linux-kernel - Re: [PATCH 01/12] extarray: define helpers for arrays defined in linker scripts

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Date:   Mon, 17 Oct 2016 11:01:13 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Peter Zijlstra <peterz@...radead.org>,
        Vegard Nossum <vegard.nossum@...cle.com>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>, stable@...r.kernel.org,
        Ming Lei <ming.lei@...onical.com>,
        Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 01/12] extarray: define helpers for arrays defined in
 linker scripts

On 10/17/2016, 10:33 AM, Peter Zijlstra wrote:
> On Sun, Oct 16, 2016 at 05:16:05PM +0200, Vegard Nossum wrote:
>> The test in this loop:
>>
>> 	for (b_fw = __start_builtin_fw; b_fw != __end_builtin_fw; b_fw++) {
>>
>> was getting completely compiled out by my gcc, 7.0.0 20160520. The result
>> was that the loop was going beyond the end of the builtin_fw array and
>> giving me a page fault when trying to dereference b_fw->name.
>>
>> This is because __start_builtin_fw and __end_builtin_fw are both declared
>> as (separate) arrays, and so gcc conludes that b_fw can never point to
>> __end_builtin_fw.
>>
> 
> Urgh, isn't that the kind of 'optimizations' we should shoot in the head
> for the kernel? Just like the -fno-strict-aliassing crap?

Unfortunately, there is no such switch for this optimization.

On the top of that, it's incorrect C according to the standard. So it is
as correct as expecting 0 printed here: 'int zero; printf("%d\n",
zero);'. It never worked, not even with gcc 4 and maybe older. We were
just lucky.

thanks,
-- 
js
suse labs