linux-kernel - Re: krealloc in kernel/params.c

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Date:	Fri, 17 Oct 2014 09:29:27 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	David Woodhouse <David.Woodhouse@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Arjun Sreedharan <arjun024@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: krealloc in kernel/params.c

Rasmus Villemoes <linux@...musvillemoes.dk> writes:
> On Wed, Oct 15 2014, Rusty Russell <rusty@...tcorp.com.au> wrote:
>
>> Rasmus Villemoes <linux@...musvillemoes.dk> writes:
>> The kzalloc-then-always-krealloc pattern is perhaps overly simplistic,
>> but this code has clearly confused people.  It worked on me...
>>
>
> I think kzalloc immediately followed by kreallocing the returned value
> is rather ugly. Other than that:

Indeed, but it's an obvious pattern.  "If not initialized, initialize".

>> -		num = mk->mp->num;
>> -		attrs = mk->mp->grp.attrs;
>> +		/* First allocation. */
>> +		mk->mp = kzalloc(sizeof(*mk->mp), GFP_KERNEL);
>> +		if (!mk->mp)
>> +			return -ENOMEM;
>
> free_module_param_attrs does not check mk->mp for being NULL before
> kfree'ing mk->mp->grp.attrs, so this will oops.

Nice catch, folded this in:

diff --git a/kernel/params.c b/kernel/params.c
index 3ebe6c64aa67..ee92e67f2cee 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -650,7 +650,8 @@ static __modinit int add_sysfs_param(struct module_kobject *mk,
 #ifdef CONFIG_MODULES
 static void free_module_param_attrs(struct module_kobject *mk)
 {
-	kfree(mk->mp->grp.attrs);
+	if (mk->mp)
+		kfree(mk->mp->grp.attrs);
 	kfree(mk->mp);
 	mk->mp = NULL;
 }

Thanks!
Rusty.
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