linux-kernel - Re: [PATCH v3 4/6] sysctl: Warn when a clamped sysctl parameter is set out of range

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Date:   Thu, 8 Mar 2018 18:11:36 +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>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 4/6] sysctl: Warn when a clamped sysctl parameter is
 set out of range

On Thu, Mar 01, 2018 at 12:43:38PM -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 |  3 +++
>  kernel/sysctl.c        | 52 ++++++++++++++++++++++++++++++++++++++++++--------
>  2 files changed, 47 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index 448aa72..3db57af 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -130,11 +130,14 @@ struct ctl_table
>   * @CTL_FLAGS_CLAMP_RANGE: Set to indicate that the entry should be
>   *	flexibly clamped to min/max range in case the user provided
>   *	an incorrect value.
> + * @CTL_FLAGS_OOR_WARNED: Set to indicate that an out of range warning
> + * 	had been issued for that entry.
>   *
>   * At most 16 different flags will be allowed.
>   */
>  enum ctl_table_flags {
>  	CTL_FLAGS_CLAMP_RANGE		= BIT(0),
> +	CTL_FLAGS_OOR_WARNED		= BIT(1),
>  };
>  
>  struct ctl_node {
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index af351ed..6c68e77 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -2505,6 +2505,7 @@ static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write,
>   * @min: pointer to minimum allowable value
>   * @max: pointer to maximum allowable value
>   * @flags: pointer to flags
> + * @name: sysctl parameter name
>   *
>   * The do_proc_dointvec_minmax_conv_param structure provides the
>   * minimum and maximum values for doing range checking for those sysctl
> @@ -2514,31 +2515,50 @@ struct do_proc_dointvec_minmax_conv_param {
>  	int *min;
>  	int *max;
>  	uint16_t *flags;
> +	const char *name;
>  };
>  
> +/* Out of range warning message */
> +#define proc_ctl_warn(type, ...)				  \
> +	pr_warn("Kernel parameter \"%s\" was set out of range [%" \
> +	#type ", %" #type "], clamped to %" #type ".\n", __VA_ARGS__)
> +

Usage of pr_*() macros are best used if and when you first
also define

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

Also if we can avoid adding a new define for a pr_warn wrapper
even better. I'd much prefer to have a helper static void routine
which gets some params and does the print than have a define.

However you choose, I just don't want a #define around a simple
pr_warn().

   Luis