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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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readme: several small improvements
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Ciro Santilli
163
README.adoc
163
README.adoc
@@ -29,7 +29,7 @@ cd linux-kernel-module-cheat
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The first configure will take a while (30 minutes to 2 hours) to clone and build, see <<benchmark-builds>> for more details.
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If you don't want to wait, you could also try to compile the examples and run them on your host computer as explained on at <<run-on-host>>, but as explained on that section, that is dangerous, limited, and will likely not work.
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If you don't want to wait, you could also try to compile the examples and run them on your host computer as explained on at <<run-kernel-modules-on-host>>, but as explained on that section, that is dangerous, limited, and will likely not work.
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After QEMU opens up, you can start playing with the kernel modules:
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@@ -613,13 +613,15 @@ When dealing with real boards, extra command line options are provided on some m
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* GRUB configuration files: https://askubuntu.com/questions/19486/how-do-i-add-a-kernel-boot-parameter
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* Raspberry pi `/boot/cmdline.txt` on a magic partition: https://raspberrypi.stackexchange.com/questions/14839/how-to-change-the-kernel-commandline-for-archlinuxarm-on-raspberry-pi-effectly
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=== Kernel command line parameters escaping
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==== Kernel command line parameters escaping
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Double quotes can be used to escape spaces as in `opt="a b"`, but double quotes themselves cannot be escaped, e.g. `opt"a\"b"`
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This even lead us to use base64 encoding with `-E`!
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=== modprobe
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=== insmod alternatives
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==== modprobe
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If you are feeling fancy, you can also insert modules with:
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@@ -665,7 +667,7 @@ Kernel modules built from the Linux mainline tree with `CONFIG_SOME_MOD=m`, are
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modprobe dummy-irq
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....
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=== myinsmod
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==== myinsmod
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https://stackoverflow.com/questions/5947286/how-to-load-linux-kernel-modules-from-c-code
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@@ -2504,6 +2506,13 @@ Can also be activated with the `panic_on_warn` boot parameter.
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=== Linux kernel tracing
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Good overviews:
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* http://www.brendangregg.com/blog/2015-07-08/choosing-a-linux-tracer.html by Brendan Greg, AKA the master of tracing. Also: https://github.com/brendangregg/perf-tools
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* https://jvns.ca/blog/2017/07/05/linux-tracing-systems/
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I hope to have examples of all methods some day, since I'm obsessed with visibility.
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==== CONFIG_PROC_EVENTS
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Logs proc events such as process creation to a link:kernel_module/netlink.c[netlink socket].
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@@ -3154,7 +3163,7 @@ devmem 0x101e9000 w 0x12345678
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which touches the register from userland through `/dev/mem`.
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==== Educational hardware models gem5
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==== gem5 educational hardware models
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TODO
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@@ -4619,78 +4628,6 @@ Boot messages start at 5 minutes, boot finishes at 10 minutes and gives a shell.
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TODO: why is the `--dtb` required despite `fs_bigLITTLE.py` having a DTB generation capability? Without it, nothing shows on terminal, and the simulation terminates with `simulate() limit reached @ 18446744073709551615`. The magic `vmlinux.vexpress_gem5_v1.20170616` works however without a DTB.
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== Insane action
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=== Run on host
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This method runs the kernel modules directly on your host computer without a VM, and saves you the compilation time and disk usage of the virtual machine method.
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It has however severe limitations, and you will soon see that the compilation time and disk usage are well worth it:
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* can't control which kernel version and build options to use. So some of the modules will likely not compile because of kernel API changes, since https://stackoverflow.com/questions/37098482/how-to-build-a-linux-kernel-module-so-that-it-is-compatible-with-all-kernel-rele/45429681#45429681[the Linux kernel does not have a stable kernel module API].
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* bugs can easily break you system. E.g.:
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** segfaults can trivially lead to a kernel crash, and require a reboot
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** your disk could get erased. Yes, this can also happen with `sudo` from userland. But you should not use `sudo` when developing newbie programs. And for the kernel you don't have the choice not to use `sudo`
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** even more subtle system corruption such as https://unix.stackexchange.com/questions/78858/cannot-remove-or-reinsert-kernel-module-after-error-while-inserting-it-without-r[not being able to rmmod]
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* can't control which hardware is used, notably the CPU architecture
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* can't step debug it with GDB easily
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Still interested?
