linux-kernel - Re: [Patch] sysctl: forbid too long numbers

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Date:	Thu, 02 Jul 2009 18:03:29 +0800
From:	Amerigo Wang <amwang@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [Patch] sysctl: forbid too long numbers

Andrew Morton wrote:
> Also, fixing this is a non-backward-compatible change which could break
> existing userspace.  Although the chances of this seem fairly small.
>
> Or are they?  One could imagine a script which was tested and developed
> on a 64-bit system, which writes a >4G number into a pseudo file.  That
> script happens to work on 32-bit systems (it might not work _well_, but
> it'll work).  With this change, the write will fail on the 32-bit
> system and the entire application could bale out or something.
>
> I'm not saying that this is a reason to avoid making the change, but
> it's all a worry and needs consideration.
>
>   

Ah, I didn't consider this situation...
Hmm... but only taking the lower 32-bits really looks strange.

> The other worrisome thing about this change is that there may well be
> existing userspace which does
>
> 	echo 42foo > /proc/whatever
>
> and the conversion to strict_strtoul() will cause that script to
> newly fail.
>
> And the chances that there are scripts which do this are pretty darned
> good - it's fairly easy to accidentally leave junk like this in strings
> when hacking stuff together in scripting languages.
>
>   

Yeah, maybe, but that is really tricky...
>
>   
>> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
>> index 756ccaf..ff2ca5c 100644
>> --- a/lib/vsprintf.c
>> +++ b/lib/vsprintf.c
>> @@ -163,11 +163,14 @@ int strict_strtoul(const char *cp, unsigned int base, unsigned long *res)
>>  	char *tail;
>>  	unsigned long val;
>>  	size_t len;
>> +	char tmp[32];
>>  
>>  	*res = 0;
>>  	len = strlen(cp);
>>  	if (len == 0)
>>  		return -EINVAL;
>> +	if (len > snprintf(tmp, "%ld", ULONG_MAX))
>> +		return -EINVAL;
>>  
>>  	val = simple_strtoul(cp, &tail, base);
>>  	if (tail == cp)
>>     
>
> And here we're doing a check for that overflow, yes?
>
> - We don't need tmp[].  vsnprintf(NULL, ...) can be used to query the
>   length of an sprintf.  See how kvasprintf() does this.
>
> - The strict_strtoul() documentation should be updated?
>
> - The above change affects strict_strtol() too.
>
> - The same change should be made to strict_strtoull() and hence
>   strict_strtoll()?
>
>   
Good points!
I agree, so maybe we only need to change this part?
Hmm, I need to check the callers of strict_strtol()...

Thank you!


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