Re: [PATCH v2 1/5] perf/sdt: ELF support for SDT

[Hemant Kumar]

Hi Namhyung,

On 10/07/2014 07:50 AM, Namhyung Kim wrote:
> Hi Hemant,
>
> On Wed, 01 Oct 2014 08:17:24 +0530, Hemant Kumar wrote:
> > This patch serves as the basic 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.
> >
> > Made the necessary changes as suggested by Namhyung Kim.
> Below are just few more nitpicks, other than that, looks good to me.
>
>
> [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))) {
> > +			ret = populate_sdt_note(&elf, ((data->d_buf) + desc_off),
> > +						nhdr.n_descsz, nhdr.n_type,
> > +						&tmp);
> > +			if (ret < 0)
> > +				goto out_ret;
> > +			list_add_tail(&tmp->note_list, sdt_notes);
> I think we can just pass the sdt_notes list into populate_sdt_note()
> instead of passing a tmp pointer.

Ok, will pass sdt_notes to populate_sdt_note.

> > +		}
> > +	}
> > +	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;
> > +
> > +	fd = open(target, O_RDONLY);
> > +	if (fd < 0)
> > +		return -EBADF;
> > +
> > +	elf = elf_begin(fd, ELF_C_READ, NULL);
> You may want to use PERF_ELF_C_READ_MMAP. :)

Sure.

> Thanks,
> Namhyung
>
>
> [...]

--
Thanks,
Hemant Kumar

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