linux-kernel - Re: [PATCH] firmware: declare __{start,end}_builtin

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Message-ID: <CAOMGZ=FwAULFx8TiJzYxS+8HqEerNCyHhK5MghoF3J5ie27Wyw@mail.gmail.com>
Date:	Sat, 25 Jun 2016 23:06:43 +0200
From:	Vegard Nossum <vegard.nossum@...il.com>
To:	Vegard Nossum <vegard.nossum@...cle.com>
Cc:	Ming Lei <ming.lei@...onical.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] firmware: declare __{start,end}_builtin_fw as pointers

On 25 June 2016 at 17:04, Vegard Nossum <vegard.nossum@...cle.com> 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 inside
> strcmp().
>
> I strongly suspect it's 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.
>
> By changing these variables from arrays to pointers, gcc can no longer
> assume that these are separate arrays.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>

Actually, the analysis seems right (by inspection of the assembly
code), but the patch is wrong and causes another crash as the
variables are not really pointers but true arrays (i.e. the linker
script provides the address of the variable, not its value).

I see the __start_foo[]/__end_foo[] idiom is used a lot in the kernel
so this could potentially be a problem in other places as well. The
best solution may be a compiler flag (if it exists). I'll play a bit
more with it to see if I can come up with something.

Vegard