[PATCH v2 1/3] 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, even though we don't handle modular sysctls. 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>
Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |   9 ++
 fs/proc/proc_sysctl.c                         | 107 ++++++++++++++++++
 include/linux/sysctl.h                        |   4 +
 init/main.c                                   |   2 +
 4 files changed, 122 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f2a93c8679e8..fd38f1e8a2bf 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4891,6 +4891,15 @@
 
 	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. Both '.' and '/' are recognized as
+			separators. 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 b6f5d459b087..7804da5f5be0 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;
@@ -1692,3 +1693,109 @@ int __init proc_sys_init(void)
 
 	return sysctl_init();
 }
+
+/* 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 vfsmount **proc_mnt = arg;
+	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++;
+
+	/*
+	 * To set sysctl options, we use a temporary mount of proc, look up the
+	 * respective sys/ file and write to it. To avoid mounting it when no
+	 * options were given, we mount it only when the first sysctl option is
+	 * found. Why not a persistent mount? There are problems with a
+	 * persistent mount of proc in that it forces userspace not to use any
+	 * proc mount options.
+	 */
+	if (!*proc_mnt) {
+		proc_fs_type = get_fs_type("proc");
+		if (!proc_fs_type) {
+			pr_err("Failed to find procfs to set sysctl from command line\n");
+			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\n");
+			return 0;
+		}
+	}
+
+	path = kasprintf(GFP_KERNEL, "sys/%s", param);
+	if (!path)
+		panic("%s: Failed to allocate path for %s\n", __func__, 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);
+		if (err == -ENOENT)
+			pr_err("Failed to set sysctl parameter '%s=%s': parameter not found\n",
+				param, val);
+		else if (err == -EACCES)
+			pr_err("Failed to set sysctl parameter '%s=%s': permission denied (read-only?)\n",
+				param, val);
+		else
+			pr_err("Error %pe opening proc file to set sysctl parameter '%s=%s'\n",
+				file, param, val);
+		goto out;
+	}
+	len = strlen(val);
+	wret = kernel_write(file, val, len, &pos);
+	if (wret < 0) {
+		err = wret;
+		if (err == -EINVAL)
+			pr_err("Failed to set sysctl parameter '%s=%s': invalid value\n",
+				param, val);
+		else
+			pr_err("Error %pe writing to proc file to set sysctl parameter '%s=%s'\n",
+				ERR_PTR(err), param, val);
+	} else if (wret != len) {
+		pr_err("Wrote only %zd bytes of %d writing to proc file %s to set sysctl parameter '%s=%s\n",
+			wret, len, path, param, val);
+	}
+
+	err = filp_close(file, NULL);
+	if (err)
+		pr_err("Error %pe closing proc file to set sysctl parameter '%s=%s\n",
+			ERR_PTR(err), param, val);
+out:
+	kfree(path);
+	return 0;
+}
+
+void do_sysctl_args(void)
+{
+	char *command_line;
+	struct vfsmount *proc_mnt = NULL;
+
+	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, &proc_mnt, 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..bee62d30ebdb 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,
 {
 }
 
+static inline 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 a48617f2e5e5..7b43118215d6 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1372,6 +1372,8 @@ 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)
-- 
2.26.0