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: kernel.yama.ptrace_scope mm.transparent_hugepage.enabled net.ipv4.conf.default.rp_filter ...etc If this goes in, it'll need to do full traversal. > 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. > - 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? > - 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. :) -Kees > > Vlastimil > > include/linux/sysctl.h | 1 + > init/main.c | 21 ++++++++++++++ > kernel/sysctl.c | 66 ++++++++++++++++++++++++++++++++++++++++++ > 3 files changed, 88 insertions(+) > > diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h > index 02fa84493f23..62ae963a5c0c 100644 > --- a/include/linux/sysctl.h > +++ b/include/linux/sysctl.h > @@ -206,6 +206,7 @@ struct ctl_table_header *register_sysctl_paths(const struct ctl_path *path, > void unregister_sysctl_table(struct ctl_table_header * table); > > extern int sysctl_init(void); > +int process_sysctl_arg(char *param, char *val, const char *unused, void *arg); > > extern struct ctl_table sysctl_mount_point[]; > > diff --git a/init/main.c b/init/main.c > index ee4947af823f..74a094c6b8b9 100644 > --- a/init/main.c > +++ b/init/main.c > @@ -1345,6 +1345,25 @@ void __weak free_initmem(void) > free_initmem_default(POISON_FREE_INITMEM); > } > > +static void do_sysctl_args(void) > +{ > +#ifdef CONFIG_SYSCTL > + size_t len = strlen(saved_command_line) + 1; > + char *command_line; > + > + command_line = kzalloc(len, GFP_KERNEL); > + if (!command_line) > + panic("%s: Failed to allocate %zu bytes\n", __func__, len); > + > + strcpy(command_line, saved_command_line); > + > + parse_args("Setting sysctl args", command_line, > + NULL, 0, -1, -1, NULL, process_sysctl_arg); > + > + kfree(command_line); > +#endif > +} > + > static int __ref kernel_init(void *unused) > { > int ret; > @@ -1367,6 +1386,8 @@ static int __ref kernel_init(void *unused) > > rcu_end_inkernel_boot(); > > + do_sysctl_args(); > + > if (ramdisk_execute_command) { > ret = run_init_process(ramdisk_execute_command); > if (!ret) > diff --git a/kernel/sysctl.c b/kernel/sysctl.c > index ad5b88a53c5a..0444656c259d 100644 > --- a/kernel/sysctl.c > +++ b/kernel/sysctl.c > @@ -1980,6 +1980,72 @@ int __init sysctl_init(void) > return 0; > } > > +/* Set sysctl value passed on kernel command line. */ > +int process_sysctl_arg(char *param, char *val, > + const char *unused, void *arg) > +{ > + size_t count; > + char *tmp; > + int err; > + loff_t ppos = 0; > + struct ctl_table *base, *child = NULL, *found = NULL; > + > + if (strncmp(param, "sysctl.", sizeof("sysctl.") - 1)) > + return 0; > + > + param += (sizeof("sysctl.") - 1); > + > + tmp = strchr(param, '.'); > + if (!tmp) { > + pr_warn("Invalid sysctl param '%s' on command line", param); > + return 0; > + } > + > + *tmp = '\0'; > + > + for (base = &sysctl_base_table[0]; base->procname != 0; base++) { > + if (strcmp(param, base->procname) == 0) { > + child = base->child; > + break; > + } > + } > + > + if (!child) { > + pr_warn("Unknown sysctl prefix '%s' on command line", param); > + return 0; > + } > + > + tmp++; > + > + for (; child->procname != 0; child++) { > + if (strcmp(tmp, child->procname) == 0) { > + found = child; > + break; > + } > + } > + > + if (!found) { > + pr_warn("Unknown sysctl param '%s.%s' on command line", param, tmp); > + return 0; > + } > + > + if (!(found->mode & 0200)) { > + pr_warn("Cannot set sysctl '%s.%s=%s' from command line - not writable", > + param, tmp, val); > + return 0; > + } > + > + > + count = strlen(val); > + err = found->proc_handler(found, 1, val, &count, &ppos); > + > + if (err) > + pr_warn("Error %d setting sysctl '%s.%s=%s' from command line", > + err, param, tmp, val); > + > + return 0; > +} > + > #endif /* CONFIG_SYSCTL */ > > /* > -- > 2.25.1 > -- Kees Cook
