Re: [PATCH v3 1/5] perf/sdt: ELF support for SDT

[Hemant Kumar]

On 10/22/2014 08:09 AM, Namhyung Kim wrote:
> Hi Hemant,
>
> On Fri, 10 Oct 2014 16:27:53 +0530, Hemant Kumar wrote:
> > This patch serves 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.
> >
> > Signed-off-by: Hemant Kumar <hemant@xxxxxxxxxxxxxxxxxx>
> Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>
>
> Just a nitpick below..
>
>
> > +static int construct_sdt_notes_list(Elf *elf, struct list_head *sdt_notes)
> > +{
> > +	GElf_Ehdr ehdr;
> > +	Elf_Scn *scn = NULL;
> > +	Elf_Data *data;
> > +	GElf_Shdr shdr;
> > +	size_t shstrndx, next;
> > +	GElf_Nhdr nhdr;
> > +	size_t name_off, desc_off, offset;
> > +	int ret = 0;
> > +
> > +	if (gelf_getehdr(elf, &ehdr) == NULL) {
> > +		ret = -EBADF;
> > +		goto out_ret;
> > +	}
> > +	if (elf_getshdrstrndx(elf, &shstrndx) != 0) {
> > +		ret = -EBADF;
> > +		goto out_ret;
> > +	}
> > +
> > +	/* Look for the required section */
> > +	scn = elf_section_by_name(elf, &ehdr, &shdr, SDT_NOTE_SCN, NULL);
> > +	if (!scn) {
> > +		ret = -ENOENT;
> > +		goto out_ret;
> > +	}
> > +
> > +	if (!(shdr.sh_type == SHT_NOTE) || (shdr.sh_flags & SHF_ALLOC)) {
> I think the below is more readable:

Yes.

> 	if ((shdr.sh_type != SHT_NOTE) || (shdr.sh_flags & SHF_ALLOC)) {

Will change to this.

> Other than that, looks good to me!
>
> Thanks,
> Namhyung
>
>
> > +		ret = -ENOENT;
> > +		goto out_ret;
> > +	}
> > +
> > +	data = elf_getdata(scn, NULL);
> > +
> > +	/* 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,
> > +						sdt_notes);
> > +			if (ret < 0)
> > +				goto out_ret;
> > +		}
> > +	}
> > +	if (list_empty(sdt_notes))
> > +		ret = -ENOENT;
> > +
> > +out_ret:
> > +	return ret;
> > +}

--
Thanks,
Hemant Kumar

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