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Date:	Sat, 21 May 2016 02:43:03 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	David Miller <davem@...emloft.net>, vegard.nossum@...il.com
Cc:	jslaby@...e.cz, stable@...r.kernel.org,
	linux-kernel@...r.kernel.org, kangjielu@...il.com, kjlu@...ech.edu,
	Sabrina Dubroca <sd@...asysnail.net>
Subject: Re: [PATCH 3.12 69/76] net: fix infoleak in rtnetlink

On 20.05.2016 18:45, David Miller wrote:
> From: Vegard Nossum <vegard.nossum@...il.com>
> Date: Fri, 20 May 2016 14:04:54 +0200
> 
>> Just out of curiosity, was this observed in practice? I could be
>> wrong, but I was under the impression that using designated
>> initializers would zero the rest of the struct, including padding.
> 
> I compiled testcases and found that the compiler does not zero out
> padding when using designated initializers.
> 
> You can do the same.
> 
> For example, on sparc 32-bit, this code:
> 
> struct foo {
> 	int a;
> 	short b;
> 	int c;
> };
> 
> extern void foo(struct foo *);
> 
> void bar(void)
> {
> 	struct foo f = { .a = 1, .b = 2, .c = 3 };
> 
> 	foo(&f);
> }
> 
> gives:
> 
> 	mov	1, %g1
> 	st	%g1, [%fp-12]
> 	mov	2, %g1
> 	sth	%g1, [%fp-8]
> 	mov	3, %g1
> 	st	%g1, [%fp-4]
> 
> It does not initialize the padding between 'b' and 'c'.

Interesting side note here is question 1 of the survey "What is C in
practice?", here:

<https://www.cl.cam.ac.uk/~pes20/cerberus/notes50-survey-discussion.html>

It seems safe right now from my understanding but we need to be careful
with future compiler optimizations, e.g. for memset, as Joseph Myers
commented on the question for future possible optimizations.

This report is also going to be presented in the C2X standard meetings,
hopefully they come up with something sensible for that.

Bye,
Hannes

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