Re: [RFC PATCH 6/6] sysctl: introduce /proc/sys/kernel/modprobe_sysctl_alias

[Masahiro Yamada]

On Fri, Jul 22, 2022 at 11:24 AM Mauricio Faria de Oliveira
<mfo@xxxxxxxxxxxxx> wrote:
>
> The goal of the earlier patches is to let sysctl userspace tools
> load the kernel module with a sysctl entry that is not available
> yet in /proc/sys/ when the tool runs (so it can become available).
>
> Let's expose this file for userspace for two reasons:
>
> 1) Allow such tools to identify that the running kernel has the
>    code which produces sysctl module aliases, so they could run
>    'modprobe sysctl:<entry>' only when it may actually help.
>
> 2) Allow an administrator to hint such tools not to do that, if
>    that is desired for some reason (e.g., rather have the tools
>    fail if something is misconfigured in a critical deployment).

This flag is just a hint.
User-space tools are still able to ignore it.

Perhaps, such administrator's choice might be specified in
tools' configuration file.

For example,

/etc/modprobe.d/forbid-sysctl-alias.conf

may specify

    blacklist:  sysctl:*

if they want to forbid sysctl aliasing.
(but I do not know if this works or not).














> Also add a module parameter for that (proc.modprobe_sysctl_alias),
> for another method that doesn't depend on sysctl tools to be set
> (that wouldn't fail them to try and set it if it's not there yet).
>
> Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxxxxx>
> ---
>  fs/proc/proc_sysctl.c  | 8 ++++++++
>  include/linux/module.h | 1 +
>  kernel/sysctl.c        | 9 +++++++++
>  3 files changed, 18 insertions(+)
>
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index ebbf8702387e..1e63819fcda8 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -33,6 +33,14 @@ static void check_struct_sysctl_device_id(void)
>         BUILD_BUG_ON(offsetof(struct sysctl_device_id, procname)
>                         != offsetof(struct ctl_table, procname));
>  }
> +
> +/*
> + * Hint sysctl userspace tools whether or not to run modprobe with sysctl alias
> + * ('modprobe sysctl:entry') if they cannot find the file '/proc/sys/.../entry'
> + */
> +int modprobe_sysctl_alias = 1;
> +module_param(modprobe_sysctl_alias, int, 0644);
> +
>  #else
>  static void check_struct_sysctl_device_id(void) {}
>  #endif
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 3010f687df19..5f565491c596 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -304,6 +304,7 @@ struct notifier_block;
>  #ifdef CONFIG_MODULES
>
>  extern int modules_disabled; /* for sysctl */
> +extern int modprobe_sysctl_alias; /* for proc sysctl */
>  /* Get/put a kernel symbol (calls must be symmetric) */
>  void *__symbol_get(const char *symbol);
>  void *__symbol_get_gpl(const char *symbol);
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 15073621cfa8..b396cfcb55fc 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1763,6 +1763,15 @@ static struct ctl_table kern_table[] = {
>                 .mode           = 0644,
>                 .proc_handler   = proc_dostring,
>         },
> +#ifdef CONFIG_PROC_SYSCTL
> +       {
> +               .procname       = "modprobe_sysctl_alias",
> +               .data           = &modprobe_sysctl_alias,
> +               .maxlen         = sizeof(modprobe_sysctl_alias),
> +               .mode           = 0644,
> +               .proc_handler   = proc_dointvec,
> +       },
> +#endif
>         {
>                 .procname       = "modules_disabled",
>                 .data           = &modules_disabled,
> --
> 2.25.1
>


--
Best Regards
Masahiro Yamada