Re: [RFC v2 2/2] kernel/sysctl: support handling command line aliases

[Kees Cook]

On Wed, Mar 25, 2020 at 01:03:45PM +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@xxxxxxx>
> ---
>  kernel/sysctl.c | 39 +++++++++++++++++++++++++++++++++++----
>  mm/page_alloc.c |  9 ---------
>  2 files changed, 35 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 18c7f5606d55..fd72853396f9 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1971,6 +1971,22 @@ static struct ctl_table dev_table[] = {
>  	{ }
>  };
>  
> +struct sysctl_alias {
> +	char *kernel_param;

const char ...

> +	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.
> + */
> +static struct sysctl_alias sysctl_aliases[] = {

static const ...

> +	{"numa_zonelist_order",		"vm.numa_zonelist_order" },
> +	{ }
> +};
> +
>  int __init sysctl_init(void)
>  {
>  	struct ctl_table_header *hdr;
> @@ -1980,6 +1996,18 @@ int __init sysctl_init(void)
>  	return 0;
>  }
>  
> +char *sysctl_find_alias(char *param)
> +{
> +	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. */
>  int process_sysctl_arg(char *param, char *val,
>  			       const char *unused, void *arg)
> @@ -1990,10 +2018,13 @@ int process_sysctl_arg(char *param, char *val,
>  	loff_t ppos = 0;
>  	struct ctl_table *ctl, *found = NULL;
>  
> -	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;
> +	} else {
> +		param = sysctl_find_alias(param);
> +		if (!param)
> +			return 0;
> +	}
>  
>  	remaining = param;
>  	ctl = &sysctl_base_table[0];
> 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";

Nice. :) Effectively: -9 lines +1 line for the using aliasing. I think
it would be worth identifying the specific requirements for a sysctl
alias to be safe to use, and likely in a comment before the alias table:

- boot param parsing must be identical to the sysctl parsing
- temporal changes must be tolerable: i.e. early_param() runs earlier
  than when the sysctl-in-boot-param runs -- must the variable be set
  before the code's other __init functions run?
- must be for a non-module code (since we don't have the dynamic support
  yet)

As it turns out, "numa_zonelist_order" has literally no effect on
anything -- it's a parsed but ignored setting:

static int __parse_numa_zonelist_order(char *s)
{
	/*
	 * We used to support different zonlists modes but they turned
	 * out to be just not useful. Let's keep the warning in place
	 * if somebody still use the cmd line parameter so that we do
	 * not fail it silently
	 */
	if (!(*s == 'd' || *s == 'D' || *s == 'n' || *s == 'N')) {
		pr_warn("Ignoring unsupported numa_zonelist_order value: %s\n", s);
		return -EINVAL;
	}
	return 0;
}

But anyway, do you have a way to generate a list of potential candidates?

-- 
Kees Cook