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

On 3/30/20 1:55 PM, Vlastimil Babka 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. 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/ (local)
[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/ (local)

Signed-off-by: Vlastimil Babka <redacted>

Boo, all the error prints should terminate with \n
Will wait for feedback before resend.