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

[Leo Yan]

Hi Mike,

On Wed, Oct 23, 2019 at 12:36:39AM +0100, Mike Leach wrote:
> 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.

Exactly, if offset < 0 it might output the incorrect trace.

> > > 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.

Good point.  Yeah, this is the root cause.

> 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.

Totally agree with this; we should generate synthetic samples without
dropping trace data.

> 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.

Will follow up this suggestion.

Very appreciate your time to review and gave out much reasonable
solution!

Thanks,
Leo Yan

> > > >  {
> > > >     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