1 00:00:00,006 --> 00:00:03,007 - [Instructor] Let's look at driver source stuff, 2 00:00:03,007 --> 00:00:05,001 a little bit in the kernel. 3 00:00:05,001 --> 00:00:06,001 First of all, let's figure 4 00:00:06,001 --> 00:00:10,007 out what ethernet devices we have. 5 00:00:10,007 --> 00:00:12,004 So we've got a few there 6 00:00:12,004 --> 00:00:14,008 and then we've got this kind of real one, 7 00:00:14,008 --> 00:00:17,000 even though we're in a VM. 8 00:00:17,000 --> 00:00:26,003 This ENP thing, let's find out more about that. 9 00:00:26,003 --> 00:00:31,004 And it tells us the driver is MLX5 core. 10 00:00:31,004 --> 00:00:35,006 All right, so let's see if there's a loadable module 11 00:00:35,006 --> 00:00:40,004 with that name. 12 00:00:40,004 --> 00:00:44,007 Yep. Sure enough, there's a module with that name loaded. 13 00:00:44,007 --> 00:00:48,005 So the driver for that ethernet was built 14 00:00:48,005 --> 00:00:51,006 as a loadable kernel module. 15 00:00:51,006 --> 00:00:55,001 Let's see if we can go find the source code for that. 16 00:00:55,001 --> 00:01:05,005 If we find dot name mlx5 underscore core. 17 00:01:05,005 --> 00:01:11,000 Oops. We don't pipe into slash name. 18 00:01:11,000 --> 00:01:12,002 All right, we've got a bunch of stuff 19 00:01:12,002 --> 00:01:15,009 underneath drivers net. 20 00:01:15,009 --> 00:01:28,004 So let's go to drivers net ethernet mellanox mlx5 core. 21 00:01:28,004 --> 00:01:29,006 Now we got lots of stuff. 22 00:01:29,006 --> 00:01:32,002 Now I already did a full build in here, 23 00:01:32,002 --> 00:01:37,005 so I got lots of object files along with the source files. 24 00:01:37,005 --> 00:01:42,007 There's a make file in here. Let's look at the make file. 25 00:01:42,007 --> 00:01:47,004 And we see here on line nine, 26 00:01:47,004 --> 00:01:51,001 obj dash value of some variable, 27 00:01:51,001 --> 00:01:55,004 plus equals mlx5 underscore core dot 0. 28 00:01:55,004 --> 00:01:57,008 Turns out that macro config, 29 00:01:57,008 --> 00:02:02,006 mlx5 underscore core will be the letter M 30 00:02:02,006 --> 00:02:04,009 when we configure this to be a module. 31 00:02:04,009 --> 00:02:07,008 So that will be OBJ dash M, 32 00:02:07,008 --> 00:02:10,008 plus equals MLX five underscore core dot 0. 33 00:02:10,008 --> 00:02:14,000 So it's going to add to the list of modules to be built, 34 00:02:14,000 --> 00:02:17,005 the mlx5 underscore core dot 0. 35 00:02:17,005 --> 00:02:22,001 And then it turns out we've got all kinds of dot 0s 36 00:02:22,001 --> 00:02:24,009 in the file here that will get added in 37 00:02:24,009 --> 00:02:29,000 to make that mlx5 underscore core. 38 00:02:29,000 --> 00:02:33,002 All right, so this is where it's built. 39 00:02:33,002 --> 00:02:35,004 Now what about our config? 40 00:02:35,004 --> 00:02:43,004 Let's grep that config mlx5 underscore core. 41 00:02:43,004 --> 00:02:45,006 That's what was in the make file. 42 00:02:45,006 --> 00:02:49,008 We're going to grep that in our dot config 43 00:02:49,008 --> 00:02:57,008 to see how our kernel is configured with respect to that. 44 00:02:57,008 --> 00:03:00,007 And there we see, it's set to M, like I said. 45 00:03:00,007 --> 00:03:04,002 It's configured to be a loadable module. 46 00:03:04,002 --> 00:03:06,002 All right, so this is a loadable module. 47 00:03:06,002 --> 00:03:09,000 There should be a module, a net function in here somewhere. 48 00:03:09,000 --> 00:03:16,003 Let's see if we can find that. 49 00:03:16,003 --> 00:03:19,000 There's a couple. Let's look at main dot C. 50 00:03:19,000 --> 00:03:22,006 That's probably the main for the driver. 51 00:03:22,006 --> 00:03:26,001 And I was looking at this before, 52 00:03:26,001 --> 00:03:31,008 but if we look at module init, 53 00:03:31,008 --> 00:03:35,001 this is 2,351 lines long. 54 00:03:35,001 --> 00:03:38,007 Second line from the bottom registers the mlx5, 55 00:03:38,007 --> 00:03:41,001 underscore init as the initialization function. 56 00:03:41,001 --> 00:03:46,005 Now let's go look at that. 57 00:03:46,005 --> 00:03:50,008 That's up on line 2,307. 