linux-kernel - Re: [PATCH v2 3/5] sysctl: Warn when a clamped sysctl parameter is set out of range

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Message-ID: <20180228005746.GZ14069@wotan.suse.de>
Date:   Wed, 28 Feb 2018 00:57:46 +0000
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 3/5] sysctl: Warn when a clamped sysctl parameter is
 set out of range

On Tue, Feb 27, 2018 at 03:49:49PM -0500, Waiman Long wrote:
> Even with clamped sysctl parameters, it is still not that straight
> forward to figure out the exact range of those parameters. One may
> try to write extreme parameter values to see if they get clamped.
> To make it easier, a warning with the expected range will now be
> printed in the kernel ring buffer when a clamped sysctl parameter
> receives an out of range value.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  include/linux/sysctl.h |  1 +
>  kernel/sysctl.c        | 55 ++++++++++++++++++++++++++++++++++++++++++--------
>  2 files changed, 48 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index eceeaee..4e4f74a2 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -128,6 +128,7 @@ struct ctl_table
>   * ctl_table flags (16 different flags, at most)
>   */
>  #define CTL_FLAGS_CLAMP_RANGE	(1 << 0) /* Clamp to min/max range */
> +#define CTL_FLAGS_OOR_WARNED	(1 << 1) /* Out-of-range warning issued */

With the enum set you can then kdocify this too.

>  struct ctl_node {
>  	struct rb_node node;
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 2b2b30c..f9f3373 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -2515,36 +2515,54 @@ static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
>   *  min: ptr to minimum allowable value
>   *  max: ptr to maximum allowable value
>   *  flags: ptr to flags
> + *  name: sysctl parameter name
>   */
>  struct do_proc_dointvec_minmax_conv_param {
>  	int *min;
>  	int *max;
>  	uint16_t *flags;
> +	const char *name;
>  };
>  
>  static int do_proc_dointvec_minmax_conv(bool *negp, unsigned long *lvalp,
>  					int *valp,
>  					int write, void *data)
>  {
> +#define SYSCTL_WARN_MSG	\
> +"Kernel parameter \"%s\" was set out of range [%d, %d], clamped to %d.\n"

Please loose this define. What about a proc_ctl_warn() which takes the
parameters and then does the checks?

> +
>  	struct do_proc_douintvec_minmax_conv_param *param = data;
>  
>  	if (write) {
>  		unsigned int val = *lvalp;
> +		bool clamped = false;
>  		bool clamp = param->flags &&
>  			   (*param->flags & CTL_FLAGS_CLAMP_RANGE);
>  
> @@ -2623,24 +2649,36 @@ static int do_proc_douintvec_minmax_conv(unsigned long *lvalp,
>  			return -EINVAL;
>  
>  		if (param->min && *param->min > val) {
> -			if (clamp)
> +			if (clamp) {
>  				val = *param->min;
> -			else
> +				clamped = true;
> +			} else {
>  				return -ERANGE;
> +			}
>  		}
>  		if (param->max && *param->max < val) {
> -			if (clamp)
> +			if (clamp) {
>  				val = *param->max;
> -			else
> +				clamped = true;
> +			} else {
>  				return -ERANGE;
> +			}
>  		}
>  		*valp = val;
> +		if (clamped && param->name &&
> +		   !(*param->flags & CTL_FLAGS_OOR_WARNED)) {
> +			pr_warn(SYSCTL_WARN_MSG, param->name,
> +				param->min ? *param->min : 0,
> +				param->max ? *param->max : UINT_MAX, val);
> +			*param->flags |= CTL_FLAGS_OOR_WARNED;

Why not just use pr_warn_once()?

  Luis