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Date:   Thu, 14 May 2020 19:10:25 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH perf/core] perf intel-pt: Fix clang build failure in
 intel_pt_synth_pebs_sample

On Thu, May 14, 2020 at 03:46:05PM -0700, Ian Rogers wrote:
> On Thu, May 14, 2020 at 3:04 PM Gustavo A. R. Silva
> <gustavoars@...nel.org> wrote:
> >
> > On Thu, May 14, 2020 at 12:06:48PM -0700, Ian Rogers wrote:
> > > On Thu, May 14, 2020 at 8:01 AM Gustavo A. R. Silva
> > > <gustavoars@...nel.org> wrote:
> > > >
> > > > On Thu, May 14, 2020 at 10:10:30AM -0300, Arnaldo Carvalho de Melo wrote:
> > > > > Em Wed, May 13, 2020 at 06:47:38PM -0500, Gustavo A. R. Silva escreveu:
> > > > > > Fix the following build failure generated with command
> > > > > > $ make CC=clang HOSTCC=clang -C tools/ perf:
> > > > > >
> > > > > > util/intel-pt.c:1802:24: error: field 'br_stack' with variable sized type 'struct branch_stack' not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
> > > > > >                         struct branch_stack br_stack;
> > > > > >                                             ^
> > > > > > 1 error generated.
> > > > > >
> > > > > > Fix this by reordering the members of struct br.
> > > > >
> > > > > Yeah, I noticed that as far back as with ubuntu 16.04's clang:
> > > > >
> > > > > clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
> > > > >
> > > > > util/intel-pt.c:1802:24: error: field 'br_stack' with variable sized type 'struct branch_stack' not at the end of a struct or class is a GNU
> > > > >       extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
> > > > >                         struct branch_stack br_stack;
> > > > >                                             ^
> > > > > 1 error generated.
> > > > >
> > > > >
> > > > > Will fold this with the bug introducing the problem to avoid bisection
> > > > > problems.
> > > > >
> > > >
> > > > I agree. Also, the commit hash of the "Fixes" tag only applies to the
> > > > perf/core branch and, I guess that might create confusion.
> > >
> > >
> > > So while this fixes the warning I believe it breaks the intent of the code.
> > >
> > > tools/perf/util/branch.h:
> > > struct branch_stack {
> > >        u64                     nr;
> > >        u64                     hw_idx;
> > >        struct branch_entry     entries[];
> > > };
> > >
> > > tools/perf/util/intel-pt.c:
> > >                struct {
> > >                        struct branch_stack br_stack;
> > >                        struct branch_entry entries[LBRS_MAX];
> > >                } br;
> > >
> > > The array in br is trying to extend branch_stack's entries array. You
> > > might have to do something like:
> > >
> > > alignas(alignof(branch_stack)) char storage[sizeof(branch_stack) +
> > > sizeof(branch_entry) * LBRS_MAX];
> > > struct branch_stack *br = &storage;
> > >
> > > malloc/free may be nicer on the eyeballs.
> > >
> >
> > Yep, I'd go for zalloc/free. There are a couple of places where dynamic
> > memory is being allocated for struct branch_stack:
> >
> > tools/perf/util/cs-etm.c-256-   if (etm->synth_opts.last_branch) {
> > tools/perf/util/cs-etm.c:257:           size_t sz = sizeof(struct branch_stack);
> > tools/perf/util/cs-etm.c-258-
> > tools/perf/util/cs-etm.c-259-           sz += etm->synth_opts.last_branch_sz *
> > tools/perf/util/cs-etm.c-260-                 sizeof(struct branch_entry);
> > tools/perf/util/cs-etm.c-261-           tidq->last_branch = zalloc(sz);
> >
> > tools/perf/util/thread-stack.c-148-     if (br_stack_sz) {
> > tools/perf/util/thread-stack.c:149:             size_t sz = sizeof(struct branch_stack);
> > tools/perf/util/thread-stack.c-150-
> > tools/perf/util/thread-stack.c-151-             sz += br_stack_sz * sizeof(struct branch_entry);
> > tools/perf/util/thread-stack.c-152-             ts->br_stack_rb = zalloc(sz);
> >
> > there is even function intel_pt_alloc_br_stack().
> >
> > Just out of curiosity, why the need of such a hack in this case (the
> > on-stack extension of branch_stack's entries array)?
> 
> My guess would be that the lbr size is an architectural constant and
> so avoiding malloc/free in what could be a hot loop was desirable.
> As this is part of a larger patch set, is this the only place this
> problem has been encountered? Perhaps a macro could perform the

Yep. I just built linux-next --which contains all the flexible-array
conversions-- with Clang --GCC doesn't catch this issue, not even GCC
10-- and I don't see any other issue like this.

I mean, I have run into these other two:

https://lore.kernel.org/lkml/20200505235205.GA18539@embeddedor/
https://lore.kernel.org/lkml/20200508163826.GA768@embeddedor/

but those are due to the erroneous application of the sizeof operator
to zero-length arrays.

> complicated stack allocation I suggested. It may be nice to save
> cycles if code this pattern is widespread and the code hot.
> 

Apparently, this is the only instace of this sort of issue in the whole
codebase.

Thanks
--
Gustavo

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