Re: [PATCH -trace] trace: fix TASK_COMM_LEN in trace event format file

[Yafang Shao <laoar.shao@gmail.com>]

On Sun, Feb 12, 2023 at 4:23 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Fri, 10 Feb 2023 15:59:21 +0000
> Yafang Shao <laoar.shao@gmail.com> wrote:
>
> > After commit 3087c61ed2c4 ("tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN"),
> > the content of the format file under
> > /sys/kernel/debug/tracing/events/task/task_newtask was changed from
> >   field:char comm[16];    offset:12;    size:16;    signed:0;
> > to
> >   field:char comm[TASK_COMM_LEN];    offset:12;    size:16;    signed:0;
> >
> > John reported that this change breaks older versions of perfetto.
> > Then Mathieu pointed out that this behavioral change was caused by the
> > use of __stringify(_len), which happens to work on macros, but not on enum
> > labels. And he also gave the suggestion on how to fix it:
> >   :One possible solution to make this more robust would be to extend
> >   :struct trace_event_fields with one more field that indicates the length
> >   :of an array as an actual integer, without storing it in its stringified
> >   :form in the type, and do the formatting in f_show where it belongs.
> >
> > The result as follows after this change,
> > $ cat /sys/kernel/debug/tracing/events/task/task_newtask/format
> >         field:char comm[16];    offset:12;      size:16;        signed:0;
> >
> > Fixes: 3087c61ed2c4 ("tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN")
> > Reported-by: John Stultz <jstultz@google.com>
> > Debugged-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> > Cc: Kajetan Puchalski <kajetan.puchalski@arm.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: John Stultz <jstultz@google.com>
> > Cc: stable@vger.kernel.org # v5.17+
> > ---
>
> The below patch appears to not break[*] anything, as I did a diff off
> all events, before and after, and it looks like a nice clean up!
>
> I had to cover the "ftrace events" which are not trace events (for example,
> trace_printk, function, function_graph). They are created by their own
> macros that are similar. But your original patch broke them.
>
> Please use this patch, and resubmit. I'll push it through my tree after
> it goes through all my normal testing. You can include:
>
> Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
>

Thanks for the suggestion. I will do it.

> [*] I haven't run it through all my tests yet.

I will run selftests/ftrace before sending it.

