On Thu, Mar 26, 2020 at 07:16:05PM +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. > > 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. Yeah, I think you can drop "RFC" from this in the next version -- you're well into getting this finalized IMO. > > .../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. Maybe add: "Both '.' and '/' are recognized as separators." > + 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; Er, I had a bunch of comments about how this should be declared static etc, but decided on a different suggestion entirely. See below... > + > +/* 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, '.', '/'); You can do the replacement against the param directly, and also avoid all the open-coded string manipulations: strreplace(param, '.', '/'); path = kasprintf(GFP_KERNEL, "sys/%s", param); if (!path) panic("%s: Failed to allocate path for %s\n", __func__, param); > + > + 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); Please check the return value of filp_close() and treat that as an error for this function too. > +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); I don't recommend sharing allocation lifetimes between two functions (process_sysctl_arg() allocs proc_mnt, and do_sysctl_args() frees it). And since you have a scoped lifetime, why allocate it or have it as a global at all? It can be stack-allocated and passed to the handler: void do_sysctl_args(void) { struct file_system_type *proc_fs_type; struct vfsmount *proc_mnt; char *command_line; proc_fs_type = get_fs_type("proc"); if (!proc_fs_type) { pr_err("Failed to mount procfs to set sysctl from command line"); return; } 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"); return; } 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); kfree(command_line); kern_unmount(proc_mnt); } And then pull the mount from (the hilariously overloaded name) "arg": static int process_sysctl_arg(char *param, char *val, const char *unused, void *arg) { struct vfsmount *proc_mnt = (struct vfsmount *)arg; char *path; if (!arg) ...fread out... etc > 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 > Looking good! I'm excited to see the next version. :) -- Kees Cook
