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

[Robert Hooker]

On Wed, Feb 22, 2017 at 1:13 AM, Colin Ian King
<colin.king at canonical.com> wrote:
> 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>
>
>
>
> --
> kernel-team mailing list
> kernel-team at lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team

Acked-by: Robert Hooker <robert.hooker at canonical.com>