linux-kernel - Re: [PATCH] ftrace: fix ftrace_event_call alignment for use with gcc 4.5

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Date:	Wed, 24 Feb 2010 14:33:20 -0500
From:	Jeff Mahoney <jeffm@...e.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ftrace: fix ftrace_event_call alignment for use with
 gcc 4.5

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On 02/24/2010 01:59 PM, Jeff Mahoney wrote:
>  GCC 4.5 introduces behavior that forces the alignment of structures to
>  use the largest possible value. The default value is 32 bytes, so if
>  some structures are defined with a 4-byte alignment and others aren't
>  declared with an alignment constraint at all - it will align at 32-bytes.
> 
>  For things like the ftrace events, this results in a non-standard array.
>  When initializing the ftrace subsystem, we traverse the _ftrace_events
>  section and call the initialization callback for each event. When the
>  structures are misaligned, we could be treating another part of the
>  structure (or the zeroed out space between them) as a function pointer.
> 
>  This patch forces the alignment for all the ftrace_event_call structures
>  to 4 bytes.
> 
>  Without this patch, the kernel fails to boot very early when built with
>  gcc 4.5.
> 
>  It's trivial to check the alignment of the members of the array, so it
>  might be worthwhile to add something to the build system to do that
>  automatically. Unfortunately, that only covers this case. I've asked one
>  of the gcc developers about adding a warning when this condition is seen.

BTW, the analysis for this is available here:

https://bugzilla.novell.com/show_bug.cgi?id=582222

- -Jeff

- -- 
Jeff Mahoney
SUSE Labs
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