linux-kernel - Re: [PATCH][GIT PULL][for 2.6.35] tracing: Add alignment to syscall metadata declarations

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Message-ID: <20100709215601.GA13985@merkur.ravnborg.org>
Date:	Fri, 9 Jul 2010 23:56:01 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zeev Tarantov <zeev.tarantov@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Maciej@...ispam.struernethosting.dk
Subject: Re: [PATCH][GIT PULL][for 2.6.35] tracing: Add alignment to
	syscall metadata declarations

On Fri, Jul 09, 2010 at 05:05:50PM -0400, Steven Rostedt wrote:
> On Fri, 2010-07-09 at 22:53 +0200, Sam Ravnborg wrote:
> 
> > But in this case we have no control of the value of "." (current address)
> > when we have processed (_ftrace_events) so it may even be at a 2 byte boundary.
> > The linker will add padding as needed to satisfy the alignmnet of
> > __syscalls_metadata - but that padding will be inbetween "." and the first
> > member in __syscalls_metadata.
> 
> Fine, but this is a separate issue.
No - this is the exact issue we are facing here.

In the linker script we have no control what-so-ever on the alignment of "."
so relying on any alignment is a potential bug.

In this particular case gcc decide to increase the alignment of
syscall_metadata from 8 to 16.

And then suddenly things fall apart because we rely on "." being
properly aligned - and it is only 4 byte aligned.

So you fixed this by forcing gcc to use a smaller alignment
for syscall_metadata (from 16 => 4).
But what happens next time when "." is two-byte aligned?
We will not change aligment down to 2...

So to fix this properly we need to make sure that "." is properly
aligned to the alignment of syscall_metadata.

In other words - the linker script needs to be fixed too.
But the fix in the .h file is required so we know the alignmnet
of syscall_metadata.

	Sam
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