Subject: Re: [PATCH v3 1/6] perf cs-etm: Fix unsigned variable comparison to zero

From: Mathieu Poirier <[email-masked]>
Date: Fri, 11 Oct 2019 14:16:06 -0600

On Sat, Oct 05, 2019 at 05:16:09PM +0800, Leo Yan wrote:
> If the u64 variable 'offset' is a negative integer, comparison it with
> bigger than zero is always going to be true because it is unsigned.
> Fix this by using s64 type for variable 'offset'.
> 
> Signed-off-by: Leo Yan <[email-masked]>
> ---
>  tools/perf/util/cs-etm.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index 4ba0f871f086..4bc2d9709d4f 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -940,7 +940,7 @@ u64 cs_etm__last_executed_instr(const struct cs_etm_packet *packet)
>  static inline u64 cs_etm__instr_addr(struct cs_etm_queue *etmq,
>  				     u64 trace_chan_id,
>  				     const struct cs_etm_packet *packet,
> -				     u64 offset)
> +				     s64 offset)

In Suzuki's reply there was two choices, 1) move the while(offset > 0) to
while (offset) or change the type of @offset to an s64.  Here we know offset
can't be negative because of the 
        tidq->period_instructions >= etm->instructions_sample_period 

in function cs_etm__sample().  As such I think option #1 is the right way to
deal with this rather than changing the type of the variable.

Mathieu

>  {
>  	if (packet->isa == CS_ETM_ISA_T32) {
>  		u64 addr = packet->start_addr;
> @@ -1372,7 +1372,7 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
>  		 * sample is reported as though instruction has just been
>  		 * executed, but PC has not advanced to next instruction)
>  		 */
> -		u64 offset = (instrs_executed - instrs_over - 1);
> +		s64 offset = (instrs_executed - instrs_over - 1);
>  		u64 addr = cs_etm__instr_addr(etmq, trace_chan_id,
>  					      tidq->packet, offset);
>  
> -- 
> 2.17.1
> 

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