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memory mapping /dev/kmem on X86_64
by Jeff Haran Dec. 19, 2013

Dec. 19, 2013

Dear Fellow Kernelnewbies, I wrote an application that calls mmap64() on /dev/kmem to memory map all of kernel virtual memory in this kernel address range from Documentation/x86/x86_64/mm.txt: ffff880000000000 - ffffc7ffffffffff (=64 TB) direct mapping of all phys. Memory I limit the top of the range to what's in the kernel variable high_memory, which I obtained for use in user space by printk()ing it from a simple kernel module that I wrote for that purpose. Sloppy, I know, but this is prototype code at this point. First thing I found was that the mmap64() call failed because my kernel (RHEL6) didn't have CONFIG_DEVKMEM defined so /dev/kmem didn't even exist. So I rebuilt the kernel with CONFIG_DEVKMEM configured. Now /dev/kmem exists, but the mmap64() call still fails due to CONFIG_STRICT_DEVMEN being configured, the purpose of which seems to be to cripple /dev/kmem and /dev/mem so that only the first MB of RAM and ioremapped registers can be accessed as part of a security thing. So I am now contemplating turning CONFIG_STRICT_DEVMEN off in my configuration and rebuilding the kernel, but I am concerned that it's still going to fail because CONFIG_X86_PAT is configured and looking at the code it appears that so long as CONFIG_X86_PAT is configured mmap access to RAM via these devices is going to be prohibited either by the range_is_allowed() function in drivers/char/mem.c (which kicks in when CONFIG_STRICT_DEVMEN is enabled) or the identical range_is_allowed() function in arch/x86/pat.c (which kicks in when CONFIG_STRICT_DEVMEM is not configured). In the latter case it appears the mem driver's call to phys_mem_access_prot_allowed() via the weak alias in mem.c will be replaced by the strong one in pat.c, which calls its range_is_allowed() function. Am I reading this right, that if I keep CONFIG_X86_PAT enabled I will not be able to mmap kernel memory regardless of my setting of CONFIG_STRICT_DEVMEM? If so, can somebody explain why this needs to be like this when CONFIG_X86_PAT is enabled? There's comments in the source about avoiding "cache aliasing" in pat.c, but I don't understand what that means. What are the downsides of disabling CONFIG_X86_PAT? Thanks, Jeff Haran

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Query about PCI base address registers
by Ulka Vaze Dec. 18, 2013

Dec. 18, 2013

Hi, In PCI configuration space there are 6 registers BAR0-5 which can hold the ioaddress or memory address used by card. However my question is - how will we know which BAR register contain ioaddress and which contain memory ? and How many of them contain the addresses. I mean shall we start reading from bar0 to 5 all to find it ? If card has 3 ports and one memory region will it be in bar0 or bar1 or bar2 ? Second is on PCI card we set it as bus master for DMA in probe function and also allocates buffer. So when is DMA actually initiated by it ? Please can anybody clarify. Thanks, -Ulka --

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Need help in configuring kernel
by m silverstri Dec. 18, 2013

Dec. 18, 2013

Hi, I am trying to build kernel with v4l2 features enable. I added these in my defconfig file (and necessary dependancy , and compile, CONFIG_VIDEO_DEV=y CONFIG_VIDEO_V4L2=y I see those are included in the generated .config file, which is what I expected: # # Multimedia core support # CONFIG_MEDIA_CAMERA_SUPPORT=y # CONFIG_MEDIA_ANALOG_TV_SUPPORT is not set # CONFIG_MEDIA_DIGITAL_TV_SUPPORT is not set # CONFIG_MEDIA_RADIO_SUPPORT is not set # CONFIG_MEDIA_RC_SUPPORT is not set # CONFIG_MEDIA_CONTROLLER is not set CONFIG_VIDEO_DEV=y CONFIG_VIDEO_V4L2=y ... My problem is when I add CONFIG_VIDEOBUF2_CORE to my defconf, I don't that include in the generated .config file (the one that I mentioned above). CONFIG_VIDEOBUF2_CORE=y I have checked that CONFIG_VIDEOBUF2_CORE has no dependency. http://cateee.net/lkddb/web-lkddb/VIDEOBUF2_CORE.html Hence , the causes my compilation fails. Any one has any idea how to fix my problem? Thank you.

