September 2012 - kernelnewbies@kernelnewbies.org

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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Finding the interrupt vector of a given IRQ
by Mark Farnell Oct. 20, 2012

Oct. 20, 2012

In the kernel, how can I find out the interrupt vector number of a given IRQ (for example, IRQ7)? Within the kernel module, I would like to manually set the IRQ using the assembly code: asm("int $<irq vector>"); and let the IRQ handler installed by a different module catch that interrupt. Is this possible? Thanks! Mark

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looking for current, online kernel-related tutorials
by Robert P. J. Day Oct. 18, 2012

Oct. 18, 2012

a *very* wide-ranging post, but i'm currently trying to collect any well-written and *current* tutorials on *any* aspect of kernel programming, device drivers and related stuff like that there -- especially those that come with working, hands-on examples. the biggest drawback with a lot of the online tutorials and the in-kernel Documentation content is that much of it is simply out-of-date and finding really good tutorials is a bit of a challenge, so if anyone has suggestions for things like that (or has written such things), by all means, post pointers to them and i'll start making a list. and what's in it for you? i'm starting some wiki pages for as many topics as i can think of: http://www.crashcourse.ca/wiki/index.php/Kernel_topics for now, things will be fairly concise, i just want to collect the content. these wiki pages will be permanently publicly available, so it's not like i'm asking anyone to do the work, at which point i'll take the results and keep them for myself. in addition, the *eventual* goal is to incorporate the results in some updated linux kernel programming courseware, which will all be released under the creative commons license so, again, at no point will any of this content suddenly disappear. at the moment, i'm working with karim yaghmour of opersys.com, who recently open-sourced all his courseware -- http://www.opersys.com/training -- and i'm working on updating the two linux courses for the 3.x kernel. when those linux course manuals are updated, they will be posted and will still be under the creative commons license. in the end, i may reorganize the wiki content if i come up with a more accessible structure, but it will never disappear. so if you know of any excellent tutorials out there, let me know, and i'll add that to the wiki where everyone can take advantage. and now, back to work ... rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================

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Question on initrd and initramfs
by Ezequiel Garcia Oct. 7, 2012

Oct. 7, 2012

Hi, This is *not* another question about initrd vs initramfs (I already read Documentation/...). That said, I'm doing some experiments with: * qemu * a very small kernel * busybox static-linked rootfs I can create an initramfs using this: $ cd initramfs $ find . | cpio -H newc -o > ../initramfs.cpio Then I use start qemu like this: $ qemu -kernel bzImage -initrd initramfs.cpio This works. Now to my first question: The kernel is compiled WITHOUT any fs driver... So how the f@#ck did he mounted initramfs? How are the files formatted? And second question: I want to repeat the above procedure but using initrd instead of initramfs, is this still supported? I'd like to use romfs or squashfs... anyone knows how to create this initrd and how to give it to qemulated kernel? This seems trivial, but I spent all morning and had no luck... :-( FWIW: My current kernel size (compressed) is around 500k! This is the smaller kernel I managed to boot. It has nothing (not even /proc) so it's pretty useless but at least it can run a shell :-) The question I'm trying to answer is: how little can it be, while still remaining useful? Thanks! Ezequiel.

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Why "lsusb" return nothing?
by Peter Teoh Oct. 3, 2012

Oct. 3, 2012

I entered "lsusb" at the command line (as root) and nothing is return, not even any error message. Doing a strace the last few lines are: open("/dev/bus/usb", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("/proc/bus/usb", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = -1 ENOENT (No such file or directory) What happened? This is Ubuntu 10.04 (it used NOT to be like that, not sure I what did wrong last time). But running a VirtualBox INSIDE this same OS, I was able to get result from "lsusb" (after enabling the USB devices in VirtualBox interface) and strace gives result: open("/dev/bus/usb/001/002", O_RDWR) = 3 ioctl(3, USBDEVFS_IOCTL, 0xbff6f75c) = -1 ENOTTY (Inappropriate ioctl for device) close(3) = 0 open("/dev/bus/usb/001/001", O_RDWR) = 3 Why the difference? -- Regards, Peter Teoh