>
> Thanks.
>
> -- Steve
>
> diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> index 4342e996bcdb..0e373222a6df 100644
> --- a/include/linux/trace_events.h
> +++ b/include/linux/trace_events.h
> @@ -270,6 +270,7 @@ struct trace_event_fields {
>                         const int  align;
>                         const int  is_signed;
>                         const int  filter_type;
> +                       const int  len;
>                 };
>                 int (*define_fields)(struct trace_event_call *);
>         };
> diff --git a/include/trace/stages/stage4_event_fields.h b/include/trace/stages/stage4_event_fields.h
> index affd541fd25e..306f39aab480 100644
> --- a/include/trace/stages/stage4_event_fields.h
> +++ b/include/trace/stages/stage4_event_fields.h
> @@ -26,7 +26,8 @@
>  #define __array(_type, _item, _len) {                                  \
>         .type = #_type"["__stringify(_len)"]", .name = #_item,          \
>         .size = sizeof(_type[_len]), .align = ALIGN_STRUCTFIELD(_type), \
> -       .is_signed = is_signed_type(_type), .filter_type = FILTER_OTHER },
> +       .is_signed = is_signed_type(_type), .filter_type = FILTER_OTHER, \
> +       .len = _len },
>
>  #undef __dynamic_array
>  #define __dynamic_array(_type, _item, _len) {                          \
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index f10bf804dd2b..f3aae2be1d53 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -1286,6 +1286,7 @@ struct ftrace_event_field {
>         int                     offset;
>         int                     size;
>         int                     is_signed;
> +       int                     len;
>  };
>
>  struct prog_entry;
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index cf3fd74fa675..60ab2f2108fe 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -114,7 +114,7 @@ trace_find_event_field(struct trace_event_call *call, char *name)
>
>  static int __trace_define_field(struct list_head *head, const char *type,
>                                 const char *name, int offset, int size,
> -                               int is_signed, int filter_type)
> +                               int is_signed, int filter_type, int len)
>  {
>         struct ftrace_event_field *field;
>
> @@ -133,6 +133,7 @@ static int __trace_define_field(struct list_head *head, const char *type,
>         field->offset = offset;
>         field->size = size;
>         field->is_signed = is_signed;
> +       field->len = len;
>
>         list_add(&field->link, head);
>
> @@ -150,14 +151,28 @@ int trace_define_field(struct trace_event_call *call, const char *type,
>
>         head = trace_get_fields(call);
>         return __trace_define_field(head, type, name, offset, size,
> -                                   is_signed, filter_type);
> +                                   is_signed, filter_type, 0);
>  }
>  EXPORT_SYMBOL_GPL(trace_define_field);
>
> +int trace_define_field_ext(struct trace_event_call *call, const char *type,
> +                      const char *name, int offset, int size, int is_signed,
> +                      int filter_type, int len)
> +{
> +       struct list_head *head;
> +
> +       if (WARN_ON(!call->class))
> +               return 0;
> +
> +       head = trace_get_fields(call);
> +       return __trace_define_field(head, type, name, offset, size,
> +                                   is_signed, filter_type, len);
> +}
> +
>  #define __generic_field(type, item, filter_type)                       \
>         ret = __trace_define_field(&ftrace_generic_fields, #type,       \
>                                    #item, 0, 0, is_signed_type(type),   \
> -                                  filter_type);                        \
> +                                  filter_type, 0);                     \
>         if (ret)                                                        \
>                 return ret;
>
> @@ -166,7 +181,7 @@ EXPORT_SYMBOL_GPL(trace_define_field);
>                                    "common_" #item,                     \
>                                    offsetof(typeof(ent), item),         \
>                                    sizeof(ent.item),                    \
> -                                  is_signed_type(type), FILTER_OTHER); \
> +                                  is_signed_type(type), FILTER_OTHER, 0);      \
>         if (ret)                                                        \
>                 return ret;
>
> @@ -1588,12 +1603,17 @@ static int f_show(struct seq_file *m, void *v)
>                 seq_printf(m, "\tfield:%s %s;\toffset:%u;\tsize:%u;\tsigned:%d;\n",
>                            field->type, field->name, field->offset,
>                            field->size, !!field->is_signed);
> -       else
> -               seq_printf(m, "\tfield:%.*s %s%s;\toffset:%u;\tsize:%u;\tsigned:%d;\n",
> +       else if (field->len)
> +               seq_printf(m, "\tfield:%.*s %s[%d];\toffset:%u;\tsize:%u;\tsigned:%d;\n",
>                            (int)(array_descriptor - field->type),
>                            field->type, field->name,
> -                          array_descriptor, field->offset,
> +                          field->len, field->offset,
>                            field->size, !!field->is_signed);
> +       else
> +               seq_printf(m, "\tfield:%.*s %s[];\toffset:%u;\tsize:%u;\tsigned:%d;\n",
> +                          (int)(array_descriptor - field->type),
> +                          field->type, field->name,
> +                          field->offset, field->size, !!field->is_signed);
>
>         return 0;
>  }
> @@ -2379,9 +2399,10 @@ event_define_fields(struct trace_event_call *call)
>                         }
>
>                         offset = ALIGN(offset, field->align);
> -                       ret = trace_define_field(call, field->type, field->name,
> +                       ret = trace_define_field_ext(call, field->type, field->name,
>                                                  offset, field->size,
> -                                                field->is_signed, field->filter_type);
> +                                                field->is_signed, field->filter_type,
> +                                                field->len);
>                         if (WARN_ON_ONCE(ret)) {
>                                 pr_err("error code is %d\n", ret);
>                                 break;
> diff --git a/kernel/trace/trace_export.c b/kernel/trace/trace_export.c
> index d960f6b11b5e..9d0a6ea4c076 100644
> --- a/kernel/trace/trace_export.c
> +++ b/kernel/trace/trace_export.c
> @@ -111,7 +111,8 @@ static void __always_unused ____ftrace_check_##name(void)           \
>  #define __array(_type, _item, _len) {                                  \
>         .type = #_type"["__stringify(_len)"]", .name = #_item,          \
>         .size = sizeof(_type[_len]), .align = __alignof__(_type),       \
> -       is_signed_type(_type), .filter_type = FILTER_OTHER },
> +       is_signed_type(_type), .filter_type = FILTER_OTHER,             \
> +       .len = _len },                                                  \
>
>  #undef __array_desc
>  #define __array_desc(_type, _container, _item, _len) __array(_type, _item, _len)



-- 
Regards
Yafang