linux-kernel - Re: [PATCH v4 2/4] sysctl: Move some boundary constants form sysctl.c to sysctl

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Date:   Wed, 20 May 2020 09:14:08 +0800
From:   Xiaoming Ni <nixiaoming@...wei.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        <keescook@...omium.org>
CC:     <mcgrof@...nel.org>, <yzaikin@...gle.com>, <adobriyan@...il.com>,
        <mingo@...nel.org>, <gpiccoli@...onical.com>, <rdna@...com>,
        <patrick.bellasi@....com>, <sfr@...b.auug.org.au>,
        <akpm@...ux-foundation.org>, <mhocko@...e.com>, <vbabka@...e.cz>,
        <tglx@...utronix.de>, <peterz@...radead.org>,
        <Jisheng.Zhang@...aptics.com>, <khlebnikov@...dex-team.ru>,
        <bigeasy@...utronix.de>, <pmladek@...e.com>,
        <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <wangle6@...wei.com>, <alex.huangjianhui@...wei.com>
Subject: Re: [PATCH v4 2/4] sysctl: Move some boundary constants form sysctl.c
 to sysctl_vals

On 2020/5/19 12:44, Tetsuo Handa wrote:
> On 2020/05/19 12:31, Xiaoming Ni wrote:
>> Some boundary (.extra1 .extra2) constants (E.g: neg_one two) in
>> sysctl.c are used in multiple features. Move these variables to
>> sysctl_vals to avoid adding duplicate variables when cleaning up
>> sysctls table.
>>
>> Signed-off-by: Xiaoming Ni <nixiaoming@...wei.com>
>> Reviewed-by: Kees Cook <keescook@...omium.org>
> 
> I feel that it is use of
> 
> 	void *extra1;
> 	void *extra2;
> 
> in "struct ctl_table" that requires constant values indirection.
> Can't we get rid of sysctl_vals using some "union" like below?
> 
> struct ctl_table {
> 	const char *procname;           /* Text ID for /proc/sys, or zero */
> 	void *data;
> 	int maxlen;
> 	umode_t mode;
> 	struct ctl_table *child;        /* Deprecated */
> 	proc_handler *proc_handler;     /* Callback for text formatting */
> 	struct ctl_table_poll *poll;
> 	union {
> 		void *min_max_ptr[2];
> 		int min_max_int[2];
> 		long min_max_long[2];
> 	};
> } __randomize_layout;
> 
> .
> 

net/decnet/dn_dev.c:
static void dn_dev_sysctl_register(struct net_device *dev, struct 
dn_dev_parms *parms)
{
	struct dn_dev_sysctl_table *t;
	int i;

	char path[sizeof("net/decnet/conf/") + IFNAMSIZ];

	t = kmemdup(&dn_dev_sysctl, sizeof(*t), GFP_KERNEL);
	if (t == NULL)
		return;

	for(i = 0; i < ARRAY_SIZE(t->dn_dev_vars) - 1; i++) {
		long offset = (long)t->dn_dev_vars[i].data;
		t->dn_dev_vars[i].data = ((char *)parms) + offset;
	}

	snprintf(path, sizeof(path), "net/decnet/conf/%s",
		dev? dev->name : parms->name);

	t->dn_dev_vars[0].extra1 = (void *)dev;

	t->sysctl_header = register_net_sysctl(&init_net, path, t->dn_dev_vars);
	if (t->sysctl_header == NULL)
		kfree(t);
	else
		parms->sysctl = t;
}

A small amount of code is not used as a boundary value when using 
extra1. This scenario may not be suitable for renaming to min_max_ptr.

Should we add const to extra1 extra2 ?

--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -124,8 +124,8 @@ struct ctl_table {
         struct ctl_table *child;        /* Deprecated */
         proc_handler *proc_handler;     /* Callback for text formatting */
         struct ctl_table_poll *poll;
-       void *extra1;
-       void *extra2;
+       const void *extra1;
+       const void *extra2;
  } __randomize_layout;


Thanks
Xiaoming Ni