July 2013 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

How to get the inode - no path_lookup
by Rishi Agrawal Sept. 17, 2015

Sept. 17, 2015

Hi All, I had a module which used the path_lookup function to print the details of any file's inode. I now want to rewrite that module in order to show some juniors how to write some code in kernel. I am using 3.4.6 kernel, I tried finding out path_lookup but google showed that it has been removed. I tried the following code then which did not work . . . dentry = kern_path_create(AT_FDCWD, filename, &path, 1); if (IS_ERR(dentry)) { printk("Failed to obtain the dentry"); return; } its not returning dentry I again tried after seeing the implementation of vfs_stat function user_path_at(AT_FDCWD, filename, lookup_flags, &path); but this also fails. I am using a proc interface to pass the filename, and copying the filename into a kernel buffer. How can I get a copy of vfs inode for a file name. -- Regards, Rishi Agrawal

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RE: Debugging techniques inside kernel
by Anand Arumugam April 9, 2014

April 9, 2014

Try using strace and see if all of the different crashes have the same or similar looking call stack. Here's a link on how to use it in case you didn't know already: http://stackoverflow.com/questions/174942/how-to-use-strace -----Original Message----- From: manty kuma Sent: 7/7/2013 8:06 PM To: Valdis Kletnieks Cc: kernelnewbies(a)kernelnewbies.org Subject: Re: Debugging techniques inside kernel Hi Valdis, Thanks for the info. I have serial logs (using minicom). This is not helping me much as randomly memory is getting crashed and everytime it changes. Even the logs before the crash are varying everytime. On Mon, Jul 8, 2013 at 11:49 AM, <Valdis.Kletnieks(a)vt.edu> wrote: On Mon, 08 Jul 2013 11:22:16 +0900, manty kuma said: > For ex : *CONFIG_SLUB_DEBUG_ON, *CONFIG_DMA_API_DEBUG > I am just told to use these configs and check. Not sure how to use them > effectively. Please share if you are aware of these kind of configs and how > to use them. To have them at one place it would be useful for debugging > beginners like me. Most _DEBUG config options are set-and-forget - you turn them on and there's no further configuration needed. They enable additional code that does additional sanity checking and if it finds a problem, it issues a printk() or a stack dump or similar which shows up in dmesg. If you're chasing memory corruption, it may be useful to use netconsole or a serial console to send out all the printk() output in real time, as often an error will be detected but the system will panic()/crash/hang in a way that your local syslog daemon is unable to write the message to disk. (I'm told that on newer UEFI based systems, you can use pstore in a similar fashion, but have not tried it myself yet).

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Debugging a Stall or a Freeze
by Salam Farhat Aug. 17, 2013

Aug. 17, 2013

I added the tracefs functionality to wrapfs. wrapfs is a stackable file system that intercepts file system calls and Tracefs basically records the VFS calls into a file on disk. wrapfs worked fine its that when I added the logging functionality it failed. The issue I am having is that When I run a workload on the mounted file system (wrapfs+tracefs) the system freezes. the mouse pointer does not even move. I set it up in virtual box and had the kernel dump the messages to a serial port which I then read from my host OS. I did this following the instructions on this page: http://linuxdeveloper.blogspot.com/2012/05/debugging-linux-kernel-over-seri… When the guest OS freezes I get the following messages seen below. I would like to know what is a good approach for debugging this issue. I am not sure what a process stall is. Is that a deadlock? [ 780.357876] BUG: soft lockup - CPU#0 stuck for 22s! [nautilus:1382] [ 780.361658] Process nautilus (pid: 1382, ti=dca12000 task=dc837230 task.ti=d) [ 780.361658] Stack: [ 780.361658] Call Trace: [ 780.361658] Code: 90 b8 43 64 03 c1 b9 40 64 03 c1 e9 49 ff ff ff 90 55 ba 0 [ 808.356372] BUG: soft lockup - CPU#0 stuck for 22s! [nautilus:1382] [ 808.360223] Process nautilus (pid: 1382, ti=dca12000 task=dc837230 task.ti=d) [ 808.360223] Stack: [ 808.360223] Call Trace: [ 808.360223] Code: ff ff 90 b8 43 64 03 c1 b9 40 64 03 c1 e9 49 ff ff ff 90 5 [ 814.876223] INFO: rcu_sched detected stall on CPU 0 (t=15000 jiffies) [ 814.876223] Process nautilus (pid: 1382, ti=dca12000 task=dc837230 task.ti=d) [ 814.876223] Stack: [ 814.876223] Call Trace: [ 814.876223] Code: 00 c3 ff ff 80 e5 10 75 ee c1 e6 18 89 b0 10 c3 ff ff 89 d Thanks. Salam

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Where is the system call table in linux kernel v3.9?
by Iker Pedrosa Aug. 12, 2013

Aug. 12, 2013

Hi Guys, I am a newbie to linux kernel and I am trying to do some of the exercises/examples of the Linux Kernel Development book by Robert Love. For the moment I'm trying to create a system call (Chapter 5) but I am unable to do the first step which states the following: "Add an entry to the end of the system call table.This needs to be done for each architecture that supports the system call (which, for most calls, is all the architectures).The position of the syscall in the table, starting at zero, is its system call number. For example, the tenth entry in the list is assigned syscall number nine." My problem is that I am unable to find the file that holds the table. In the book the file that needs to be changed is the entry.S but it no longers exists in v3.9. In an example that I have found on the internet, which is done using v3.0, the file to change is syscall_table_32.S. But I've got the same problem, it doesn't exist. So anybody can help me to find the table? I know that I should be using v2.6 of the kernel but I don't know if that version will work with the distribution that I'm using. The question is also in stackoverflow so if someone wants to answer there I won't have any problem. The link to the page is the following: http://stackoverflow.com/questions/17652555/where-is-the-system-call-table-… -- Iker Pedrosa <ikerpedrosam(a)gmail.com>

