March 2015 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

How to handle Hotplug with UIO userspace driver
by Mandeep Sandhu Sept. 28, 2017

Sept. 28, 2017

Hi All, We're working on a custom PCIe based hardware, which can be hotplugged into the system. We're using the UIO approach of having a minimal kernel driver which basically handles the interrupt and provides a mmap of the device memory to userspace and userspace driver which mmap's the provided memory and manages the device. We're using systemd and udev rules to spawn off a user process whenever the hardware is plugged into the system. Now my question is, how can we gracefully inform the userspace process if someone pulls out the hardware (sitting on a PCI slot). Even with full hotplug support, there can always be a case of "surprise removal", where someone yanks the board without going through the whole "attention button" routine. While the kernel driver has no problem with hot-removal, I see that if I "unregister" our UIO driver in response to the removal, then the mapping for the hardware's physical memory would become "invalid" (done by UIO on unregsiter). This could possibly cause the user-space process to crash due to illegal memory access. Is there a "standard" way to handle this scenario, i.e for hotpluggable hardware using UIO? Basically, we'd want to "unregister" the UIO device only after the userspace process is informed of the removal so that it could unmap/close the device and exit gracefully. We could try doing it using some systemd trick, where we put a dependency on one of the UIO created files (/dev/uioX), so that when the file is destroyed, systemd could kill/shutdown the process. Although, there's no guarantee that the userspace process won't try to access the mmaped space during that interval, so this isn't a reliable solution either. Any hints, thoughts? Thanks., -mandeep

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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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Eudyptula challenge task 6 reply
by harshkdev May 5, 2015

May 5, 2015

Hi all, I have sent solution of task 6 on 8/feb/2015.Till now didn't get reply.Do I resend the email or wait. I thought eudyptula is ignoring the reply. Regards Harsh Jain

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Is it all right for Netfilter modules to use control buffers (skb->cb)?
by Alberto Leiva April 28, 2015

April 28, 2015

Hi I was assuming yes, since I've seen at least one module doing this. An example is nf_defrag_ipv6 (ipv6_defrag() calls nf_ct_frag6_gather() which calls NFCT_FRAG6_CB()). What confuses me is that the IPv6 subsystem seems to have its own control buffer: - ipv6_rcv() calls IP6CB(): http://lxr.free-electrons.com/source/net/ipv6/ip6_input.c?v=3.17#L102 - then it calls the netfilter chain, which might include nf_defrag_ipv6: http://lxr.free-electrons.com/source/net/ipv6/ip6_input.c?v=3.17#L186 - then ip6_rcv_finish() comes into play, which calls dst_input(), which might well call ip6_input(), which calls ip6_input_finish(). - ip6_input_finish() once again dereferences the IPv6 version of the control buffer: http://lxr.free-electrons.com/source/net/ipv6/ip6_input.c?v=3.17#L219 If Netfilter modules are allowed to wash out control buffers, how does the IPv6 private data seems to be surviving the Netfilter chain? Thanks

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Kernel thread scheduling
by Vincenzo Scotti April 10, 2015

April 10, 2015

Hello, I am actually studying kernel threads, and I have some doubts about them. Let's take for example this snippet of code static int thread_function(void *data) { while (!kthread_should_stop()) { schedule(); } pr_err("Stopped"); return 0; } This way it works just fine, and waits until I call kthread_stop on it. But if I comment out that schedule() call, it just hangs my system when I load it (it is part of a module). I see that the loop-schedule-wakeup pattern is used among all the others kernel threads. But I don't get why I need to call the scheduler explicitly. I know that the kernel is fully preemptible, and in my interpretation I thought that it could stop every running thread, even in kernel space, using a timer-based interrupt handler, to give cpu to other threads. Doesn't this pattern resemble a voluntary preemption model? Where am I wrong?

