September 2013 - kernelnewbies@kernelnewbies.org

WEP/WPA/WPA2 implementation in software
by Prabhunath G Sept. 13, 2013

Sept. 13, 2013

Dear All, Suppose there is no hardware support for encryption/decryption for Wi-Fi devices. Will the data be encrypted/decrypted by software implementation. If yes, is it done at the application level or at the kernel level (mac80211). Kindly clarify. If the question has to be rephrased, please do it Regards, Prabhunath G

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Linux process and threads, CPU Affinity
by manty kuma Sept. 13, 2013

Sept. 13, 2013

I have few queries related to threads and Process scheduling in Linux. 1. When my process goes into sleep and wakes back, is it always that it will be scheduled on the same CPU that it got scheduled before? 2. When i create a thread from the process, Will it also be executed on the same CPU always? Even if other CPU's are free and sleeping. I would like to know the mechanism in Linux in specific. Also i am creating the threads through pthread library. I am facing a random hangup issue which is always not reproducible. Need this information to proceed in the right direction. I have also posted a query for the same SO. But no constructive reply. Hence re-posting it here. http://stackoverflow.com/questions/18779947/linux-threads-and-process-cpu-a…

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Could I get the user space fd according to my struct filp?
by Grissiom Sept. 13, 2013

Sept. 13, 2013

Hi all, If I have done `fd_install(fd, filp)`, could I retrieve the fd with the filp? I think I could iterate over the fdtable but is there any API to do that in a more reliable way? -- Cheers, Grissiom

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Re: Getting struct page pointer from virtual address
by ajay saini Sept. 12, 2013

Sept. 12, 2013

More information : - Linux kernel version : 2.6.32 (But I would like a method which is portable to other higher versions as well) - I tried using follow_page, but this function is not exported from the kernel so, can't use it. (Any reason why this function is not exported??) Thanks Ajay ________________________________ From: ajay saini <ajay_saini1006(a)yahoo.co.in> To: "kernelnewbies(a)kernelnewbies.org" <kernelnewbies(a)kernelnewbies.org> Sent: Tuesday, 3 September 2013 1:21 PM Subject: Getting struct page pointer from virtual address Hey, I am working on a linux kernel module and I have a virtual address and mm (struct mm_struct) for a process in this module. I can find the virtaul memory area to which this address belongs to by using find_vma. Is there a function in the linux kernel which I can use in this module (i.e. exported from the kernel) to get struct page pointer for this virtual address. Thanks Ajay

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802.11 b/g/n device data sheet
by Prabhunath G Sept. 12, 2013

Sept. 12, 2013

Dear All, I need to analyse the driver for any 802.11 b/g/n device. Motivation is to contribute in some form in the near future. Request you to get me the datasheet that is availalbe. Tried googling for a few datasheets but couldn't find. Thanks Prabhunath G

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lg_local_lock issue
by Saket Sinha Sept. 12, 2013

Sept. 12, 2013

I am facing some issue on a filesystem-driver I ported from 2.6.18 kernel to 3.8. Now a lot has changed in the VFS since 2.6.18 to 3.8, which has caused a lot of problem for this driver. One of the biggest core change is the change in Path resolution. *LET ME GIVE SOME Basic Information* Path resolution is the finding a dentry corresponding to a path name string, by performing a path walk. Paths are resolved by walking the namespace tree, starting with the first component of the pathname (eg. root or cwd) with a known dentry, then finding the child of that dentry, which is named the next component in the path string. Then repeating the lookup from the child dentry and finding its child with the next element, and so on. Kindly have a look at the below image- http://lwn.net/images/ns/kernel/dentry.png In my driver code it is done here in the function- int get_full_path_d(const struct dentry *dentry, char *real_path) at the below link- https://github.com/disdi/hepunion/blob/master/fs/hepunion/helpers.c#L375 *I would like to point out here that I had dcache_lock on 2.6.18 kernel to protect this lookup but on 3.8 kernel, I haven't found a suitable replacement * The entire code flow chart of my driver can be found here- https://github.com/disdi/hepunion/issues/6 Since 2.6.38, RCU is used to make a significant part of the entire path walk (including dcache look-up) completely "store-free" (so, no locks, atomics, or even stores into cachelines of common dentries). This is known as "rcu-walk" path walking. rcu-walk uses a d_seq protected snapshot. When looking up a child of this parent snapshot, we open d_seq critical section on the child before closing d_seq critical section on the parent. This gives an interlocking ladder of snapshots to walk down. Something like the below link- http://spatula-city.org/~im14u2c/images/man_running_up_crumbling_stairs.jpg *PROBLEM* Since this filesystem does not follow RCU-lookup as of now or since I have not done anything to upgrade our driver to tell VFS whether we follow RCU or not and simply ported the driver to 3.8 kernel by changing the kernel functions or APIs but the respective replacements. Now there is no compilier error but at runtime, I find myself caught in some endless loop and in the stack or dmesg I see something called *lg_local_lock.* The entire kernel stack of my driver can be found here - https://github.com/disdi/hepunion/issues/5 I searched and found the following link describing it thoroughly- http://lwn.net/Articles/401738/ To sum up 1. This thing has come up with the new RCU-lookup change. 2. lglocks is to *protect the list of open files* which is attached to each superblock structure. Since this filesystem driver is failing when I do a "ls in mount point" and if we do a strace on "ls" we have open("/scratch", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3 and in the stack after it hangs, it shows lg_lock 3.And most importanly "The real reason that the per-superblock open files list exists is to let the kernel check for writable files when a filesystem is being remounted read-only." I have a union of filesystems in this filesystem, where one is Read Only and the other is Read-Write. I think I am violating some of this kernel protection mechanism and find mydriver stuck. If you can have a look at this issue, I shall be grateful to you. Regards, Saket Sinha

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inode of anonymity file
by Grissiom Sept. 12, 2013

Sept. 12, 2013

Hi all, I found that `anon_inode_getfd` and `anon_inode_getfile` could create anonymity files that I could implement my own read/write etc without both writing "drivers" nor expose the structure into the file system. But when I dig into the source code, I fount all the files created by them will share the same inode. Will it causing problems? Will other anon files interfere with my own anon files? If one anon file get deleted or closed, will I lost the inode of my own anon file? Is seems that none of these bad things happened, but why? -- Cheers, Grissiom

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IPFIX discussion
by Varun Sharma Sept. 12, 2013

Sept. 12, 2013

Hi, Is there any one who knows about IPFIX ? I want to discuss about some question on ipfix . Varun

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stop build of kernel when there is a compilation error when using make -j
by Kevin Wilson Sept. 11, 2013

Sept. 11, 2013

Hello, When I run "make" for a kernel tree, it stops immediately on the first compilation error. However, when I use "make -j 4" and there an error, it does not stop immediately because of the parallel build using several CPU cores Is there a way to cause "make -j 4" to stop on the first compilation error ? regards, Kevin

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by GMAIL Sept. 11, 2013

Sept. 11, 2013

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