[PATCH][SRU][XENIAL] ftrace/x86: Set ftrace_stub to weak to prevent gcc from using short jumps to it

[Colin Ian King]

On 21/02/17 19:54, Colin King wrote:
> From: Steven Rostedt <rostedt at goodmis.org>
> 
> BugLink: 
> 
> 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 <hjl.tools at gmail.com>
> 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 <matt at codeblueprint.co.uk>
> Reported-by: Namhyung Kim <namhyung at kernel.org>
> Tested-by: Matt Fleming <matt at codeblueprint.co.uk>
> Reviewed-by: Masami Hiramatsu <mhiramat at kernel.org>
> Signed-off-by: Steven Rostedt <rostedt at goodmis.org>
> ---
>  arch/x86/kernel/mcount_64.S | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/mcount_64.S b/arch/x86/kernel/mcount_64.S
> index ed48a9f..6192422 100644
> --- a/arch/x86/kernel/mcount_64.S
> +++ b/arch/x86/kernel/mcount_64.S
> @@ -182,7 +182,8 @@ 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)
>  
> 

Forgot t add:

cherry-pick from commit 8329e818f14926a6040df86b2668568bde342ebf
Signed-off-by: Colin Ian King <colin.king at canonical.com>