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kernel customisation issue
by Laxmi Meena Dec. 17, 2013

Dec. 17, 2013

how to compile kernel without module support? When I try disabling the "Enable Loadable module support" option and rebuild the kernel. I get following error "Enable kernel module support and then compile the kernel" Thanks in advance. -Laxmi

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postgresql/libpq-fe.h
by Robert Clove Dec. 17, 2013

Dec. 17, 2013

Hi all, I am getting this error error: postgresql/libpq-fe.h: No such file or directory i have done yum install postgresql-devel then also i am getting this error please guide. thanks

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help in developing soft and hardlockup detection tool
by Vipul Jain Dec. 16, 2013

Dec. 16, 2013

Hi, I would like to write a kernel module that will induce the softlockup and hardlockup on the cpu core(s). Below is my logic and was wondering if some one can help me verify and guide me creating a thread and other stuff for implementing the logic. softlockup: on given cpu number. 1. disable kernel preemption 2. keep looping for 21 seconds (as per kernel Documentation it takes 20 seconds to detect and I would like to recover the system once its detected). 3. release the cpu hardlockup on given cpu number. 1. disable interrrupts. 2. keep looping for 11 seconds. 3. enable interrupts and release cpu. Regards, Vipul.

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Kernel compiling and installation
by Venkat Swaminathan Dec. 16, 2013

Dec. 16, 2013

Currently i am trying to work with my kernel : I am trying to make my chipset(ICH7 - M) to support AHCI in my debian 6 (linux-source-2.6.32). With mentioned link's ref :http://tartarus.org/~ds/quirk-ich-force-ahci.patch <http://tartarus.org/%7Eds/quirk-ich-force-ahci.patch> i cooked the quirk.c in my work machine and compiled the KERNEL make SUBDIRS=drivers/pci/ and make module SUBDIRS=drivers/pci/ After successful compilation i copied the "pci-stub.ko" from linux-source-2.6.32/drivers/pci to test machine's :"/lib/modules/2.6.32-5-686/kernel/drivers/pci" path and then set quirks.ich_force_ahci=1 in /etc/grub/grub.conf and rebooted the system But after reboot my IDE is using ata_PIIX driver not AHCI driver. Don't know what is error.request some guidance on applying the patch Regards Venkat.S

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network register on /proc fs
by Hatt Tom Dec. 16, 2013

Dec. 16, 2013

hi: Does the network relevent register on /proc fs ? when does it register ? which entry of /proc will be used by ifconfig" command ? Thanks! -- Best Regards!

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create sysfs class attribute and sub directories
by Jeshwanth Kumar N K Dec. 15, 2013

Dec. 15, 2013

Hello List :), I have a small question about sysfs. I am creating a kernel module to print some versions in sysfs attributes. So the directory tree would be. /sys/class/versions/main/version /dev/version So, versions is my class directory, inside that two directories main and dev, both directory should contains version attribute. I am little familiar with creating with sysfs class attribute using struct class, but how to create the directory (like main/dev) and create attributes in both directory ? I have one more question, Is sysfs is right place to display version information ? Thanks in Advance :) -- Regards Jeshwanth Kumar N K Bangalore, India

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A question about ip_options's option type
by Guibin(Bill) Tian Dec. 14, 2013

Dec. 14, 2013

Hi, maybe it's a naive question. But I am confused here. For the options field of an IP header, as for the data in this field, according to document and the code in the method of "ip_options_compile", the first byte is the options type data and the second byte is the length of the option. But I traced back the method of "ip_options_get_from_user", there is no place that assign these two values to the __data field. I checked some example of socket API "setsockopt", it also just converts the target options data pointer into a char * pointer and passes down to " ip_options_get_from_user", still it doesn't set these two values. Can anyone help me on this? Because I am trying to add my own options to IP header at the IP layer for some experiment purposes. struct linger linger = { 0 }; linger.l_onoff = 1; linger.l_linger = 30; status = setsockopt(serverSocket, SOL_SOCKET, SO_LINGER, (const char *) &linger, sizeof(linger));

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