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

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Date:	Thu, 25 Feb 2010 09:09:25 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jeff Mahoney <jeffm@...e.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ftrace: fix ftrace_event_call alignment for use with
 gcc 4.5

On Wed, 2010-02-24 at 14:33 -0500, Jeff Mahoney wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> 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
> 

Thanks! I'll apply this and push it forward (as well as to stable)

-- Steve

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