Re: [PATCH v2 perf/core] perf tools: Replace zero-length array with flexible-array

From: Arnaldo Carvalho de Melo
Date: Tue May 19 2020 - 21:42:16 EST

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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@xxxxxxxxxx>
> ---
> 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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