netdev - Re: [PATCH] sysctl: fix min/max handling in __do_proc_doulongvec

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Date:	Thu, 07 Oct 2010 09:37:32 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	Robin Holt <holt@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Willy Tarreau <w@....eu>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	James Morris <jmorris@...ei.org>,
	"Pekka Savola \(ipv6\)" <pekkas@...core.fi>,
	Patrick McHardy <kaber@...sh.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>
Subject: Re: [PATCH] sysctl: fix min/max handling in __do_proc_doulongvec_minmax()

Eric Dumazet <eric.dumazet@...il.com> writes:

> Le jeudi 07 octobre 2010 à 17:25 +0800, Américo Wang a écrit :
>> >>
>> >
>> >Here is the final one.
>> 
>> Oops, that one is not correct. Hopefully this one
>> is correct.
>> 
>> --------------->
>> 
>> Eric D. noticed that we may trigger an OOPS if we leave ->extra{1,2}
>> to NULL when we use proc_doulongvec_minmax().
>> 
>> Actually, we don't need to store min/max values in a vector,
>> because all the elements in the vector should share the same min/max
>> value, like what proc_dointvec_minmax() does.
>> 
>
> If we assert same min/max limits are to be applied to all elements,
> a much simpler fix than yours would be :
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index f88552c..8e45451 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -2485,7 +2485,7 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
>  		kbuf[left] = 0;
>  	}
>  
> -	for (; left && vleft--; i++, min++, max++, first=0) {
> +	for (; left && vleft--; i++, first=0) {
>  		unsigned long val;
>  
>  		if (write) {
>
>
> Please dont send huge patches like this to 'fix' a bug,
> especially on slow path.
>
> First we fix the bug, _then_ we can try to make code more 
> efficient or more pretty or shorter.
>
> So the _real_ question is :
>
> Should the min/max limits should be a single pair,
> shared by all elements, or a vector of limits.

The difference between long handling and int handling is a
usability issue.  I don't expect we will be exporting new
vectors via sysctl, so the conversion of a handful of vectors
from int to long is where this is most likely to be used.

I skimmed through all of what I presume are the current users
aka linux-2.6.36-rcX and there don't appear to be any users
of proc_dounlongvec_minmax that use it's vector properties there.

Which doubly tells me that incrementing the min and max pointers
is not what we want to do.

Eric


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