linux-kernel - Re: [PATCH] perf script: Replace zero-length array with flexible-array

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Date:   Fri, 8 May 2020 14:37:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf script: Replace zero-length array with
 flexible-array

On Thu, May 07, 2020 at 02:06:15PM -0500, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
> 
> struct foo {
>         int stuff;
>         struct boo array[];
> };
> 
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
> 
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
> 
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
> 
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
> 
> This issue was found with the help of Coccinelle.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  arch/x86/events/intel/bts.c              |    2 +-
>  arch/x86/events/intel/uncore.h           |    2 +-
>  include/linux/perf_event.h               |    4 ++--
>  kernel/events/callchain.c                |    2 +-
>  kernel/events/internal.h                 |    2 +-
>  tools/perf/bench/sched-messaging.c       |    2 +-
>  tools/perf/builtin-inject.c              |    2 +-
>  tools/perf/builtin-script.c              |    2 +-
>  tools/perf/builtin-timechart.c           |    2 +-
>  tools/perf/util/annotate.h               |    4 ++--
>  tools/perf/util/branch.h                 |    2 +-
>  tools/perf/util/cputopo.h                |    2 +-
>  tools/perf/util/dso.h                    |    4 ++--
>  tools/perf/util/event.h                  |    2 +-
>  tools/perf/util/jitdump.c                |    2 +-
>  tools/perf/util/jitdump.h                |    6 +++---
>  tools/perf/util/ordered-events.h         |    2 +-
>  tools/perf/util/pstack.c                 |    2 +-
>  tools/perf/util/symbol.h                 |    2 +-
>  tools/perf/util/unwind-libunwind-local.c |    2 +-
>  20 files changed, 25 insertions(+), 25 deletions(-)
> 

Arnaldo, do you want this, or should I take it?