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Has anyone worked with PF_RING
by Robert Clove Aug. 8, 2013

Aug. 8, 2013

Hi ALL, I have to understand the PF_RING part. I mean why it is being used,is it available in kernel freely? What are the benefits of PF_RING and what are the disadvantages of using it? Why we normally don't use it? If anyone has any clues please share..... Thanks

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Not able to get crash dump
by neha naik Aug. 6, 2013

Aug. 6, 2013

Hi All, I have loaded the linux crashdump on ubuntu machine. I can manually generate the crashdump by the 'echo c > /proc/sysrq-trigger'. However, i am having a panic in a module i have written, which is not generating a core dump. I simply see the stack in the console and it kind of hangs there. I have to manually power it off and power it on ... Can someone explain why this happens? Is it because the kernel has gone into such a state that it cannot even follow the procedure for crash dump. Regards, Neha

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Re: Getting Started
by Valdis.Kletnieks＠vt.edu Aug. 5, 2013

Aug. 5, 2013

On Wed, 31 Jul 2013 10:32:59 +0530, you said: > I am a computer science student. I want to contribute to the open source > projects, debugging. As it is my first time, i need some guidance. (a) You're better off replying to the list, as you then get answers from others besides me. Redirecting back to the list. (b) If you want to actually *help*, and have it set on being the kernel rather than any one of thousands of deserving userspace projects, your best bet is 1) learn to build and install a self-compiled kernel. 2) just get a copy of the 'linux-next' tree 3) Update and build kernels every few days 4) watch them fail (and fail they will - I have at least 3 bugs I've tripped over in the past week to report still) 5) Use 'git bisect' to identify the patch that caused the failure, and report it to the appropriate people. Quite frankly, the kernel needs less half-baked patches from novices, and more qualified testers. And you can get up to speed on testing a heck of a lot faster than you can learn all the ins and outs of kernel code hacking. And if you're ambitious, you can always add "(6) include a patch fixing the problem" once you get better at it...

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How kernel handle interrupts
by Woody Wu Aug. 5, 2013

Aug. 5, 2013

Hi, List Where is the Kernel code that handles external interrupts? I want to have a look at it but haven't found out where it is. Actually, I have some basic questions about interrupt handling in Linux. 1. After Kernel's ISR received an interrupt, I believe it will invoke a handler defined in a device driver if any. But it should be the device driver's responsibility or kernel ISR's responsibility to clear (or acknowledge) the interrupt? 2. My device, an AX88796B network controller, asserting the interrupt line in a level-triggered manner. Now I met problem with the device that might caused by the CPU interrupt mode is not set as level-triggered by edge trigger. My CPU is Samsung S3C2410, an ARM920T powered one. Does anyone know usually where and how should I do this kind of setting? Thanks in advance. -- woody I can't go back to yesterday - because I was a different person then.

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pulling in patches mentioned in mails (linux-kernel@vger.kernel.org)
by kernel neophyte Aug. 4, 2013

Aug. 4, 2013

Hi, I am sorry if this has been asked before, I tried to look around but did not get an solution and hence the mail. In this mail from Mel Gorman: https://lkml.org/lkml/2013/7/15/362 He mentions a set of patches: https://lkml.org/lkml/2013/7/15/292 https://lkml.org/lkml/2013/7/15/383 https://lkml.org/lkml/2013/7/15/322 Could someone please teach me 1) how do I find out where is the tree he is working on (i.e. is he working on linux-next or ... ) 2) Where do I find these patches mentioned ? Currently I cut copy paste in a vim manually and its really frustrating, could some one please point out the right way of pulling these patches. via git ? Thanks a lot for your time in advance. -Neophyte PS: This is my first ever mail in public, please pardon my grammatical errors or choice of words.

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Read BIOS
by Satya Prakash Prasad July 31, 2013

July 31, 2013

I am trying to write a code to read BIOS details. Below is the program but I believe that I am not reading BIOS details. Can anyone please correct me in my understanding: Note I found that /proc/ioports gives the I/O ports details like: [root@ioport]# cat /proc/ioports 0000-001f : dma1 0020-0021 : pic1 0040-0043 : timer0 0050-0053 : timer1 ........and so on.................. #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/io.h> #include <ctype.h> #include <string.h> main(int argc, char *argv[]) { unsigned char c; if(argc != 2) { printf("-1. Invalid args : usage %s start-end address\n", argv[0]); exit(-1); } char * temp = argv[1]; while(*temp != '\0' && (isxdigit(*temp) || *temp=='-')) temp++; char * delim = strchr(argv[1], '-'); if(*temp != '\0' || !delim ) { printf("-2. Invalid args : usage %s start-end address\n", argv[0]); exit(-2); } *delim = '\0'; int start = (int)strtol(argv[1], '\0', 16), end = (int)strtol(delim+1, '\0', 16), index = 0; if(start < 0 || end <= 0 ) { printf("-3. Invalid args : usage %s start-end address\n", argv[0]); exit(-3); } if(iopl(3) == -1) { printf("-4. Unable to set privilege level\n"); exit(-4); } if(ioperm(start, end - start, 1) == -1) { printf("-5. Unable to set access permission bits\n"); exit(-5); } for(index = start ; index <= end ; index ++) { c = inb(index); printf("%X", c); } ioperm(start, end - start, 0); printf("\n"); } So my program dumps the data in-between the ports assigned for devices: [root@ioport]# ./a.out 0000-001f 01000007D0FFFFFFFFFFFFF0100000 7D0FFFFFFFFFFFFF Please let me know if I am in the right path or not and guide me appropriately. Regards, Prakash

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