Re: [PATCH V6 08/15] ftrace: Add perf text poke events for ftrace trampolines

[Adrian Hunter <adrian.hunter@intel.com>]

On 21/04/20 4:45 pm, Peter Zijlstra wrote:
> On Sun, Apr 05, 2020 at 11:13:20PM +0300, Adrian Hunter wrote:
>> Add perf text poke events for ftrace trampolines when created and when
>> freed.
> 
> Maybe also put in a little more detail on the various events. Because
> arch_ftrace_update_trampoline() can also generate text_poke_bp() events,
> to update an existing trampoline.
> 
> A diagram, like with the kprobes thing perhaps.

How about adding this:

There can be 3 text_poke events for ftrace trampolines:

1. NULL -> trampoline
   By ftrace_update_trampoline() when !ops->trampoline
   Trampoline created

2. [e.g. on x86] CALL rel32 -> CALL rel32
   By arch_ftrace_update_trampoline() when ops->trampoline and
                        ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP
   [e.g. on x86] via text_poke_bp() which generates text poke events
   Trampoline-called function target updated

3. trampoline -> NULL
   By ftrace_trampoline_free() when ops->trampoline and
                 ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP
   Trampoline freed

> 
>> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
>> ---
>>  kernel/trace/ftrace.c | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
>> index 398d58b8ffa5..3318f7eca76c 100644
>> --- a/kernel/trace/ftrace.c
>> +++ b/kernel/trace/ftrace.c
>> @@ -2792,6 +2792,13 @@ static void ftrace_trampoline_free(struct ftrace_ops *ops)
>>  {
>>  	if (ops && (ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP) &&
>>  	    ops->trampoline) {
>> +		/*
>> +		 * Record the text poke event before the ksymbol unregister
>> +		 * event.
>> +		 */
>> +		perf_event_text_poke((void *)ops->trampoline,
>> +				     (void *)ops->trampoline,
>> +				     ops->trampoline_size, NULL, 0);
>>  		perf_event_ksymbol(PERF_RECORD_KSYMBOL_TYPE_OOL,
>>  				   ops->trampoline, ops->trampoline_size,
>>  				   true, FTRACE_TRAMPOLINE_SYM);
>> @@ -6816,6 +6823,13 @@ static void ftrace_update_trampoline(struct ftrace_ops *ops)
>>  		perf_event_ksymbol(PERF_RECORD_KSYMBOL_TYPE_OOL,
>>  				   ops->trampoline, ops->trampoline_size, false,
>>  				   FTRACE_TRAMPOLINE_SYM);
>> +		/*
>> +		 * Record the perf text poke event after the ksymbol register
>> +		 * event.
>> +		 */
>> +		perf_event_text_poke((void *)ops->trampoline, NULL, 0,
>> +				     (void *)ops->trampoline,
>> +				     ops->trampoline_size);
>>  	}
>>  }
>>  
>> -- 
>> 2.17.1
>>