1 00:00:00,000 --> 00:00:05,007 (upbeat music) 2 00:00:05,007 --> 00:00:08,004 - [Instructor] We've got some good challenges for you here. 3 00:00:08,004 --> 00:00:10,008 So we got to do some kernel configuration. 4 00:00:10,008 --> 00:00:15,006 I'm just going to use make menuconfig. 5 00:00:15,006 --> 00:00:21,000 The first one is remove ReiserFS support. 6 00:00:21,000 --> 00:00:23,005 And that's a file system. 7 00:00:23,005 --> 00:00:26,004 So we look at the top level here. 8 00:00:26,004 --> 00:00:29,007 We find file systems 9 00:00:29,007 --> 00:00:33,007 and we look for ReiserFS and we find it right there. 10 00:00:33,007 --> 00:00:38,000 It's currently by default chosen as a module. 11 00:00:38,000 --> 00:00:42,003 It is deprecated though, so I'm going to hit space bar. 12 00:00:42,003 --> 00:00:44,001 That turns it into an asterisk. 13 00:00:44,001 --> 00:00:47,001 I hit space bar again, turns it off. 14 00:00:47,001 --> 00:00:49,008 So that removes it. 15 00:00:49,008 --> 00:00:52,008 And then I can get out of file systems here by moving 16 00:00:52,008 --> 00:00:55,003 with my arrow key over to exit. 17 00:00:55,003 --> 00:01:00,008 And the next one was choosing debug atomic sleep. 18 00:01:00,008 --> 00:01:03,000 So where could that be? 19 00:01:03,000 --> 00:01:06,007 Now one thing you could do with these is you could do a grep 20 00:01:06,007 --> 00:01:10,004 throughout the kernel source looking at K config files. 21 00:01:10,004 --> 00:01:12,009 And I do that sometimes. 22 00:01:12,009 --> 00:01:15,006 You could try a search in here. 23 00:01:15,006 --> 00:01:18,006 I'll leave that up to you how you find it. 24 00:01:18,006 --> 00:01:20,007 But I believe debug atomic sleep 25 00:01:20,007 --> 00:01:23,007 is underneath kernel hacking. 26 00:01:23,007 --> 00:01:27,009 So there's kernel hacking 27 00:01:27,009 --> 00:01:33,006 and where would it be under there? 28 00:01:33,006 --> 00:01:37,005 Okay, I don't see it at this level. 29 00:01:37,005 --> 00:01:40,001 Scheduler? 30 00:01:40,001 --> 00:01:41,003 No. 31 00:01:41,003 --> 00:01:45,004 Alright, so this is the process of trying to find stuff. 32 00:01:45,004 --> 00:01:46,002 Let's see. 33 00:01:46,002 --> 00:01:47,004 It's debug atomic sleep. 34 00:01:47,004 --> 00:01:50,005 That's locking. 35 00:01:50,005 --> 00:01:53,006 So how about lock debugging? 36 00:01:53,006 --> 00:01:58,002 We're looking for debug atomic sleep. 37 00:01:58,002 --> 00:02:02,007 How about sleep inside atomic section checking. 38 00:02:02,007 --> 00:02:04,005 Let's see if that could be it. 39 00:02:04,005 --> 00:02:06,009 We go to help. 40 00:02:06,009 --> 00:02:11,004 And the top there says config debug_atomic_sleep. 41 00:02:11,004 --> 00:02:13,004 So we could have searched for that. 42 00:02:13,004 --> 00:02:16,005 Maybe did that, you found it quickly, alright? 43 00:02:16,005 --> 00:02:19,008 And we want to turn that on. 44 00:02:19,008 --> 00:02:22,000 So we exit out of the help here. 45 00:02:22,000 --> 00:02:25,003 We hit the space bar, that turns it on. 46 00:02:25,003 --> 00:02:28,002 And then we got some questions 47 00:02:28,002 --> 00:02:31,006 about disable heap randomization. 48 00:02:31,006 --> 00:02:34,007 It's another challenging one to find. 49 00:02:34,007 --> 00:02:37,008 I'm going to the top level here 50 00:02:37,008 --> 00:02:44,005 and I believe that's under memory management options. 51 00:02:44,005 --> 00:02:48,006 We're looking for disable heap randomization. 52 00:02:48,006 --> 00:02:49,004 There it is. 53 00:02:49,004 --> 00:02:51,004 It's off. 54 00:02:51,004 --> 00:02:53,001 The challenge didn't say to turn it on, 55 00:02:53,001 --> 00:02:56,001 it just said is it safe for recent distros. 56 00:02:56,001 --> 00:02:58,000 And what's the default? 57 00:02:58,000 --> 00:03:00,006 So let's do the help. 58 00:03:00,006 --> 00:03:04,003 Okay, so it was off, so the default was off. 59 00:03:04,003 --> 00:03:06,004 That's also known as n. 60 00:03:06,004 --> 00:03:09,003 So we see that equals n there. 