linux-kernel - Re: 2.6.35 regression

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Date:	Thu, 08 Jul 2010 10:17:22 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	Zeev Tarantov <zeev.tarantov@...il.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: 2.6.35 regression

On Thu, 2010-07-08 at 16:08 +0200, Sam Ravnborg wrote:

> In vmlinux.lds.h we have the following code:
> #define TRACE_SYSCALLS() VMLINUX_SYMBOL(__start_syscalls_metadata) = .; \
>                          *(__syscalls_metadata)                         \
>                          VMLINUX_SYMBOL(__stop_syscalls_metadata) = .;
> 
> But there is nothing that guarantee that
> __syscalls_metadata starts at the address
> assigned to __start_syscalls_metadata.
> 
> The will align __syscalls_metadata accoding to the
> largest member in that section.
> 
> We need to do one of two things:
> 1) Make sure __start_syscalls_metadata is properly aligned
> 2) or make the code robust against misaligned symbols.

1 is much easier to do than 2, so I would say we need to do 1.

Hmm, looking at include/linux/syscalls.h:

#define SYSCALL_TRACE_ENTER_EVENT(sname)				\
	static const struct syscall_metadata __syscall_meta_##sname;	\
	static struct ftrace_event_call					\
	__attribute__((__aligned__(4))) event_enter_##sname;		\
	static struct trace_event enter_syscall_print_##sname = {	\
		.trace                  = print_syscall_enter,		\
	};								\
	static struct ftrace_event_call __used				\
	  __attribute__((__aligned__(4)))				\
	  __attribute__((section("_ftrace_events")))			\
	  event_enter_##sname = {					\

The __syscall_meta_##sname is not forced aligned. The forced 4 byte
alignment compacts the code in a nice array, as the event_enter_##sname
is done.


> 
> > 
> > Zeev, can you try to reproduce it with gcc 4.4.
> > 
> > And for now could you send me the output of this:
> > 
> >  objdump -Dr --start-addr 0x`nm vmlinux | grep __start_syscalls_metadata | cut -d' ' -f 1` \
> >     --stop-addr 0x`nm vmlinux | grep __stop_syscalls_metadata | cut -d' ' -f 1` vmlinux
> > 
> This output would be great to have just to check if my assumption above is correct.

Yeah, that's why I asked about it. I want to confirm it too. I think
adding the align attribute to the meta data will fix it.

-- Steve


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