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Facing trouble in creating a packet in kernel space
by Rifat Rahman Oct. 2, 2012

Oct. 2, 2012

Hello there, I need to mangle rtp packets in kernel space. So far I am new in kernel module programming. I am trying to implement a module for netfilter hooks. For the first time as exercise, I am trying to write smaller modules. Let me explain what I am actually doing now. I have an echo client and server. The server runs on port 6000. Both are on different machines (May be VMs in bridge filter mode). The client sends udp message and the server just echoes it back. Let us suppose the client sends "some message" as data. Then now I am trying to write a module for the client machine that will append "12345" after the data so that the server will get "some message12345" and echo it back. Now there are various things I did faced. I relied on the NF_IP_POST_ROUTING hook. At first, I copied the data to a temporary storage, and then add 12345 with that. Then I increase skb->tail using skb_put(). Then I memset() 0 to the packet data, and copy the temporary storage with that. Then as the procedure, NF_ACCEPT is returned. There are certain checking points like the udph->dest == 6000 etc. etc. When I use skb_put(), my system hangs out after two or three minutes. When I dmesg to be certain that everything goes right, I find it OK. But, suppose once I send a message like "This is a pretty big message" and another time I send "small message" then I get just "small message12345g message" that means, the bigger message is stored somewhere I don't understand. I tried with skb_add_data() but that works incorrectly here, I understand it's my fault. I just can't figure it out. Now, one thing came in my mind, if it's not possible, should I create new packets for that data appending? I find skb->end - skb->tail is not so big. But ultimately I have to merge two or three packets into one packet and then skb_put() will not suffice for me. Then the point comes, I can use alloc_skb(), skb_reserve(), skb_header_pointer() and other skb manipulation functions, but I don't understand how can I drop the packet got (should I return NF_DROP?) and how can I route my created packets in the packet flowing path? I saw in the xtables_addons for help (file xt_ECHO.c) but found they used ip_route_me_harder() for it. But the function they used is quite different, it is their own. Then I need suggestions about how to route the newly created packet in kernel space. This is the point I am stuck for a few days. If anyone can help me, I will be highly grateful. I understand my knowledge is poor about Linux kernel, but I need a definite way to understand, otherwise the code is not helping me much. Thanks in advance. -- Rifat Rahman

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what's with "get_cpu_ptr" and "put_cpu_ptr"?
by Robert P. J. Day Sept. 30, 2012

Sept. 30, 2012

at the moment, i'm working my way through numerous kernel topics in aid of tracking updates for LKD3 and making personal notes. for instance, here's my wiki page for per-CPU variables: http://www.crashcourse.ca/wiki/index.php/Per-CPU_variables as you can see, lots of links to docs and tutorials and excerpts from header and source files, just for my own benefit for the time being and to be organized later. but toward the end, you can read where i'm a bit puzzled by two macros i ran across just this morning that i had never seen before, and defined in <linux/percpu.h>: #define get_cpu_ptr(var) ({ \ preempt_disable(); \ this_cpu_ptr(var); }) #define put_cpu_ptr(var) do { \ (void)(var); \ preempt_enable(); \ } while (0) i don't recall ever seeing a reference to those two macros anywhere i've read, and a tree-wide search shows *very* few uses: $ grep -rw get_cpu_ptr * include/linux/percpu.h:#define get_cpu_ptr(var) ({ \ kernel/events/core.c: cpuctx = get_cpu_ptr(pmu->pmu_cpu_context); kernel/events/core.c: cpuctx = get_cpu_ptr(pmu->pmu_cpu_context); kernel/events/core.c: cpuctx = get_cpu_ptr(pmu->pmu_cpu_context); $ grep -rw put_cpu_ptr * include/linux/percpu.h:#define put_cpu_ptr(var) do { \ kernel/events/core.c: put_cpu_ptr(pmu->pmu_cpu_context); kernel/events/core.c: put_cpu_ptr(pmu->pmu_cpu_context); kernel/events/core.c: put_cpu_ptr(pmu->pmu_cpu_context); $ that's it, that appears to be the extent of usage in the entire kernel source tree. i'm sure i'll figure them out given a bit more time, but anyone want to clarify the purpose of those macros? and perhaps whether they're rarely used because normal kernel programmers shouldn't be using them? thanks. rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================

