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

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Date:	Fri, 09 Jul 2010 16:46:14 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sam Ravnborg <sam@...nborg.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, 2010-07-09 at 22:33 +0200, Sam Ravnborg wrote:
> On Fri, Jul 09, 2010 at 03:56:42PM -0400, Steven Rostedt wrote:

> This looks like a fix that just hide the real bug.
> If I remember the original report correct the problem is
> that the symbol:
> 
>     __start_syscalls_metadata
> 
> Does not point to a valid syscall entry.
> 
> The symbol is assigned in vmlinux.lds.h like this:
> #define TRACE_SYSCALLS() VMLINUX_SYMBOL(__start_syscalls_metadata) = .; \
>                          *(__syscalls_metadata)                         \
>                          VMLINUX_SYMBOL(__stop_syscalls_metadata) = .;
> 
> Now consider what is happening if we have the following scanario:
> 
> . equals 0x1004 so __start_syscalls_metadata is set to 0x1004
> But __syscall_metadata require 8 byte alignment so it starts at 0x1008.


I did not know that the linker could start a section at a half a word
size. That seems to me to be a linker bug.

If a word for a box is 8 bytes than the linker had better start sections
on 8 byte boundaries. Otherwise I would think other things may break.

For 4 byte word boxes, this should be safe anyway.

-- Steve



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