Re: [RFC] kexec: Add bootconfig support to get_command_line

[Usama Arif <usamaarif642@gmail.com>]

On 21/08/2025 04:28, Baoquan He wrote:
> Hi,
> 
> On 08/11/25 at 04:37pm, Usama Arif wrote:
>> If the kernel is enabled with CONFIG_BOOT_CONFIG_FORCE, it will
>> always append the bootconfig at the start of the kernel commandline.
>> If reuse-cmdline option is used in kexec, the bootconfig will be
>> repeatedly appended at the start of the commandline on every kexec.
>> As there is a limit on the size of kernel commandline that can be
>> used, --reuse-cmdline option will break kexec depending on the
>> size of bootconfig on repeated kexec.
>>
>> Bootconfig is embedded in the kernel, so it should not be considered
>> when reusing kernel command line.
>> Modify get_command_line() to first attempt reading kernel parameters
>> from /proc/bootconfig before falling back to /proc/cmdline. This allows
>> kexec to use the original boot parameters without bootconfig which
>> is embedded in the kernel.
> 
> Thanks for the great fix. While I have some questions about the change.
> 
> 1, if we can read kernel parameters from /proc/bootconfig, does it mean
>    we will skip other kernel parameters, e.g I set some parameters in
>    grub, meanwhile set bootconfg? Is this allowed?

Hi Baoquan,

Thanks for the questions.

/proc/bootconfig treats the parameters from bootconfig separate from the rest.
i.e. parameters from grub will be coming after "# Parameters from bootloader:"
in /proc/bootconfig. It shouldn't skip grub parameters. For e.g. on my machine

cat /proc/bootconfig 
kernel.csdlock_debug = "1"
kernel.mitigations = "off"
# Parameters from bootloader:
# crashkernel=.....


kernel.csdlock_debug = "1"
kernel.mitigations = "off"
comes from bootconfig, while the crashkernel and the rest come from bootloader [1]

[1] https://elixir.bootlin.com/linux/v6.16.1/source/Documentation/filesystems/proc.rst#L739

> 
> 2. If /proc/bootconfig includes the whole kernel parameters of the
>    runnign kernel, setup_boot_config() will parse and append bootconfig
>    to kernel cmdline again, right? But it doesn't matter.

/proc/bootconfig contains the whole parameters, but the bootconfig is seperated by "# Parameters from bootloader:".
If we repeatedly kexec using /proc/cmdline, we will continuously append bootconfig,
and wont be able to kexec anymore once the commandline gets too big..

> 
> Sorry, I am not familiar with bootconfig.
> 
> Thanks
> Baoquan
>