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

Mathieu Poirier
2019-10-11T20:16:14Z

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 <leo.yan@linaro.org>
> ---
> 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