November 2017 - kernelnewbies@kernelnewbies.org

Invoking a system call from within the kernel
by Demi Marie Obenour Nov. 19, 2017

Nov. 19, 2017

I am looking to write my first driver. This driver will create a single character device, which can be opened by any user. The device will support one ioctl: long ioctl_syscall(int fd, long syscall, long args[6]); This is simply equivalent to: syscall(syscall, args[0], args[1], args[2], args[3], args[4], args[5], args[6]); and indeed I want it to behave *identically* to that. That means that ptracers are notified about the syscall (and given the opportunity to update its arguments), and that seccomp_bpf filters are applied. Furthermore, it means that all arguments to the syscall need full validation, as if they came from userspace (because they do). Is there an in-kernel API that allows one to invoke an arbitrary syscall with arguments AND proper ptrace/seccomp_bpf filtering? If not, how difficult would it be to create one? Sincerely, Demi Obenour

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My first patch - patch for adl_pci9118.c
by Fabian Baumanis Nov. 18, 2017

Nov. 18, 2017

Hi there, this morning i created my first patch for the Linux kernel. It fixes a style issue which was reported by using checkpatch.pl on drivers/staging/comedi/drivers/adl_pci9118.c Although it's a very small patch, i hope this is useful. My question now is: how can i submit the patch to the developers? Best regards and have a nice weekend, Fabian

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How to add a haradware breakpoint on a physical memory address?
by kipade Nov. 17, 2017

Nov. 17, 2017

I found some memory was rewrite by gpu module, but i have not found where it layout. I just use samples hw_breakpoint module for memory monitor. However, i found it only monitor on a virtual address, in another words, if two variable point the same physical memory, it can't be detected if I just monitor on the other variable. any hints will be very very appreciated.

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Is there a to prevent a file descriptor being closed
by ayaka Nov. 16, 2017

Nov. 16, 2017

Hello I am writing a driver to operation with the dma-buf, a problem I meet is that a dma-buf maybe closed by the other driver or userspace application, then the same number of the file descriptor will be assigned with the other thing, its private data is different. I watch what the drm system do. The drm system doesn't use the file descriptor but the its private data (from dma_buf_get()) to check whether it is the same buffer. Is there a reference in the file descriptor? And is there a way to increase the reference to a file descriptor. I have designed a way to track the usage of the dma-buf of my driver, there is not a problem for the driver to release them. But I just want the new dma-buf is assigned with new file descriptor number, not re-use the closed one. Thank you.

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Dangling/orphaned shared library as MAP_DENYWRITE result ( BUG)
by Lev Olshvang Nov. 16, 2017

Nov. 16, 2017

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sleep
by Tobin C. Harding Nov. 16, 2017

Nov. 16, 2017

Is there any 'non-process' context apart from interrupt context? I had a re-read of sleep sections in ldd3 and in Robert Love's book but am still not totally clear on this. The reason for the question is understanding when we cannot sleep. thanks in advance, Tobin.

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Linux release & branch management
by Anand Arumugam Nov. 16, 2017

Nov. 16, 2017

Hello All, I am looking for some good wiki or documentation about about Linux branches and release management. I have read in the web about the benevolent dictator model <https://git-scm.com/book/en/v2/Distributed-Git-Distributed-Workflows#wfdiag…> of git is used by Linus. But what I'm looking for is the actual logistical details like what are the official blessed repos/branches that Linus' lieutenants use and where does the patch submitted by any contributor get to and how does it get to, etc. The recent email thread on submitting patches to Linus triggered this question for me. Thanks, Anand.

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git pull
by Tobin C. Harding Nov. 15, 2017

Nov. 15, 2017

Hi Greg, This is totally asking a favour, feel free to ignore. How do you format your [GIT PULL] emails to Linus? Do you create a tag and then run a git command to get the email? I tried to do it manually and failed pretty hard (as you no doubt will notice on LKML). thanks, Tobin.

