Re: [PATCH v2 1/3] kernel/sysctl: support setting sysctl parameters from kernel command line

[Masami Hiramatsu <mhiramat@kernel.org>]

On Tue, 14 Apr 2020 13:32:20 +0200
Vlastimil Babka <vbabka@suse.cz> wrote:

> A recently proposed patch to add vm_swappiness command line parameter in
> addition to existing sysctl [1] made me wonder why we don't have a general
> support for passing sysctl parameters via command line. Googling found only
> somebody else wondering the same [2], but I haven't found any prior discussion
> with reasons why not to do this.
> 
> Settings the vm_swappiness issue aside (the underlying issue might be solved in
> a different way), quick search of kernel-parameters.txt shows there are already
> some that exist as both sysctl and kernel parameter - hung_task_panic,
> nmi_watchdog, numa_zonelist_order, traceoff_on_warning. A general mechanism
> would remove the need to add more of those one-offs and might be handy in
> situations where configuration by e.g. /etc/sysctl.d/ is impractical.
> 
> Hence, this patch adds a new parse_args() pass that looks for parameters
> prefixed by 'sysctl.' and tries to interpret them as writes to the
> corresponding sys/ files using an temporary in-kernel procfs mount. This
> mechanism was suggested by Eric W. Biederman [3], as it handles all dynamically
> registered sysctl tables, even though we don't handle modular sysctls. Errors
> due to e.g. invalid parameter name or value are reported in the kernel log.
> 
> The processing is hooked right before the init process is loaded, as some
> handlers might be more complicated than simple setters and might need some
> subsystems to be initialized. At the moment the init process can be started and
> eventually execute a process writing to /proc/sys/ then it should be also fine
> to do that from the kernel.
> 
> Sysctls registered later on module load time are not set by this mechanism -
> it's expected that in such scenarios, setting sysctl values from userspace is
> practical enough.
> 
> [1] https://lore.kernel.org/r/BL0PR02MB560167492CA4094C91589930E9FC0@BL0PR02MB5601.namprd02.prod.outlook.com/
> [2] https://unix.stackexchange.com/questions/558802/how-to-set-sysctl-using-kernel-command-line-parameter
> [3] https://lore.kernel.org/r/87bloj2skm.fsf@x220.int.ebiederm.org/
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>

[...]

> diff --git a/init/main.c b/init/main.c
> index a48617f2e5e5..7b43118215d6 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1372,6 +1372,8 @@ static int __ref kernel_init(void *unused)
>  
>  	rcu_end_inkernel_boot();
>  
> +	do_sysctl_args();
> +

Ah, I see. Since the sysctl is designed to be called after all __init calls were
done, it shouldn't use bootconfig directly because bootconfig is full of __init
call. OK, anyway we can use "kernel.sysctl" prefixed bootconfig for these.

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>