linux-kernel - Re: [PATCH] tracing: properly align linker defined symbols

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Message-ID: <20100710134849.GA15800@merkur.ravnborg.org>
Date:	Sat, 10 Jul 2010 15:48:49 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Zeev Tarantov <zeev.tarantov@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [PATCH] tracing: properly align linker defined symbols

On Sat, Jul 10, 2010 at 07:34:05AM -0400, Steven Rostedt wrote:
> On Sat, 2010-07-10 at 13:18 +0300, Zeev Tarantov wrote:
> > On Sat, Jul 10, 2010 at 09:35, Sam Ravnborg <sam@...nborg.org> wrote:
> 
> > > +/*
> > > + * Align to a 32 byte boundary equal to the
> > > + * alignment gcc 4.5 uses for a struct
> > > + */
> > > +#define STRUCT_ALIGN() . = ALIGN(32)
> > > +
> 
> What I'm nervous about is when gcc 4.8 decides to up the alignment to
> 64.
> 
> Maybe we should have both patches, just to be safe.

Another approach could be to just stop playing games with alignment
and use the fact that __stop_syscalls_metadata point to next byte
after last entry.

Something like the below untested patch.

But to fix the current regression I prefer the simpler
patch that just fixup the aligment.

	Sam


diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index 34e3580..50e9606 100644
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -79,15 +79,19 @@ static struct syscall_metadata *find_syscall_meta(unsigned long syscall)
 	stop = (struct syscall_metadata *)__stop_syscalls_metadata;
 	kallsyms_lookup(syscall, NULL, NULL, NULL, str);
 
-	for ( ; start < stop; start++) {
+	/*
+	 * start may point a few bytes before first entry
+	 * stop points at first byte after last entry
+	 */
+	for (stop--; stop >= start; stop--) {
 		/*
 		 * Only compare after the "sys" prefix. Archs that use
 		 * syscall wrappers may have syscalls symbols aliases prefixed
 		 * with "SyS" instead of "sys", leading to an unwanted
 		 * mismatch.
 		 */
-		if (start->name && !strcmp(start->name + 3, str + 3))
-			return start;
+		if (stop->name && !strcmp(stop->name + 3, str + 3))
+			return stop;
 	}
 	return NULL;
 }
--
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