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

From: Hemant Kumar
Date: Tue Oct 07 2014 - 02:10:33 EST


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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