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Date:   Tue, 19 May 2020 22:42:09 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     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>,
        Ian Rogers <irogers@...gle.com>, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH v2 perf/core] perf tools: Replace zero-length array with
 flexible-array

Em Fri, May 15, 2020 at 12:29:26PM -0500, Gustavo A. R. Silva escreveu:
> 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[];
> };

Thanks, applied.

- Arnaldo
 
> 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>
> ---
> Changes in v2:
>  - Remove changes in tools/perf/util/branch.h
> 
>  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/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 +-
>  14 files changed, 18 insertions(+), 18 deletions(-)
> 
> diff --git a/tools/perf/bench/sched-messaging.c b/tools/perf/bench/sched-messaging.c
> index 97e4a4fb33624..71d830d7b9231 100644
> --- a/tools/perf/bench/sched-messaging.c
> +++ b/tools/perf/bench/sched-messaging.c
> @@ -40,7 +40,7 @@ struct sender_context {
>  	unsigned int num_fds;
>  	int ready_out;
>  	int wakefd;
> -	int out_fds[0];
> +	int out_fds[];
>  };
>  
>  struct receiver_context {
> diff --git a/tools/perf/builtin-inject.c b/tools/perf/builtin-inject.c
> index 53932db97a79d..4a6de4b03ac0b 100644
> --- a/tools/perf/builtin-inject.c
> +++ b/tools/perf/builtin-inject.c
> @@ -51,7 +51,7 @@ struct perf_inject {
>  struct event_entry {
>  	struct list_head node;
>  	u32		 tid;
> -	union perf_event event[0];
> +	union perf_event event[];
>  };
>  
>  static int output_bytes(struct perf_inject *inject, void *buf, size_t sz)
> diff --git a/tools/perf/builtin-script.c b/tools/perf/builtin-script.c
> index ecc8bd4c5e57d..4c0837efaceba 100644
> --- a/tools/perf/builtin-script.c
> +++ b/tools/perf/builtin-script.c
> @@ -2449,7 +2449,7 @@ static int __cmd_script(struct perf_script *script)
>  struct script_spec {
>  	struct list_head	node;
>  	struct scripting_ops	*ops;
> -	char			spec[0];
> +	char			spec[];
>  };
>  
>  static LIST_HEAD(script_specs);
> diff --git a/tools/perf/builtin-timechart.c b/tools/perf/builtin-timechart.c
> index c76f84b174c4b..4e380e7b52307 100644
> --- a/tools/perf/builtin-timechart.c
> +++ b/tools/perf/builtin-timechart.c
> @@ -128,7 +128,7 @@ struct sample_wrapper {
>  	struct sample_wrapper *next;
>  
>  	u64		timestamp;
> -	unsigned char	data[0];
> +	unsigned char	data[];
>  };
>  
>  #define TYPE_NONE	0
> diff --git a/tools/perf/util/annotate.h b/tools/perf/util/annotate.h
> index 2d88069d64283..0a0cd4f32175e 100644
> --- a/tools/perf/util/annotate.h
> +++ b/tools/perf/util/annotate.h
> @@ -144,7 +144,7 @@ struct annotation_line {
>  	u32			 idx;
>  	int			 idx_asm;
>  	int			 data_nr;
> -	struct annotation_data	 data[0];
> +	struct annotation_data	 data[];
>  };
>  
>  struct disasm_line {
> @@ -227,7 +227,7 @@ void symbol__calc_percent(struct symbol *sym, struct evsel *evsel);
>  struct sym_hist {
>  	u64		      nr_samples;
>  	u64		      period;
> -	struct sym_hist_entry addr[0];
> +	struct sym_hist_entry addr[];
>  };
>  
>  struct cyc_hist {
> diff --git a/tools/perf/util/cputopo.h b/tools/perf/util/cputopo.h
> index 7bf6b811f715e..6201c3790d868 100644
> --- a/tools/perf/util/cputopo.h
> +++ b/tools/perf/util/cputopo.h
> @@ -22,7 +22,7 @@ struct numa_topology_node {
>  
>  struct numa_topology {
>  	u32				nr;
> -	struct numa_topology_node	nodes[0];
> +	struct numa_topology_node	nodes[];
>  };
>  
>  struct cpu_topology *cpu_topology__new(void);
> diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
> index 9553a1fd9e8a8..e3e9e3b772970 100644
> --- a/tools/perf/util/dso.h
> +++ b/tools/perf/util/dso.h
> @@ -137,7 +137,7 @@ struct dso_cache {
>  	struct rb_node	rb_node;
>  	u64 offset;
>  	u64 size;
> -	char data[0];