61 00:03:09,003 --> 00:03:13,005 That means the default is not on. 62 00:03:13,005 --> 00:03:18,001 And it says here on ancient distros, 63 00:03:18,001 --> 00:03:22,002 post 2000 ones, off is usually a safe choice. 64 00:03:22,002 --> 00:03:24,006 So we could turn it off. 65 00:03:24,006 --> 00:03:26,006 That would be okay. 66 00:03:26,006 --> 00:03:30,002 Alright, so we found that documentation 67 00:03:30,002 --> 00:03:34,005 and then we want to find KVM. 68 00:03:34,005 --> 00:03:36,003 Is that enabled as a module? 69 00:03:36,003 --> 00:03:38,004 What'll be the module name? 70 00:03:38,004 --> 00:03:40,004 So that's kernel virtual machines. 71 00:03:40,004 --> 00:03:43,007 Let's look under virtualization. 72 00:03:43,007 --> 00:03:46,006 There's KVM right there. 73 00:03:46,006 --> 00:03:49,001 It's enabled as a module, yes. 74 00:03:49,001 --> 00:03:52,002 And then we do a help on that. 75 00:03:52,002 --> 00:03:56,001 And here in documentation it says the module 76 00:03:56,001 --> 00:03:58,007 will be called KVM. 77 00:03:58,007 --> 00:04:02,000 So if we do an LS mod, we could see if KVM is loaded. 78 00:04:02,000 --> 00:04:04,007 Otherwise we could do a mod probe on that. 79 00:04:04,007 --> 00:04:08,004 What device file will refer to it. 80 00:04:08,004 --> 00:04:11,001 And it says right here a character device node 81 00:04:11,001 --> 00:04:13,007 or file will be called dev/kvm. 82 00:04:13,007 --> 00:04:15,000 There you go. 83 00:04:15,000 --> 00:04:18,000 Alright, so we did a bunch of config related stuff. 84 00:04:18,000 --> 00:04:21,003 We're going to exit out of here. 85 00:04:21,003 --> 00:04:23,004 Let's build bzImage. 86 00:04:23,004 --> 00:04:26,009 Now if you did change any configuration options 87 00:04:26,009 --> 00:04:29,004 and you had already built it, it might take a while 88 00:04:29,004 --> 00:04:32,006 to build it again, but I built it 89 00:04:32,006 --> 00:04:33,008 and didn't change anything, 90 00:04:33,008 --> 00:04:37,001 so I'm just going to do the command to do it again. 91 00:04:37,001 --> 00:04:40,009 And it shouldn't take long. 92 00:04:40,009 --> 00:04:50,004 In fact, we'll time it and we'll spell it correctly. 93 00:04:50,004 --> 00:04:52,008 Alright, it only took 11 and a half seconds. 94 00:04:52,008 --> 00:04:55,003 It says it's right there. 95 00:04:55,003 --> 00:05:06,003 Let's see how big it is. 96 00:05:06,003 --> 00:05:11,005 So it is about 14 meg. 97 00:05:11,005 --> 00:05:20,008 Let's look how big the kernel image we boot from, 98 00:05:20,008 --> 00:05:24,004 how big that file is. 99 00:05:24,004 --> 00:05:27,009 And it's pretty close to the same size, right? 100 00:05:27,009 --> 00:05:29,009 And then what about VM Linux? 101 00:05:29,009 --> 00:05:31,004 Well, it turns out VM Linux 102 00:05:31,004 --> 00:05:35,001 is in the top level kernel directory. 103 00:05:35,001 --> 00:05:39,000 And if we look at that, 104 00:05:39,000 --> 00:05:42,000 that file is much, much bigger. 105 00:05:42,000 --> 00:05:47,008 So let's see, that's like 397 meg. 106 00:05:47,008 --> 00:05:51,006 And if we ask what kind of file is that? 107 00:05:51,006 --> 00:05:58,009 It's actually a statically linked x86-64 executable. 108 00:05:58,009 --> 00:06:01,005 You need the VM Linux file sometimes 109 00:06:01,005 --> 00:06:02,006 for like debugging the kernel 110 00:06:02,006 --> 00:06:07,002 'cause it's got the symbols in it and so forth. 111 00:06:07,002 --> 00:06:10,000 But otherwise you don't need it 112 00:06:10,000 --> 00:06:14,001 and you're booting with the bzImage file renamed 113 00:06:14,001 --> 00:06:15,008 and put in boot. 114 00:06:15,008 --> 00:06:17,001 Alright, I hope that was helpful. 115 00:06:17,001 --> 00:06:19,008 I hope you learned a lot about the kernel. 116 00:06:19,008 --> 00:06:24,000 It's really useful and cool stuff, thanks.