute. 03:55.950 --> 04:05.650 So we can also step further through the program and notice that how RSI receives the address of message. 04:05.880 --> 04:11.040 A message which prints the Hello World on the screen and exits. 04:11.040 --> 04:19.860 And notice also how Rip Rip points every time to the next instruction to execute. 04:19.950 --> 04:28.740 And here we also I want to discuss here is also some additional GDB commands here which is the break 04:28.740 --> 04:35.340 here or b sets a break point as we have done before in this lecture. 04:35.340 --> 04:39.570 So for example, we can also disable this uh, break point here. 04:39.570 --> 04:42.720 So disable break point. 04:42.990 --> 04:44.070 Uh, break. 04:45.940 --> 04:46.660 Point. 04:46.660 --> 04:49.270 And after that, you will enter the number in this case, actually. 04:49.360 --> 04:53.350 As you can see, this is the main is not the breakpoint number here. 04:54.530 --> 05:00.980 But also we also can, after disabling it, enable the break point again. 05:03.430 --> 05:04.150 Like this. 05:04.480 --> 05:06.910 And we also have the continue. 05:08.380 --> 05:14.590 Or see this is the continue the execution until the next breakpoint. 05:14.620 --> 05:21.400 As you can see here, we don't have we don't have breakpoints except that main breakpoint here with. 05:23.190 --> 05:26.160 And that's why our program exited normally. 05:26.160 --> 05:28.020 So our program done executed. 05:28.020 --> 05:35.580 And we also have the next we have this is to step over the current line and stop at the next line. 05:35.760 --> 05:38.360 So we also have help. 05:38.370 --> 05:42.310 This shows the help here, help or age. 05:43.180 --> 05:43.660 So. 05:45.070 --> 05:46.120 As you can see here. 05:46.120 --> 05:55.210 And we also have to enable this is enables the text user interface and to disable it, you can simply 05:55.210 --> 06:04.300 write why disable and we can also which you learned in previous lecture actually um, print and also 06:04.300 --> 06:07.750 print the value of a variable registers and so on. 06:08.480 --> 06:11.480 So now let's actually see some examples here. 06:11.510 --> 06:14.270 Uh, now we will print, uh, p. 06:14.570 --> 06:15.530 Racks. 06:15.560 --> 06:16.520 Racks here. 06:16.520 --> 06:21.320 And as you can see, there is a no registers because our program is already done executing. 06:21.380 --> 06:22.460 Now we will. 06:22.880 --> 06:24.710 Let's actually start this again. 06:24.740 --> 06:27.590 Clear the screen and gdb. 06:28.190 --> 06:28.580 Hello. 06:29.400 --> 06:34.800 And as you can see here, we started our, uh, debugging process from zero again. 06:34.800 --> 06:37.500 And now what we're going to do is we will start. 06:38.910 --> 06:40.410 We must set a break point. 06:40.620 --> 06:41.370 Break. 06:42.320 --> 06:43.890 Break main. 06:44.250 --> 06:48.640 And we can also enable or disable the break point as we did in previous. 06:48.660 --> 06:50.640 As you learned in here. 06:51.270 --> 06:51.960 In order to. 06:53.260 --> 06:53.680 Run it. 06:53.680 --> 06:59.290 So we will disable, as you can see here, when we set the break main here, we are getting this one 06:59.290 --> 07:03.820 number here so we can use it to disable break point. 07:03.820 --> 07:06.280 And after that we will set the number one. 07:06.280 --> 07:10.810 And as you can see, it's disabled and we can also delete the break point. 07:10.810 --> 07:13.360 So we disabled it but didn't delete it. 07:13.360 --> 07:17.320 So after that we can also enable a break point. 07:17.320 --> 07:26.710 So but if you deleted it, you need to define a Neve break Neve break here enable break point. 07:26.710 --> 07:31.840 And as you can see here, we also have the break point here enabled. 07:31.840 --> 07:34.870 And now we will delete the break point here. 07:34.870 --> 07:38.020 And after that you will enter the number and this is the list. 07:38.030 --> 07:45.610 So now if we enable this break point right now, enable break point one, we will get an error because 07:45.610 --> 07:47.710 the no break point number one. 07:47.710 --> 07:51.070 So now we will add our break point again. 07:51.070 --> 07:56.680 And after that we will run here or run our main program. 07:56.680 --> 08:01.690 And we will also use the info registers here that we did here. 08:01.690 --> 08:05.590 And as you can see, that's so this line is not executed. 08:05.590 --> 08:12.610 And now we will also get the info registers, info registers here and now. 08:14.920 --> 08:17.650 We will use this print function here. 08:17.770 --> 08:25.540 Now p here and this dollar sign rax and as you can see, we have this. 08:26.020 --> 08:26.740 And. 08:27.840 --> 08:29.250 P message here. 08:29.460 --> 08:34.510 As you can see, it's void because there's no message here, which the message is not stored. 08:35.190 --> 08:40.650 Variable type stored in the memory. 08:40.950 --> 08:44.020 So we need to get that memory address to print it out. 08:44.040 --> 08:45.930 In this case here. 08:49.700 --> 08:53.210 Of which we learned that in previous lecture. 08:53.210 --> 08:58.640 We did some examples to print the Hello world message using the GDB. 08:59.030 --> 09:07.430 And one important remark about the GDP is to properly use it, you must insert a function prologue and 09:07.430 --> 09:09.870 the function epilogue in your code. 09:09.890 --> 09:13.670 So I will show in the next sections how to do that. 09:13.670 --> 09:20.870 And in later sections we will discuss function, prologue and function epilogues when we talk about 09:20.870 --> 09:21.860 stack frames. 09:21.860 --> 09:27.980 So for short information, for short programs such as our Hello World program, there is no problem. 09:27.980 --> 09:35.810 But with longer and bigger and fancy programs, GDB will show an expected behavior if there is no prologue 09:35.810 --> 09:36.980 or epilogue. 09:36.980 --> 09:41.810 So play around with gdb refer to the online manual for example. 09:41.810 --> 09:46.190 You can also type the man gdb here man. 09:47.840 --> 09:48.380 EDB. 09:49.670 --> 09:50.900 So here. 09:51.950 --> 09:54.560 And in this case, I didn't get it any. 09:54.890 --> 09:55.610 So it's okay. 09:55.610 --> 10:02.660 But you can also find or find a lot of online manuals and get familiar with GDB because even when you 10:02.660 --> 10:09.470 use a graphical user interface debugger, some functionality may not be available or may not want to 10:09.470 --> 10:16.670 install the graphical user interface debugger on your system because GDB does all the job at 10 or 10 10:16.670 --> 10:17.390 out of ten. 10:17.390 --> 10:24.560 And now in the next lecture we will also create some assembly code, a slightly improved version of 10:24.560 --> 10:25.250 Hello World. 10:25.250 --> 10:27.200 So I'm meeting you in next lecture.