> +	char data[];
>  };
>  
>  struct auxtrace_cache;
> @@ -209,7 +209,7 @@ struct dso {
>  	struct nsinfo	*nsinfo;
>  	struct dso_id	 id;
>  	refcount_t	 refcnt;
> -	char		 name[0];
> +	char		 name[];
>  };
>  
>  /* dso__for_each_symbol - iterate over the symbols of given type
> diff --git a/tools/perf/util/event.h b/tools/perf/util/event.h
> index b8289f160f070..6ae01c3c2ffa7 100644
> --- a/tools/perf/util/event.h
> +++ b/tools/perf/util/event.h
> @@ -79,7 +79,7 @@ struct sample_read {
>  
>  struct ip_callchain {
>  	u64 nr;
> -	u64 ips[0];
> +	u64 ips[];
>  };
>  
>  struct branch_stack;
> diff --git a/tools/perf/util/jitdump.c b/tools/perf/util/jitdump.c
> index e3ccb0ce1938d..32bb05e03fb2d 100644
> --- a/tools/perf/util/jitdump.c
> +++ b/tools/perf/util/jitdump.c
> @@ -57,7 +57,7 @@ struct debug_line_info {
>  	unsigned long vma;
>  	unsigned int lineno;
>  	/* The filename format is unspecified, absolute path, relative etc. */
> -	char const filename[0];
> +	char const filename[];
>  };
>  
>  struct jit_tool {
> diff --git a/tools/perf/util/jitdump.h b/tools/perf/util/jitdump.h
> index f2c3823cc81a4..ab2842def83df 100644
> --- a/tools/perf/util/jitdump.h
> +++ b/tools/perf/util/jitdump.h
> @@ -93,7 +93,7 @@ struct debug_entry {
>  	uint64_t addr;
>  	int lineno;	    /* source line number starting at 1 */
>  	int discrim;	    /* column discriminator, 0 is default */
> -	const char name[0]; /* null terminated filename, \xff\0 if same as previous entry */
> +	const char name[]; /* null terminated filename, \xff\0 if same as previous entry */
>  };
>  
>  struct jr_code_debug_info {
> @@ -101,7 +101,7 @@ struct jr_code_debug_info {
>  
>  	uint64_t code_addr;
>  	uint64_t nr_entry;
> -	struct debug_entry entries[0];
> +	struct debug_entry entries[];
>  };
>  
>  struct jr_code_unwinding_info {
> @@ -110,7 +110,7 @@ struct jr_code_unwinding_info {
>  	uint64_t unwinding_size;
>  	uint64_t eh_frame_hdr_size;
>  	uint64_t mapped_size;
> -	const char unwinding_data[0];
> +	const char unwinding_data[];
>  };
>  
>  union jr_entry {
> diff --git a/tools/perf/util/ordered-events.h b/tools/perf/util/ordered-events.h
> index 0920fb0ec6cc7..75345946c4b9e 100644
> --- a/tools/perf/util/ordered-events.h
> +++ b/tools/perf/util/ordered-events.h
> @@ -29,7 +29,7 @@ typedef int (*ordered_events__deliver_t)(struct ordered_events *oe,
>  
>  struct ordered_events_buffer {
>  	struct list_head	list;
> -	struct ordered_event	event[0];
> +	struct ordered_event	event[];
>  };
>  
>  struct ordered_events {
> diff --git a/tools/perf/util/pstack.c b/tools/perf/util/pstack.c
> index 80ff41fc45be8..a1d1e4ef6257e 100644
> --- a/tools/perf/util/pstack.c
> +++ b/tools/perf/util/pstack.c
> @@ -15,7 +15,7 @@
>  struct pstack {
>  	unsigned short	top;
>  	unsigned short	max_nr_entries;
> -	void		*entries[0];
> +	void		*entries[];
>  };
>  
>  struct pstack *pstack__new(unsigned short max_nr_entries)
> diff --git a/tools/perf/util/symbol.h b/tools/perf/util/symbol.h
> index 93fc43db1be3c..ff4f4c47e1484 100644
> --- a/tools/perf/util/symbol.h
> +++ b/tools/perf/util/symbol.h
> @@ -55,7 +55,7 @@ struct symbol {
>  	u8		inlined:1;
>  	u8		arch_sym;
>  	bool		annotate2;
> -	char		name[0];
> +	char		name[];
>  };
>  
>  void symbol__delete(struct symbol *sym);
> diff --git a/tools/perf/util/unwind-libunwind-local.c b/tools/perf/util/unwind-libunwind-local.c
> index b4649f5a0c2f1..9aededc0bc06f 100644
> --- a/tools/perf/util/unwind-libunwind-local.c
> +++ b/tools/perf/util/unwind-libunwind-local.c
> @@ -243,7 +243,7 @@ struct eh_frame_hdr {
>  	 *    encoded_t fde_addr;
>  	 * } binary_search_table[fde_count];
>  	 */
> -	char data[0];
> +	char data[];
>  } __packed;
>  
>  static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
> -- 
> 2.26.2
> 

-- 

- Arnaldo

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