On Mon, Mar 30, 2020 at 01:55:33PM +0200, 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. > > 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> > --- > .../admin-guide/kernel-parameters.txt | 9 ++ > fs/proc/proc_sysctl.c | 100 ++++++++++++++++++ > include/linux/sysctl.h | 4 + > init/main.c | 2 + > 4 files changed, 115 insertions(+) > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > index c07815d230bc..81ff626fc700 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. 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 c75bb4632ed1..653188c9c4c9 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 +1726,102 @@ 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 = *((struct vfsmount **)arg); I would just make this: struct vfsmount **proc_mnt = (struct vfsmount **)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++; > + > + if (!proc_mnt) { if (!*proc_mnt) { I would also add a comment here to explain that this is doing an on-demand mount so that it doesn't have to mount proc if there are not sysctl parameters. > + proc_fs_type = get_fs_type("proc"); > + if (!proc_fs_type) { > + pr_err("Failed to find procfs to set sysctl from command line"); > + return 0; > + } > + proc_mnt = kern_mount(proc_fs_type); *proc_mnt = kern_mount(proc_fs_type); > + put_filesystem(proc_fs_type); > + if (IS_ERR(proc_mnt)) { if (IS_ERR(*proc_mnt)) { > + pr_err("Failed to mount procfs to set sysctl from command line"); > + return 0; > + } > + *((struct vfsmount **)arg) = proc_mnt; Then drop this line. > + } > + > + 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); 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", > + param, val); > + else if (err == -EACCES) > + pr_err("Failed to set sysctl parameter '%s=%s': permission denied (read-only?)", > + param, val); > + else > + pr_err("Error %pe opening proc file to set sysctl parameter '%s=%s'", > + 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", > + param, val); > + else > + pr_err("Error %pe writing to proc file to set sysctl parameter '%s=%s'", > + ERR_PTR(err), 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); > + } > + > + err = filp_close(file, NULL); > + if (err) > + pr_err("Error %pe closing proc file to set sysctl parameter '%s=%s'", > + 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..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) As with the others in the no-op case: 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 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 > Otherwise, yes, looks good! -- Kees Cook