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why so little usage of DEFINE_SEMAPHORE()?
by Robert P. J. Day Sept. 29, 2012

Sept. 29, 2012

following up on my earlier post, i'm summarizing the current use of semaphores in the kernel and quite a lot of docs are going to have to cleanse themselves of now-obsolete references to all of DECLARE_MUTEX[_LOCKED] and init_MUTEX[_LOCKED], since the current API consists simply of: * DEFINE_SEMAPHORE() * sema_init() but a quick grep for the former produces only: $ grep -rw DEFINE_SEMAPHORE * drivers/macintosh/adb.c:static DEFINE_SEMAPHORE(adb_probe_mutex); drivers/staging/comedi/drivers/usbduxfast.c:static DEFINE_SEMAPHORE(start_stop_sem); drivers/staging/comedi/drivers/usbduxsigma.c:static DEFINE_SEMAPHORE(start_stop_sem); drivers/staging/comedi/drivers/dt9812.c:static DEFINE_SEMAPHORE(dt9812_mutex); drivers/staging/comedi/drivers/usbdux.c:static DEFINE_SEMAPHORE(start_stop_sem); drivers/staging/csr/drv.c:DEFINE_SEMAPHORE(udi_mutex); drivers/staging/csr/io.c:DEFINE_SEMAPHORE(Unifi_instance_mutex); drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c:static DEFINE_SEMAPHORE(bnx2x_prev_sem); include/linux/semaphore.h:#define DEFINE_SEMAPHORE(name) \ kernel/printk.c:static DEFINE_SEMAPHORE(console_sem); $ ignoring what's under drivers/staging/ (which is generally a bit of a mess at the best of times), the above shows that there is almost *no* usage of DEFINE_SEMAPHORE(), which surprised me somewhat, but i can imagine a couple reasons. first, a lot of semaphores have been replaced by the newer and simpler mutex. also, rather than semaphores being created standalone, they're normally defined as part of a structure, and when an instance of that structure is declared, it's naturally initialized with sema_init() in the appropriate initialization code. so i'm not surprised that the usage of DEFINE_SEMAPHORE() is dropping, i'm just surprised that it's almost non-existent. thoughts? rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================

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Link an out-of-tree module into vmlinux
by Alexei Colin Sept. 28, 2012

Sept. 28, 2012

Hello, Does the build system support building built-in components of vmlinux out-of-tree? That is, I know that these two are possible: (1) build out-of-tree code as a loadable kernel module (.ko) (use the M=/path/to/out-of-tree/dir) (2) build an in-tree module as a statically built-in component of vmlinux instead of as a LKM (use the CONFIG_<module>=y, or otherwise add the object files to obj-y instead of obj-m) The question is can you build an an _out-of-tree_ module as a statically built-in component of vmlinux? It seems you can't do it without modifying the root Makefile. That is, the reason (2) works is because the directory that will have built-in.o in it is in-tree and explicitly referenced by the root Makefile. I would hope that the M=dir parameter would effectively add the dir to the list, but it seems that it does not, spoiling the whole party: the built-in.o is correctly created, but it is not picked up by the vmlinux link-line. The only workaround I have working is to add an ugly out-of-tree path into the list of directories in the root Makefile. I could not find this use-case in the neither in the Kbuild documentation nor online. Thank you in advance. -alexei

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I/O event notification mechanism - select() or epoll() ?
by Rahul Bedarkar Sept. 28, 2012

Sept. 28, 2012

Hi, I have to capture kernel udev events through libudev. One of API provides asynchronous events by reading on file descriptor. I found that there two ways to do that. 1) select call 2) epoll I also found that epoll is smarter and meant to replace old select call. What are your thoughts on these ? Which one is better ? Thanks Rahul B.

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