On 3/17/20 10:29 PM, Kees Cook wrote: > On Tue, Mar 17, 2020 at 02:21:05PM +0100, 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. > > I'd like to see stuff like this (as you say, you've found some > redundancies here which could be cleaned up a bit). I think the reason > it hasn't happened before is that the answers have mostly revolved > around "just set it in your initramfs". :P > >> [...] >> Hence, this patch adds a new parse_args() pass that looks for parameters >> prefixed by 'sysctl.' and searches for them in the sysctl ctl_tables. When >> found, the respective proc handler is invoked. The search is just a naive >> linear one, to avoid using the whole procfs layer. It should be acceptable, >> as the cost depends on number of sysctl. parameters passed. > > I think this needs reconsidering: this RFC only searches 1 level deep, > but sysctls are a tree. For example: Yes, that was an oversight, easily fixed. > kernel.yama.ptrace_scope > mm.transparent_hugepage.enabled Hm, transparent_hugepage is in sysfs (/sys/kernel/mm), but not sysctl, at least in my case the sysctl tool doesn't list it. Yours does? Yay for consistency. > net.ipv4.conf.default.rp_filter > ...etc > > If this goes in, it'll need to do full traversal. Right. >> The main limitation of avoiding the procfs layer is however that sysctls >> dynamically registered by register_sysctl_table() or register_sysctl_paths() >> cannot be set by this method. > > Correct. And I like what you've done in the code: announce any unhandled > sysctls. > >> 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. > > I agree about placement. > >> >> [1] https://lore.kernel.org/linux-doc/BL0PR02MB560167492CA4094C91589930E9FC0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ >> [2] https://unix.stackexchange.com/questions/558802/how-to-set-sysctl-using-kernel-command-line-parameter >> >> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> >> --- >> Hi, >> >> this is an early RFC so I can get feedback whether to pursue this idea further, >> before trying the more complicated stuff with dynamically registered sysctls. >> For those I have some unanswered questions: >> - Support them at all? > > Maybe? It seems excessive for the initial version. OK >> - Do so by an internal procfs mount again, that was removed by 61a47c1ad3a4 ? >> Or try to keep it simple. > > I think you can walk the registered sysctl structures themselves, yes? I should be able to, yeah. >> - If sysctls are dynamically registered at module load, process the command >> line sysctl arguments again? - this would be rather complicated I guess. > > If it does get supported, perhaps saving them somewhere for > register_sysctl_table() to walk when it gets called? > > I like the idea if just for having to build less boiler plate for > supporting things that I've had to plumb to both boot_params and sysctl. > :) Thanks, I will pursue the idea further then :) Vlastimil