58 00:03:50,008 --> 00:03:53,005 So this is the function that gets called 59 00:03:53,005 --> 00:03:55,005 when the module is loaded. 60 00:03:55,005 --> 00:03:59,002 So you can see it does maybe a little warning, 61 00:03:59,002 --> 00:04:03,008 does some sort of random bits, registers some stuff, 62 00:04:03,008 --> 00:04:08,001 all right, registers it as a PCI driver 63 00:04:08,001 --> 00:04:11,007 and then just check it for errors throughout. 64 00:04:11,007 --> 00:04:14,005 And if it gets an error, it does some cleanup stuff 65 00:04:14,005 --> 00:04:17,000 and then it returns the error. 66 00:04:17,000 --> 00:04:20,005 So that could be minus one or minus two or minus three 67 00:04:20,005 --> 00:04:23,002 or something like that, depending on what the error was. 68 00:04:23,002 --> 00:04:27,000 All right, so that's the mellanox driver 69 00:04:27,000 --> 00:04:29,004 for our ethernet card. 70 00:04:29,004 --> 00:04:37,008 Let's look at that mem dot C. 71 00:04:37,008 --> 00:04:40,007 And let's use cscope. 72 00:04:40,007 --> 00:04:44,006 All right, so we know the name of a function. 73 00:04:44,006 --> 00:04:50,000 Let's look for the definition. How about write null. 74 00:04:50,000 --> 00:04:52,002 Only one of those apparently, it brings us right to it. 75 00:04:52,002 --> 00:04:53,004 Boom, just like that. 76 00:04:53,004 --> 00:04:56,007 Notice read null is right above it. 77 00:04:56,007 --> 00:05:00,009 So just like we saw, the right null just returns the number 78 00:05:00,009 --> 00:05:06,008 of bytes you asked for and read null just returns zero. 79 00:05:06,008 --> 00:05:09,000 Pretty simple, right? 80 00:05:09,000 --> 00:05:13,005 Now how does it know to use these functions? 81 00:05:13,005 --> 00:05:20,005 Well, let me show you a little bit more. 82 00:05:20,005 --> 00:05:22,009 So what a character driver does is, 83 00:05:22,009 --> 00:05:27,005 it registers a struct file operations 84 00:05:27,005 --> 00:05:31,005 and in that struct, it has the address of the functions 85 00:05:31,005 --> 00:05:37,003 that it wants to implement that are part of the file API. 86 00:05:37,003 --> 00:05:42,005 So null implements lseek, read, write, 87 00:05:42,005 --> 00:05:46,005 some iterations, a splice and a U ring command, right? 88 00:05:46,005 --> 00:05:48,003 There's potentially a lot more things, 89 00:05:48,003 --> 00:05:52,002 like M map, didn't implement. 90 00:05:52,002 --> 00:05:55,001 So these are just the ones it implemented. 91 00:05:55,001 --> 00:05:57,008 So you should be able to do any of these with dev null. 92 00:05:57,008 --> 00:05:59,000 If you do one of the others, 93 00:05:59,000 --> 00:06:00,006 it'll just do a default kind of thing 94 00:06:00,006 --> 00:06:02,008 and that might be returning 95 00:06:02,008 --> 00:06:06,003 with an error saying not implemented. 96 00:06:06,003 --> 00:06:08,001 So there is an lseek. 97 00:06:08,001 --> 00:06:11,006 Why don't we just go ahead and look at that. 98 00:06:11,006 --> 00:06:17,000 That's called when you try to seek on a device. 99 00:06:17,000 --> 00:06:20,007 And what the null one does is just set the position to zero. 100 00:06:20,007 --> 00:06:23,000 Every time you call, every time you try to move it, oh, 101 00:06:23,000 --> 00:06:27,002 move the pointer to 100, offset to 100 into the file, 102 00:06:27,002 --> 00:06:30,001 it just sets it to zero and returns back zero, 103 00:06:30,001 --> 00:06:33,004 saying you're at zero no matter what you do. 104 00:06:33,004 --> 00:06:36,009 And the comment here is kind of interesting. 105 00:06:36,009 --> 00:06:41,003 It says now you can open in append mode. 106 00:06:41,003 --> 00:06:43,008 So append mode needed the lseek. 107 00:06:43,008 --> 00:06:46,005 So you can't really append the dev null. 108 00:06:46,005 --> 00:06:47,003 You try to append, 109 00:06:47,003 --> 00:06:49,003 it'll just, you know, when you do the write, 110 00:06:49,003 --> 00:06:51,007 just say it did it write like we saw. 111 00:06:51,007 --> 00:06:55,003 But the open should work. There you go. 112 00:06:55,003 --> 00:07:00,000 That was delving into some kernel source code for drivers.