Re: [PATCH v4 1/3] perf/sdt : Raw SDT parsing functions

Namhyung Kim
2014-08-29T07:22:26Z

Hi Hemant,

On Thu, 28 Aug 2014 03:14:20 +0530, Hemant Kumar wrote:
> This patch serves as the initial support to identify and list SDT events in binaries.
> When programs containing SDT markers are compiled, gcc with the help of assembler
> directives identifies them and places them in the section ".note.stapsdt". To find these
> markers from the binaries, one needs to traverse through this section and parse the
> relevant details like the name, type and location of the marker. Also, the original
> location could be skewed due to the effect of prelinking. If that is the case, the
> locations need to be adjusted.
>
> The functions in this patch open a given ELF, find out the SDT section, parse the
> relevant details, adjust the location (if necessary) and populate them in a list.
>

[SNIP]
> + /* Get the SDT notes */
> + for (offset = 0; (next = gelf_getnote(data, offset, &nhdr, &name_off,
> + &desc_off)) > 0; offset = next) {
> + if (nhdr.n_namesz == sizeof(SDT_NOTE_NAME) &&
> + !memcmp(data->d_buf + name_off, SDT_NOTE_NAME,
> + sizeof(SDT_NOTE_NAME))) {
> + val = populate_sdt_note(&elf, ((data->d_buf) + desc_off),
> + nhdr.n_descsz, nhdr.n_type,
> + &tmp);
> + if (!val)
> + list_add_tail(&tmp->note_list, sdt_notes);
> + if (val == -ENOMEM) {
> + ret = val;
> + goto out_ret;
> + }

It seems populate_sdt_note() can failed with other error than ENOMEM.
So I think it'd be better changing it like:

ret = populate_sdt_note(...);
if (ret < 0)
goto out_ret;

list_add_tail(...);

So no need to use the 'val' variable.

> + }
> + }
> + if (list_empty(sdt_notes))
> + ret = -ENOENT;
> +
> +out_ret:
> + return ret;
> +}
> +
> +/*
> + * get_sdt_note_list() : Takes two arguments "head" and "target", where head
> + * is the head of the SDT events' list and "target" is the file name as to
> + * where the SDT events should be looked for. This opens the file, initializes
> + * the ELF and then calls construct_sdt_notes_list.
> + */
> +int get_sdt_note_list(struct list_head *head, const char *target)
> +{
> + Elf *elf;
> + int fd, ret;

Just a nitpick. It'd be better setting ret to -EBADF IMHO.

> +
> + fd = open(target, O_RDONLY);
> + if (fd < 0)
> + return -EBADF;
> +
> + symbol__elf_init();

This is really need? I guess it's not harmful but no need to call it
whenever we check sdt note in every file. A single call to
simbole__init() can be placed in the cmd_list() instead.

> + elf = elf_begin(fd, ELF_C_READ, NULL);
> + if (!elf) {
> + ret = -EBADF;
> + goto out_close;
> + }
> + ret = construct_sdt_notes_list(elf, head);
> + elf_end(elf);
> +
> +out_close:
> + close(fd);
> + return ret;
> +}
> +
> +/*
> + * is_an_elf() : Returns 'true' if the file is an elf and 'false' otherwise
> + */
> +bool is_an_elf(char *file)
> +{
> + int fd;
> + Elf *elf;
> + bool ret = true;
> +
> + fd = open(file, O_RDONLY);
> + if (fd < 0) {
> + ret = false;
> + goto out_ret;
> + }
> +
> + symbol__elf_init();

Ditto.

Thanks,
Namhyung

> + elf = elf_begin(fd, ELF_C_READ, NULL);
> + if (!elf) {
> + ret = false;
> + goto out_close;
> + }
> + if (elf_kind(elf) != ELF_K_ELF)
> + ret = false;
> +
> + elf_end(elf);
> +
> +out_close:
> + close(fd);
> +out_ret:
> + return ret;
> +}
> +
> void symbol__elf_init(void)
> {
> elf_version(EV_CURRENT);
> diff --git a/tools/perf/util/symbol.h b/tools/perf/util/symbol.h
> index 615c752..83be31a 100644
> --- a/tools/perf/util/symbol.h
> +++ b/tools/perf/util/symbol.h
> @@ -294,4 +294,23 @@ int compare_proc_modules(const char *from, const char *to);
> int setup_list(struct strlist **list, const char *list_str,
> const char *list_name);
>
> +struct sdt_note {
> + char *name;
> + char *provider;
> + bool bit32;
> + union {
> + Elf64_Addr a64[3];
> + Elf32_Addr a32[3];
> + } addr;
> + struct list_head note_list;
> +};
> +
> +int get_sdt_note_list(struct list_head *head, const char *target);
> +bool is_an_elf(char *file);
> +
> +#define SDT_BASE_SCN ".stapsdt.base"
> +#define SDT_NOTE_SCN ".note.stapsdt"
> +#define SDT_NOTE_TYPE 3
> +#define SDT_NOTE_NAME "stapsdt"
> +
> #endif /* __PERF_SYMBOL */