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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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readme: explain how to use dynamic debugging for boot messages
run: pass the console_msg_format=syslog boot parameter by default, to allow seeing what is the priority of messages on the terminal. Document printk better.
This commit is contained in:
Ciro Santilli
70
README.adoc
70
README.adoc
@@ -300,17 +300,55 @@ every time.
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To automate that, use the methods described at: <<init>>
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=== Message control
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=== printk
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We use `printk` a lot, and it shows on the QEMU terminal by default. If that annoys you (e.g. you want to see stdout separately), do:
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We use `printk` a lot, and it shows on the terminal by default, along with stdout and what you type.
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Hide all `printk` messages:
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....
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dmesg -n 1
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....
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or equivalently:
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....
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echo 1 > /proc/sys/kernel/printk
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....
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See also: https://superuser.com/questions/351387/how-to-stop-kernel-messages-from-flooding-my-console
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When in <<graphic-mode>>, you can scroll up a bit on the default TTY with:
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Do it with a <<kernel-command-line-parameters>> to affect the boot itself:
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....
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./run -e 'loglevel=5'
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....
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and now only boot warning messages or worse show, which is useful to identify problems.
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Our default `printk` format is:
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....
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<LEVEL>[TIMESTAMP] MESSAGE
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....
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e.g.:
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....
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<6>[ 2.979121] Freeing unused kernel memory: 2024K
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....
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where:
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* `LEVEL`: higher means less serious
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* `TIMESTAMP`: seconds since boot
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This format is selected by the following boot options:
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* `console_msg_format=syslog`: add the `<LEVEL>` part
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* `printk.time=y`: add the `[TIMESTAMP]` part
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Scroll up in <<graphic-mode>>:
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....
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Shift-PgUp
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@@ -321,15 +359,15 @@ but I never managed to increase that buffer:
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* https://askubuntu.com/questions/709697/how-to-increase-scrollback-lines-in-ubuntu14-04-2-server-edition
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* https://unix.stackexchange.com/questions/346018/how-to-increase-the-scrollback-buffer-size-for-tty
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The superior alternative is to use text mode or a telnet connection.
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The superior alternative is to use text mode and GNU screen or tmux.
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==== pr_debug
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https://stackoverflow.com/questions/28936199/why-is-pr-debug-of-the-linux-kernel-not-giving-any-output/49835405#49835405
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Not printable by default without recompile.
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Debug messages are not printable by default without recompiling.
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But the awesome `CONFIG_DYNAMIC_DEBUG=y` option allows us to do:
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But the awesome `CONFIG_DYNAMIC_DEBUG=y` option which we enable by default allows us to do:
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....
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echo 8 > /proc/sys/kernel/printk
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@@ -345,13 +383,19 @@ and we have a shortcut at:
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Syntax: https://www.kernel.org/doc/html/v4.11/admin-guide/dynamic-debug-howto.html
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Wildcards are also accepted, e.g. enable all messages from all files:
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....
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echo 'file * +p' > /sys/kernel/debug/dynamic_debug/control
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....
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TODO: why is this not working:
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....
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func sys_init_module +p' > /sys/kernel/debug/dynamic_debug/control
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echo 'func sys_init_module +p' > /sys/kernel/debug/dynamic_debug/control
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....
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For modules, we can use:
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Enable messages in specific modules:
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....
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echo 8 > /proc/sys/kernel/printk
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@@ -359,12 +403,20 @@ echo 'module myprintk +p' > /sys/kernel/debug/dynamic_debug/control
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insmod /myprintk.ko
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....
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which should now contain the `pr_debug` message:
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which now outputs the `pr_debug` message:
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....
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printk debug
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....
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Enable `pr_debug` for boot messages as well, before we can reach userland and write to `/proc`:
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....
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./run -e 'dyndbg="file * +p" loglevel=8'
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....
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Get ready for the noisiest boot ever, I think it overflows the `printk` buffer and funny things happen.
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=== Module documentation
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....
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3
run
3
run
@@ -10,12 +10,11 @@ debug=false
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kgdb=false
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kvm=false
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# norandmaps: Don't use address space randomization. Equivalent to echo 0 > /proc/sys/kernel/randomize_va_space.
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# printk.time=y: log in format: "[time ] msg" for all printk messages.
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# nokaslr:
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# - https://unix.stackexchange.com/questions/397939/turning-off-kaslr-to-debug-linux-kernel-using-qemu-and-gdb
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# - https://stackoverflow.com/questions/44612822/unable-to-debug-kernel-with-qemu-gdb/49840927#49840927
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# Turned on by default since v4.12
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extra_append='nokaslr norandmaps printk.devkmsg=on printk.time=y'
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extra_append='console_msg_format=syslog nokaslr norandmaps printk.devkmsg=on printk.time=y'
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extra_append_after_dash=
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extra_flags=
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extra_flags_qemu=
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