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thread polling for timestamp on socket's error queue doesn't wake up
by Ricardo Biehl Nov. 15, 2017

Nov. 15, 2017

I've notice that polling for timestamp in socket's error queue from a thread that hasn't sended the packet results in not waking the polling thread when an event arrives. A simple proof of that is the following code. When compiled with SEND_IN_SAME_THREAD defined, the program will poll on the same thread it has sent the packet. Otherwise the waiter thread polls and the main thread send packets. Please first look in the two SEND_IN_SAME_THREAD preprocessor conditionals before check the rest of the code. Is this the expected behavior? Should I block waiting for packet to be sent while I could send more packets? kernel documentation about network timestamping (see section 1.3): https://www.kernel.org/doc/Documentation/networking/timestamping.txt kernel version: 4.12.14 glibc version: 2.24 /* * 10/11/2017 * polling for timestamp on socket's error queue */ #include <arpa/inet.h> /* hton*() */ #include <poll.h> /* poll() */ #include <pthread.h> /* pthread_*() */ #include <string.h> /* memset() */ #include <sys/types.h> /* socket() recv() */ #include <sys/socket.h> /* socket() recv() */ #include <unistd.h> /* close() */ #include <linux/net_tstamp.h> /* timestamp stuff */ #define PORT 8080 static int do_poll(int sfd) { struct pollfd pfd; /* prepare */ memset(&pfd, 0, sizeof(pfd)); pfd.fd = sfd; pfd.events = POLLERR; /* poll */ return poll(&pfd, 1, -1); } static void do_recv(int sfd) { char tmp; while (recv(sfd, &tmp, sizeof(tmp), MSG_ERRQUEUE) == sizeof(tmp)); } static void do_send(int sfd) { struct sockaddr_in saddr; char tmp = 'a'; /* prepare */ saddr.sin_family = AF_INET; saddr.sin_port = htons(PORT); saddr.sin_addr.s_addr = htonl(INADDR_LOOPBACK); sendto(sfd, &tmp, sizeof(tmp), 0, (struct sockaddr*) &saddr, sizeof(saddr)); sleep(1); } static void* waiter(void *data) { int sfd = *((int*) data); for (;;) { #ifdef SEND_IN_SAME_THREAD do_send(sfd); #endif do_poll(sfd); do_recv(sfd); } return NULL; } static int set_ts_opt(int sfd) { unsigned int opt; /* set timestamp option */ opt = SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_TX_SCHED | SOF_TIMESTAMPING_OPT_CMSG; if (setsockopt(sfd, SOL_SOCKET, SO_TIMESTAMPING, (char*) &opt, sizeof(opt)) == -1) return -1; return 0; } static int open_socket(void) { int sfd; int tmp; /* open socket in non-blocking mode */ sfd = socket(AF_INET, SOCK_DGRAM|SOCK_NONBLOCK, 0); if (sfd == -1) return -1; tmp = set_ts_opt(sfd); if (tmp == -1) goto _go_close_socket; return sfd; _go_close_socket: close(sfd); return -1; } int main(int argc, char **argv) { pthread_t waiter_thread; int sfd; if ((sfd = open_socket()) == -1) return 1; /* create thread and wait for it to terminate */ pthread_create(&waiter_thread, NULL, waiter, &sfd); #ifndef SEND_IN_SAME_THREAD for (;;) do_send(sfd); #endif pthread_join(waiter_thread, NULL); return 0; } Cheers! -- Ricardo Biehl Pasquali

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memory usage between kernel versions
by Mike Harless Nov. 14, 2017

Nov. 14, 2017

Hi, I've got an embedded product that is currently using a 3.14 kernel, with all user apps compiled with gcc 4.9 against that kernel. When I replace the 3.14 kernel with a 4.9 kernel, and leave everything else the same on the product, I see my memory available drop by over 10MB when looking at /proc/meminfo. The size difference between the 3.14 and 4.9 kernels is only about 1MB, so I'm trying to figure out where the rest went, and if I've possibly configured somehting in the 4.9 kernel differently that's causing the change. Looking at smaps files for various processes, I'm seeing some Rss differences when some common libraries are loaded and I'm wondering if it's really different, or just different accounting/fetching. For instance, with libc and the same process with both kernels: 3.14 kernel: 4c010000-4c0d0000 r-xp 00000000 00:0e 1177 /lib/libc-2.20-2014.11.so Size: 768 kB Rss: 416 kB Pss: 12 kB 4.9 kernel: 4c010000-4c0d0000 r-xp 00000000 00:0e 1177 /lib/libc-2.20-2014.11.so Size: 768 kB Rss: 768 kB Pss: 12 kB Is this difference in Rss something to keep looking into, or are there other thoughts or suggestions on where to look? Thanks! --Mike

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