Re: [RFC] kernel/sysctl: support setting sysctl parameters from kernel command line
From: Kees Cook <hidden>
Date: 2020-03-17 21:29:18
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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@BL0PR02MB5601.namprd02.prod.outlook.com/ (local) [2] https://unix.stackexchange.com/questions/558802/how-to-set-sysctl-using-kernel-command-line-parameter Signed-off-by: Vlastimil Babka <redacted> --- 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
quoted hunk ↗ jump to hunk
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