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Date: Wed, 01 Apr 2015 19:31:50 +0200
From: Heinrich Schuchardt <xypron.glpk@gmx.de>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: Michal Nazarewicz <mina86@mina86.com>, Ingo Molnar <mingo@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>, Joe Perches <joe@perches.com>,
        Josh Hunt <johunt@akamai.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Rusty Russell <rusty@rustcorp.com.au>,
        Daniel Walter <dwalter@google.com>,
        David Rientjes <rientjes@google.com>,
        Kees Cook <keescook@chromium.org>,
        "David S. Miller" <davem@davemloft.net>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Aaron Tomlin <atomlin@redhat.com>, Prarit Bhargava <prarit@redhat.com>,
        Eric B Munson <emunson@akamai.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Sam Ravnborg <sam@ravnborg.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] sysctl: detect overflows when converting to int
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On 01.04.2015 00:45, Andrew Morton wrote:
> On Sun, 29 Mar 2015 21:28:29 +0200 Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
> 
>> When converting unsigned long to int overflows may occur.
>> These currently are not detected when writing to the sysctl
>> file system.
>>
>> E.g. on a system where int has 32 bits and long has 64 bits
>>   echo 0x800001234 > /proc/sys/kernel/threads-max
>> has the same effect as
>>   echo 0x1234 > /proc/sys/kernel/threads-max
>>
>> The patch adds the missing check in do_proc_dointvec_conv.
>>
>> With the patch an overflow will result in an error EINVAL when
>> writing to the the sysctl file system.
> 
> hm, why fix this?  There's a small risk of breaking
> accidentally-working userspace, but I expect we can live with that.
> 
> But how big a problem is this, really?  This behaviour is quite
> expected, after all.
> 

The typical user of a Linux system has never read the Kernel code and
possibly has limited programming experience.
Furthermore in Documentation/sysctl/kernel.txt there is no hint that
only 32-bit integers can be used.
So why should this typical user expect that on a 64-bit system
+3000000000 is considered a negative number?

Now that we know this is a bug why shouldn't we fix it?

Best regards

Heinrich