Re: [RFC v3 2/2] kernel/sysctl: support handling command line aliases

[Kees Cook <keescook@chromium.org>]

On Thu, Mar 26, 2020 at 07:16:06PM +0100, Vlastimil Babka wrote:
> We can now handle sysctl parameters on kernel command line, but historically
> some parameters introduced their own command line equivalent, which we don't
> want to remove for compatibility reasons. We can however convert them to the
> generic infrastructure with a table translating the legacy command line
> parameters to their sysctl names, and removing the one-off param handlers.
> 
> This patch adds the support and makes the first conversion to demonstrate it,
> on the (deprecated) numa_zonelist_order parameter.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> Changes in v3:
> - constify some things according to Kees
> - expand the comment of sysctl_aliases to note on different timings
> 
>  fs/proc/proc_sysctl.c | 48 ++++++++++++++++++++++++++++++++++++-------
>  mm/page_alloc.c       |  9 --------
>  2 files changed, 41 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index 8ee3273e4540..3a861e0a7c7e 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -1729,6 +1729,37 @@ int __init proc_sys_init(void)
>  
>  struct vfsmount *proc_mnt = NULL;
>  
> +struct sysctl_alias {
> +	const char *kernel_param;
> +	const char *sysctl_param;
> +};
> +
> +/*
> + * Historically some settings had both sysctl and a command line parameter.
> + * With the generic sysctl. parameter support, we can handle them at a single
> + * place and only keep the historical name for compatibility. This is not meant
> + * to add brand new aliases. When adding existing aliases, consider whether
> + * the possibly different moment of changing the value (e.g. from early_param
> + * to the moment do_sysctl_args() is called) is an issue for the specific
> + * parameter.
> + */
> +static const struct sysctl_alias sysctl_aliases[] = {
> +	{"numa_zonelist_order",		"vm.numa_zonelist_order" },
> +	{ }
> +};
> +
> +const char *sysctl_find_alias(char *param)

This should be "static" too.

> +{
> +	const struct sysctl_alias *alias;
> +
> +	for (alias = &sysctl_aliases[0]; alias->kernel_param != NULL; alias++) {
> +		if (strcmp(alias->kernel_param, param) == 0)
> +			return alias->sysctl_param;
> +	}
> +
> +	return NULL;
> +}
> +
>  /* Set sysctl value passed on kernel command line. */
>  static int process_sysctl_arg(char *param, char *val,
>  			       const char *unused, void *arg)
> @@ -1741,15 +1772,18 @@ static int process_sysctl_arg(char *param, char *val,
>  	loff_t pos = 0;
>  	ssize_t wret;
>  
> -	if (strncmp(param, "sysctl", sizeof("sysctl") - 1))
> -		return 0;
> -
> -	param += sizeof("sysctl") - 1;
> +	if (strncmp(param, "sysctl", sizeof("sysctl") - 1) == 0) {
> +		param += sizeof("sysctl") - 1;
>  
> -	if (param[0] != '/' && param[0] != '.')
> -		return 0;
> +		if (param[0] != '/' && param[0] != '.')
> +			return 0;
>  
> -	param++;
> +		param++;
> +	} else {
> +		param = (char *) sysctl_find_alias(param);
> +		if (!param)
> +			return 0;
> +	}
>  
>  	if (!proc_mnt) {
>  		proc_fs_type = get_fs_type("proc");
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3c4eb750a199..de7a134b1b8a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5460,15 +5460,6 @@ static int __parse_numa_zonelist_order(char *s)
>  	return 0;
>  }
>  
> -static __init int setup_numa_zonelist_order(char *s)
> -{
> -	if (!s)
> -		return 0;
> -
> -	return __parse_numa_zonelist_order(s);
> -}
> -early_param("numa_zonelist_order", setup_numa_zonelist_order);
> -
>  char numa_zonelist_order[] = "Node";
>  
>  /*
> -- 
> 2.25.1
> 

Otherwise, yay, I love it! :)

-- 
Kees Cook