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....
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cd kernel_module
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./make-host.sh
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....
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If the compilation of any of the C files fails because of kernel or toolchain differences that we don't control on the host, just rename it to remove the `.c` extension and try again:
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....
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mv broken.c broken.c~
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./build_host
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....
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Once you manage to compile, and have come to terms with the fact that this may blow up your host, try it out with:
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....
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sudo insmod hello.ko
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# Our module is there.
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sudo lsmod | grep hello
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# Last message should be: hello init
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dmest -T
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sudo rmmod hello
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# Last message should be: hello exit
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dmesg -T
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# Not present anymore
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sudo lsmod | grep hello
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....
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Once you are done with this method, you must clean up the in-tree build objects before you decide to do the right thing and move on to the superior `./build` Buildroot method:
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....
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cd "kernel_module"
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./make-host.sh clean
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....
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otherwise they will cause problems.
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=== Hello host
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Minimal host build system sanity check example.
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....
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cd hello_host
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make
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insmod hello.ko
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dmesg
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rmmod hello.ko
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dmesg
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....
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== Buildroot
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=== Custom Buildroot options
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@@ -5181,6 +5118,76 @@ gem5:
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** https://stackoverflow.com/questions/47997565/gem5-system-requirements-for-decent-performance/48941793#48941793
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** https://github.com/gem5/gem5/issues/25
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== Run kernel modules on host
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This method runs the kernel modules directly on your host computer without a VM, and saves you the compilation time and disk usage of the virtual machine method.
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It has however severe limitations, and you will soon see that the compilation time and disk usage are well worth it:
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* can't control which kernel version and build options to use. So some of the modules will likely not compile because of kernel API changes, since https://stackoverflow.com/questions/37098482/how-to-build-a-linux-kernel-module-so-that-it-is-compatible-with-all-kernel-rele/45429681#45429681[the Linux kernel does not have a stable kernel module API].
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* bugs can easily break you system. E.g.:
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** segfaults can trivially lead to a kernel crash, and require a reboot
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** your disk could get erased. Yes, this can also happen with `sudo` from userland. But you should not use `sudo` when developing newbie programs. And for the kernel you don't have the choice not to use `sudo`.
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** even more subtle system corruption such as https://unix.stackexchange.com/questions/78858/cannot-remove-or-reinsert-kernel-module-after-error-while-inserting-it-without-r[not being able to rmmod]
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* can't control which hardware is used, notably the CPU architecture
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* can't step debug it with <<gdb,GDB>> easily. The alternatives are JTAG or <<kgdb>>, but those are less reliable, and JTAG requires extra hardware.
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Still interested?
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....
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cd kernel_module
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./make-host.sh
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....
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If the compilation of any of the C files fails because of kernel or toolchain differences that we don't control on the host, just rename it to remove the `.c` extension and try again:
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....
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mv broken.c broken.c~
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./build_host
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....
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Once you manage to compile, and have come to terms with the fact that this may blow up your host, try it out with:
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....
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sudo insmod hello.ko
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# Our module is there.
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sudo lsmod | grep hello
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# Last message should be: hello init
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dmest -T
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sudo rmmod hello
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# Last message should be: hello exit
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dmesg -T
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# Not present anymore
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sudo lsmod | grep hello
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....
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Once you are done with this method, you must clean up the in-tree build objects before you decide to do the right thing and move on to the superior `./build` Buildroot method:
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....
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cd "kernel_module"
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./make-host.sh clean
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....
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otherwise they will cause problems.
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=== Hello host
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Minimal host build system sanity check example.
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....
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cd hello_host
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make
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insmod hello.ko
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dmesg
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rmmod hello.ko
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dmesg
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....
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== Conversation
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=== kmod
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@@ -5321,7 +5328,7 @@ QEMU automatically adds a second CPU to the DTB!
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The action seems to be happening at: `hw/arm/virt.c`.
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<<gem5-fs-biglittle>> 2a9573f5942b5416fb0570cf5cb6cdecba733392 can also generate its own DTB.
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<<gem5-fs_biglittle>> 2a9573f5942b5416fb0570cf5cb6cdecba733392 can also generate its own DTB.
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=== Directory structure
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