[RFC v3 1/2] kernel/sysctl: support setting sysctl parameters from kernel command line

[Vlastimil Babka]

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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
[2] https://unix.stackexchange.com/questions/558802/how-to-set-sysctl-using-kernel-command-line-parameter
[3] https://lore.kernel.org/r/87bloj2skm.fsf@xxxxxxxxxxxxxxxxxxxxx/

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
Changes in v3:
- use temporary procfs mount as Eric suggested. Seems to be the better option
  after all. Naming wise it simply converts all . to / - according to strace the
  sysctl tool seems to be doing the same.

Since the major change, I'm sending another RFC. If this approach is ok, then
it probably needs just some tweaks to the various error prints, and then
converting the rest of existing on-off aliases (if I come up with an idea how
to find them all). Thanks for all the feedback so far.

 .../admin-guide/kernel-parameters.txt         |  9 ++
 fs/proc/proc_sysctl.c                         | 90 +++++++++++++++++++
 include/linux/sysctl.h                        |  4 +
 init/main.c                                   |  2 +
 4 files changed, 105 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index c07815d230bc..0c7e032e7c2e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4793,6 +4793,15 @@
 
 	switches=	[HW,M68k]
 
+	sysctl.*=	[KNL]
+			Set a sysctl parameter, right before loading the init
+			process, as if the value was written to the respective
+			/proc/sys/... file. Unrecognized parameters and invalid
+			values are reported in the kernel log. Sysctls
+			registered later by a loaded module cannot be set this
+			way.
+			Example: sysctl.vm.swappiness=40
+
 	sysfs.deprecated=0|1 [KNL]
 			Enable/disable old style sysfs layout for old udev
 			on older distributions. When this option is enabled
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index c75bb4632ed1..8ee3273e4540 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -14,6 +14,7 @@
 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/bpf-cgroup.h>
+#include <linux/mount.h>
 #include "internal.h"
 
 static const struct dentry_operations proc_sys_dentry_operations;
@@ -1725,3 +1726,92 @@ int __init proc_sys_init(void)
 
 	return sysctl_init();
 }
+
+struct vfsmount *proc_mnt = NULL;
+
+/* Set sysctl value passed on kernel command line. */
+static int process_sysctl_arg(char *param, char *val,
+			       const char *unused, void *arg)
+{
+	char *path;
+	struct file_system_type *proc_fs_type;
+	struct file *file;
+	int len;
+	int err;
+	loff_t pos = 0;
+	ssize_t wret;
+
+	if (strncmp(param, "sysctl", sizeof("sysctl") - 1))
+		return 0;
+
+	param += sizeof("sysctl") - 1;
+
+	if (param[0] != '/' && param[0] != '.')
+		return 0;
+
+	param++;
+
+	if (!proc_mnt) {
+		proc_fs_type = get_fs_type("proc");
+		if (!proc_fs_type) {
+			pr_err("Failed to mount procfs to set sysctl from command line");
+			return 0;
+		}
+		proc_mnt = kern_mount(proc_fs_type);
+		put_filesystem(proc_fs_type);
+		if (IS_ERR(proc_mnt)) {
+			pr_err("Failed to mount procfs to set sysctl from command line");
+			proc_mnt = NULL;
+			return 0;
+		}
+	}
+
+	len = 4 + strlen(param) + 1;
+	path = kmalloc(len, GFP_KERNEL);
+	if (!path)
+		panic("%s: Failed to allocate %d bytes t\n", __func__, len);
+
+	strcpy(path, "sys/");
+	strcat(path, param);
+	strreplace(path, '.', '/');
+
+	file = file_open_root(proc_mnt->mnt_root, proc_mnt, path, O_WRONLY, 0);
+	if (IS_ERR(file)) {
+		err = PTR_ERR(file);
+		pr_err("Error %d opening proc file %s to set sysctl parameter '%s=%s'",
+			err, path, param, val);
+		goto out;
+	}
+	len = strlen(val);
+	wret = kernel_write(file, val, len, &pos);
+	if (wret < 0) {
+		err = wret;
+		pr_err("Error %d writing to proc file %s to set sysctl parameter '%s=%s'",
+			err, path, param, val);
+	} else if (wret != len) {
+		pr_err("Wrote only %ld bytes of %d  writing to proc file %s to set sysctl parameter '%s=%s'",
+			wret, len, path, param, val);
+	}
+
+	filp_close(file, NULL);
+out:
+	kfree(path);
+	return 0;
+}
+
+void do_sysctl_args(void)
+{
+	char *command_line;
+
+	command_line = kstrdup(saved_command_line, GFP_KERNEL);
+	if (!command_line)
+		panic("%s: Failed to allocate copy of command line\n", __func__);
+
+	parse_args("Setting sysctl args", command_line,
+		   NULL, 0, -1, -1, NULL, process_sysctl_arg);
+
+	if (proc_mnt)
+		kern_unmount(proc_mnt);
+
+	kfree(command_line);
+}
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 02fa84493f23..5f3f2a00d75f 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);
+void do_sysctl_args(void);
 
 extern struct ctl_table sysctl_mount_point[];
 
@@ -236,6 +237,9 @@ static inline void setup_sysctl_set(struct ctl_table_set *p,
 {
 }
 
+void do_sysctl_args(void)
+{
+}
 #endif /* CONFIG_SYSCTL */
 
 int sysctl_max_threads(struct ctl_table *table, int write,
diff --git a/init/main.c b/init/main.c
index ee4947af823f..a91ea166a731 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1367,6 +1367,8 @@ 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)
-- 
2.25.1