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Fix incorrect assumption that -- is needed in kernel CLI

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Ciro Santilli
2023-01-12 13:40:04 +00:00
parent ddeb36db14
commit 99c0c25ad0
2 changed files with 59 additions and 30 deletions
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84
README.adoc
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@@ -3256,6 +3256,12 @@ The most direct way to replace `init` with our own is to just use the `init=` <<
This just counts every second forever and does not give you a shell.
And to pass command line arguments to the program, you can use `--kernel-cli-after-dash`:
....
./run --kernel-cli 'init=/bin/sh' --kernel-cli-after-dash '-c "echo asdf; pwd; ls"'
....
This method is not very flexible however, as it is hard to reliably pass multiple commands and command line arguments to the init with it, as explained at: xref:init-environment[xrefstyle=full].
For this reason, we have created a more robust helper method with the `--eval` option:
@@ -3435,55 +3441,66 @@ at the very end of the boot logs.
=== Init environment
Documented at https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html[]:
Documented at https://www.kernel.org/doc/html/v5.9/admin-guide/kernel-parameters.html[]:
____
The kernel parses parameters from the kernel command line up to "--"; if it doesn't recognize a parameter and it doesn't contain a '.', the parameter gets passed to init: parameters with '=' go into init's environment, others are passed as command line arguments to init. Everything after "--" is passed as an argument to init.
____
And you can try it out with:
And you can try it out with our link:userland/linux/init_env_poweroff.c[] program:
....
./run --kernel-cli 'init=/lkmc/linux/init_env_poweroff.out' --kernel-cli-after-dash 'asdf=qwer zxcv'
./run --kernel-cli 'init=/lkmc/linux/init_env_poweroff.out loglevel=8 go_in_env=val1 with_dot.before=val2' \
--kernel-cli-after-dash 'go_in_arg=val3 with_dot.after=val4 zxcv'
....
From the <<dry-run,generated QEMU command>>, we see that the kernel CLI at LKMC 69f5745d3df11d5c741551009df86ea6c61a09cf now contains:
From the <<dry-run,generated QEMU command>>, we see that the kernel CLI at LKMC 69f5745d3df11d5c741551009df86ea6c61a09cf now contains (or at least would if it weren't for https://github.com/cirosantilli/linux-kernel-module-cheat/issues/110 oh God), manually indented for readability:
....
init=/lkmc/linux/init_env_poweroff.out console=ttyS0 -- lkmc_home=/lkmc asdf=qwer zxcv
<6>[ 0.000000] Command line: root=/dev/vda nopat console_msg_format=syslog nokaslr norandmaps panic=-1 printk.devkmsg=on printk.time=y rw
init=/lkmc/linux/init_env_poweroff.out loglevel=8 go_in_env=val1 with_dot.before=val2 -- go_in_arg=val3 with_dot.after=val4 zxcv
....
and the init program outputs:
....
args:
/lkmc/linux/init_env_poweroff.out
lkmc_home=/lkmc
asdf=qwer
zxcv
env:
HOME=/
TERM=linux
<6>[ 2.728992] Run /lkmc/linux/init_env_poweroff.out as init process
<7>[ 2.729587] with arguments:
<7>[ 2.729820] /lkmc/linux/init_env_poweroff.out
<7>[ 2.730085] lkmc_home=/lkmc
<7>[ 2.730259] go_in_arg=val3
<7>[ 2.730432] with_dot.after=val4
<7>[ 2.730623] zxcv
<7>[ 2.730767] with environment:
<7>[ 2.730960] HOME=/
<7>[ 2.731115] TERM=linux
<7>[ 2.731267] go_in_env=val
....
Source: link:userland/linux/init_env_poweroff.c[].
As of the Linux kernel v5.7 (possibly earlier, I've skipped a few releases), boot also shows the init arguments and environment very clearly, which is a great addition:
Tested as of the Linux kernel v5.7 and possibly earlier, boot also shows the init arguments and environment very clearly when using <<printk>> at `loglevel=8`, which is a great addition:
....
<6>[ 0.309984] Run /sbin/init as init process
<7>[ 0.309991] with arguments:
<7>[ 0.309997] /sbin/init
<7>[ 0.310004] nokaslr
<7>[ 0.310010] -
<7>[ 0.310016] with environment:
<7>[ 0.310022] HOME=/
<7>[ 0.310028] TERM=linux
<7>[ 0.310035] earlyprintk=pl011,0x1c090000
<7>[ 0.310041] lkmc_home=/lkmc
<6>[ 2.777265] Run /lkmc/linux/init_env_poweroff.out as init process
<7>[ 2.777729] with arguments:
<7>[ 2.777953] /lkmc/linux/init_env_poweroff.out
<7>[ 2.778212] lkmc_home=/lkmc
<7>[ 2.778391] go_in_arg=val2
<7>[ 2.778554] zxcv
<7>[ 2.778705] with environment:
<7>[ 2.778901] HOME=/
<7>[ 2.779065] TERM=linux
<7>[ 2.779223] go_in_env=val1
....
As we can see, the passing of environment variables to init is a bit wonky:
* `init` and `loglevel` didn't go because they were recognized
* `go_in_env` went because it wasn't
so things can always break in future kernel releases... The only way to guard from this a bit is to use some unique prefix and hope the kernel never decides to use it. We of course use `lkmc_*` for our variables.
And it is also impossible to pass environment variables that contain dots in them: our `with_dot.before` is simply gone. Not a common use case, but still, quite insane.
==== init arguments
The annoying dash `-` gets passed as a parameter to `init`, which makes it impossible to use this method for most non custom executables.
@@ -6088,7 +6105,7 @@ See also:
* https://unix.stackexchange.com/questions/48601/how-to-display-the-linux-kernel-command-line-parameters-given-for-the-current-bo
* https://askubuntu.com/questions/32654/how-do-i-find-the-boot-parameters-used-by-the-running-kernel
The arguments are documented in the kernel documentation: https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html
The arguments are documented in the kernel documentation: https://www.kernel.org/doc/html/v5.9/admin-guide/kernel-parameters.html
When dealing with real boards, extra command line options are provided on some magic bootloader configuration file, e.g.:
@@ -7667,6 +7684,17 @@ No, there no official documentation: https://stackoverflow.com/questions/1521393
[[seq-file]]
==== seq_file
TODO these examples broke in kernel 5.0 and we didn't notice because they just added a flimsy warning and nothing else:
* https://stackoverflow.com/questions/25399112/how-to-use-a-seq-file-in-linux-kernel-modules/45100058
* https://stackoverflow.com/questions/60570242/seq-file-not-working-properly-after-next-returns-null
Fix with ifdef for kernel 4/5. The error message is:
....
seq_file: buggy .next function next [module-name] did not update position index
....
Writing trivial read <<file-operations>> is repetitive and error prone. The `seq_file` API makes the process much easier for those trivial cases:
....

5
rootfs_overlay/lkmc/gem5_exit.sh
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@@ -1,4 +1,5 @@
#!/bin/sh
# To be able to do init=/gem5_exit.sh, since kernel CLI argument passing is too messy:
# https://cirosantilli.com/linux-kernel-module-cheat#init-arguments
# This should not be needed anymore after we learned that it should be -- instead of -.
# You should be instead able to just use something init=/bin/sh -- -c 'm5 exit'
# https://github.com/cirosantilli/linux-kernel-module-cheat/pull/226
m5 exit