How to transfer kernel data to user data using procfs
Integrating into the /proc filesystem,Fortune cookies through the /proc filesystem,Listing 2 illustrates reading from and then writing to a virtual file in /proc. This example checks and then enables IP forwarding within the kernel’s TCP/IP stack.,Later you see how to use the read_proc and write_proc commands to plug in functions for reading and writing the virtual file.
[root @plato] # ls / proc 1 2040 2347 2874 474 fb mdstat sys 104 2061 2356 2930 9 filesystems meminfo sysrq - trigger 113 2073 2375 2933 acpi fs misc sysvipc 1375 21 2409 2934 buddyinfo ide modules tty 1395 2189 2445 2935 bus interrupts mounts uptime 1706 2201 2514 2938 cmdline iomem mtrr version 179 2211 2515 2947 cpuinfo ioports net vmstat 180 2223 2607 3 crypto irq partitions 181 2278 2608 3004 devices kallsyms pci 182 2291 2609 3008 diskstats kcore self 2 2301 263 3056 dma kmsg slabinfo 2015 2311 2805 394 driver loadavg stat 2019 2337 2821 4 execdomains locks swaps [root @plato 1] # ls / proc / 1 auxv cwd exe loginuid mem oom_adj root statm task cmdline environ fd maps mounts oom_score stat status wchan [root @plato] # cat / proc / 1 / cmdline init [root @plato] #
We have seen a very simple example for a /proc file where we only read the file /proc/helloworld. It's also possible to write in a /proc file. It works the same way as read, a function is called when the /proc file is written. But there is a little difference with read, data comes from user, so you have to import data from user space to kernel space (with copy_from_user or get_user)
Kernel space device drivers usually implement directories and file that show through /sys or /proc. Can the long running user space programs do this as well?,You can however provide similar interfaces from userspace, for example using FIFOs; see mkfifo for details. You can see an implementation of this in sysvinit with its initctl FIFO.,Surely they can. You can mount anything (actual disk filesystems, fuse filesystems, overlayfs, tmpfs, bind mounts, etc) below /sys or /proc, extending and/or replacing the interfaces provided by the kernel., 1 procfs is just a synthetic filesystem, that's nothing special about it. You can use fuse to implement a synthetic filesystem. Call your fs "procfs", and yuppie, you have "provided" procfs. – mosvy Dec 29 '18 at 4:27
# unshare - m # touch / tmp / foo # mount - B / tmp / foo / proc / 1 / status # echo FOR GREAT JUSTICE > /proc/ 1 / status # cat / proc / 1 / status
The proc entry can also be used to pass data to the kernel by writing into the kernel, so there can be two kinds of proc entries.,proc files can also be used to control and modify kernel behavior on the fly. The proc files need to be writable in this case.,An entry that reads as well as writes data into and from kernel space.,In this tutorial, we will see Procfs in Linux.
proc_create("etx_proc", 0666, NULL, & proc_fops);
We have seen a very simple example for a /proc file where we only read the file /proc/helloworld. It's also possible to write in a /proc file. It works the same way as read, a function is called when the /proc file is written. But there is a little difference with read, data comes from user, so you have to import data from user space to kernel space (with copy_from_user or get_user) , The reason for copy_from_user or get_user is that Linux memory (on Intel architecture, it may be different under some other processors) is segmented. This means that a pointer, by itself, does not reference a unique location in memory, only a location in a memory segment, and you need to know which memory segment it is to be able to use it. There is one memory segment for the kernel, and one for each of the processes.
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