From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.linuxfoundation.org ([140.211.169.12]:39630 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750713AbdAXH4G (ORCPT ); Tue, 24 Jan 2017 02:56:06 -0500 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Matt Fleming , Namhyung Kim , Masami Hiramatsu , Steven Rostedt , Jiri Slaby Subject: [PATCH 4.4 01/42] ftrace/x86: Set ftrace_stub to weak to prevent gcc from using short jumps to it Date: Tue, 24 Jan 2017 08:55:11 +0100 Message-Id: <20170124075509.354196270@linuxfoundation.org> In-Reply-To: <20170124075509.299412838@linuxfoundation.org> References: <20170124075509.299412838@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: stable-owner@vger.kernel.org List-ID: 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Steven Rostedt commit 8329e818f14926a6040df86b2668568bde342ebf upstream. Matt Fleming reported seeing crashes when enabling and disabling function profiling which uses function graph tracer. Later Namhyung Kim hit a similar issue and he found that the issue was due to the jmp to ftrace_stub in ftrace_graph_call was only two bytes, and when it was changed to jump to the tracing code, it overwrote the ftrace_stub that was after it. Masami Hiramatsu bisected this down to a binutils change: 8dcea93252a9ea7dff57e85220a719e2a5e8ab41 is the first bad commit commit 8dcea93252a9ea7dff57e85220a719e2a5e8ab41 Author: H.J. Lu Date: Fri May 15 03:17:31 2015 -0700 Add -mshared option to x86 ELF assembler This patch adds -mshared option to x86 ELF assembler. By default, assembler will optimize out non-PLT relocations against defined non-weak global branch targets with default visibility. The -mshared option tells the assembler to generate code which may go into a shared library where all non-weak global branch targets with default visibility can be preempted. The resulting code is slightly bigger. This option only affects the handling of branch instructions. Declaring ftrace_stub as a weak call prevents gas from using two byte jumps to it, which would be converted to a jump to the function graph code. Link: http://lkml.kernel.org/r/20160516230035.1dbae571@gandalf.local.home Reported-by: Matt Fleming Reported-by: Namhyung Kim Tested-by: Matt Fleming Reviewed-by: Masami Hiramatsu Signed-off-by: Steven Rostedt Cc: Jiri Slaby Signed-off-by: Greg Kroah-Hartman --- arch/x86/kernel/mcount_64.S | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/arch/x86/kernel/mcount_64.S +++ b/arch/x86/kernel/mcount_64.S @@ -180,7 +180,8 @@ GLOBAL(ftrace_graph_call) jmp ftrace_stub #endif -GLOBAL(ftrace_stub) +/* This is weak to keep gas from relaxing the jumps */ +WEAK(ftrace_stub) retq END(ftrace_caller)