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Questions about the zoned page frame allocator and fix mapped addresses
by Sunny Shah April 1, 2015

April 1, 2015

Hello, I have few more questions from my reading of "Understanding the Linux Kernel", chapter "Memory Management". - The book says, about releasing page frames to the per CPU cache - "no page frame is ever released to the cold cache: the kernel always assumes the freed page frame is hot with respect to the hardware cache". What is the reason for this decision ? - It is possible for a page to be in ZONE_NORMAL and yet have it's PG_reserved flag cleared. Is this correct ? - The function "fix_to_virt" for fix-mapped linear addresses does the following: return (0xfffff000UL - (idx << PAGE_SHIFT)); Why are the upper 4096 bytes not used, and the addressing starts from the top of the virtual address space - 4096 ? - The book says "each fix-mapped linear address maps one page frame of the physical memory". Shouldn't it be "maps one *physical location* of memory" rather than one page frame ? - My understanding is that the kernel page table entries for addresses > 896 MB would be empty and those addresses would be mapped using separate data structures used for temporary and permanent kernel mappings and non-contiguous page frame allocation. Is this wrong ? Thanks, Sunny

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mmap text file
by Ssagarr Patil April 1, 2015

April 1, 2015

Hi, I am trying to achieve faster file writes using mmap. so I am trying the follwowing: /* open file */ while (loop) { /* get the current file size of output file */ filesize = lseek(outfd, 0, SEEK_END); /* Increase file size */ ftruncate(outfd, (off_t)filesize + (off_t)buf.size); /* mmap destination file */ if((dest = mmap(0, buf.size, PROT_WRITE, MAP_SHARED, outfd, filesize)) == (void *) -1) { printf( "Error mapping ouput file: %s: %d, %d\n", argv[1], i, filesize); perror("MMAP Failed\n"); goto error; } memcpy(dest, src, size); munmap(dest); } I am able to only mmap it once next time it fails to mmap, any pointers why it fails ? Regards, --Sagar

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The Eudyptula Challenge - Sending task
by Leonardo Ariel Carreras Rodríguez March 31, 2015

March 31, 2015

Hello everyone, I am trying to send Task 1. I already verified that my files do what they are supposed to. I am using mutt to send emails, so I have some questions. 1. Because gmail gives an advice about not using less secure apps, I am using another email address. Does Little care about the email or the ID is enough? 2. I attached as text using mutt the files that were required, but I am not sure if that is ok. An email was sent to Little from that 'new' account (not the one from which I started the challenge). 3. How long does it take to know if the task was completed, or at least that is waiting for correction? Regards, Leonardo Carreras

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How to auto-load kernel module when peripheral
by Woody Wu March 31, 2015

March 31, 2015

Hi, I want my usb-serial converter can be automatically ready to use after it plugged into an USB port. Currently I have to manually do 'modprobe ftdi_sio' before use it, otherwise, no module will be loaded and there is no a /dev/ttyUSB0 appearing. I am using mdev instead of udev in this embedded system, and full pathname of mdev had already put in /proc/sys/kernel/hotplug file. I also already added a line into my mdev.conf: ttyUSB* root:tty 660 To make it clear, the problem is when I inserted the convert, I did not see /dev/ttyUSB0 was created and 'lsmod' also not list the 'ftdi_sio' module. If I check the kernel message at the time of inserting the device, I can see: [ 43.425000] usb 1-1: new full speed USB device number 2 using s3c2416-ohci [ 43.585000] usb 1-1: New USB device found, idVendor=0403, idProduct=6001 [ 43.585000] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3 [ 43.585000] usb 1-1: Product: FT232R USB UART [ 43.585000] usb 1-1: Manufacturer: FTDI [ 43.585000] usb 1-1: SerialNumber: A900aMbV How do I do? Thanks in advance! -- Life is the only flaw in an otherwise perfect nonexistence -- Schopenhauer woody public key at http://subkeys.pgp.net:11371 (narkewoody(a)gmail.com)

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Re: Can add new module without upgrade the kernel?
by Woody Wu March 31, 2015

March 31, 2015

Sorry, I did not write a subject. On Monday, 30 March 2015, Woody Wu <narkewoody(a)gmail.com> wrote: > Hi, > > I have a kernel already run on production, but I then realized that I need > to add one or two driver to it. But I hope I can avoid to upgrade the > kernel image for those already running products, I hope I can only extend > the kernel by add the driver modules to the root file system. Is that > possible? The current kernel has already compiled with the loadable modules > options, but for the drivers that I want now the old config is 'no'. > > Thanks in advance. > -woody > > > -- > Sent from Gmail Mobile > -- Sent from Gmail Mobile

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