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

[Mike Leach]

Hi Leo,

Two points here - both related.

On Tue, 22 Oct 2019 at 06:10, Leo Yan <leo.yan@xxxxxxxxxx> wrote:
>
> Hi Mathieu,
>
> On Fri, Oct 11, 2019 at 02:16:06PM -0600, Mathieu Poirier wrote:
> > 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@xxxxxxxxxx>
> > > ---
> > >  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)
> >
Issue 1:

OK - it appears that cs_etm__instr_addr() is supposed to be returning
the address within the current trace sample of the instruction related
to offset.
For T32 - then if offset < 0, packet->start_addr is returned - not
good but at least within the current trace range
For A32/A64 - if offset < 0 then an address _before_
packet->start_addr is returned - clearly wrong and possibly a
completely invalid address that was never actually traced.

> > 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.
>
> I took sometime to use formulas to prove that 'offset' is possible to
> be a negative value :)
>
> Just paste the updated commit log at here for review:
>
>   Pi: period_instructions
>   Ie: instrs_executed
>   Io: instrs_over
>   Ip: instructions_sample_period
>
>   Pi' = Pi + Ie   -> New period_instructions equals to the old
>                      period_instructions + instrs_executed
>   Io  = Pi' - Ip  -> period_instructions - instructions_sample_period
>
>   offset = Ie - Io - 1
>          = Ie - (Pi' - Ip) -1
>          = Ie - (Pi + Ie - Ip) -1
>          = Ip - Pi - 1
>
> In theory, if Ip (instructions_sample_period) is small enough and Pi
> (period_instructions) is bigger than Ip, then it will lead to the
> negative value for 'offset'.
>
> So let's see below command:
>
>   perf inject --itrace=i1il128 -i perf.data -o perf.data.new
>
> With this command, 'offset' is very easily to be a negative value when
> handling packets; this is because if use the inject option 'i1', then
> instructions_sample_period equals to 1; so:
>
>   offset = 1 - Pi - 1
>          = -Pi
>
> Any Pi bigger than zero leads 'offset' to a negative value.
>
> Thanks,
> Leo Yan
>

Issue 2:

Assuming I have understood the logic of this code correctly - there is
an issue were sample_period < period_instructions as you say -
but I believe the problem is in the logic of the sampling function itself.

Suppose we have a sample_period of 4.

Now on an initial pass through the function, period_instructions must
be 0. (i.e. none left over from the previous pass.)
Suppose also that the number of instructions executed in this sample
is 10 - thus updating period_instructions.
Therefore:
instr_over = 10 - 4 -> 6
offset = 10 - 6 - 1 -> 3.
We therefore call cs_etm_instr_addr to find the address an offset of 3
instructions from the start of the trace sample and synthesize the
sample.
After this we set period_instructions to the instr_over value of 6.

Next pass, assume 10 instructions in the trace sample again.
period_instructions = 6 + 10 -> 16
instr_over = 16 - 4 -> 12
offset = 10 - 12 - 1 -> -3  - the negative value your formulae predict.

This implies that the sample we want is actually in the previous trace
packet - which I believe is in fact the case - as explained below.

My reading of the code is that cs_etm__sample() is called once per
trace range packet extracted from the decoder - and a trace range
packet represents N instructions_executed.
Further I am assuming that instructions_sample_period represents the
desired periodicity of the instruction samples - i.e. 1 sample every
instructions_sample_period number of instructions.

Thus my conclusion here is that where M = instructions_executed +
period_instructions, the function should generate quotient ( M /
instructions_sample_period ) samples and set period_instructions to M
mod instructions_sample_period on exit, ensuring period_instructions
is never larger than the sample_period.

i.e. loop to generate multiple samples until instr_over and therefore
the output value of period_instructions is less than the value of
instructions_sample_period - for the example above, with 10
instructions and a periodicity of 4, we generate 2 samples with a
remainder of 2 instructions carried forwards.

In short leave offset as unsigned and fix the logic of the
cs_etm__sample() function.

Regards

Mike

> > >  {
> > >     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
> > >



--
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK