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    SubjectRe: [RFC PATCH v1 1/2] perf/sdt : Listing of SDT markers by perf
    Date
    Hi Hemant,

    On Mon, 24 Feb 2014 14:45:43 +0530, Hemant Kumar wrote:
    > This patch enables perf to list the SDT markers present in a system. It looks
    > in dsos given by ldconfig --print-cache and for other binaries, it looks into
    > the PATH environment variable. After preparing a list of the binaries, then
    > it starts searching for SDT markers in them.
    > To find the SDT markers, first an elf section named .note.stapsdt is searched
    > for. And then the SDT notes are retreived one by one from that section.
    > To counter the effect of prelinking, the section ".stapsdt.base" is searched.
    > If its found, then the location of the SDT marker is adjusted.
    >
    > All these markers' info is written into a cache file
    > "/var/cache/perf/perf-sdt.cache".
    > Since, the presence of SDT markers is quite common these days, hence, its better
    > to make them visible to a user easily. Also, creating a cache file will help a user
    > to probe (to be implemented) these markers without much hussle. This cache file will
    > hold most of the SDT markers.
    > The format of each entry is -
    >
    > %provider : marker : file_path : build_id : location : semaphore_loc
    >
    > % - marks the beginning of each entry.
    > provider - The provider name of the SDT marker.
    > marker - The marker name of the SDT marker.
    > file_path - Full/absolute path of the file in which this marker is present.
    > location : Adjusted location of the SDT marker inside the program.
    > semaphore_loc : The semaphore address if present otherwise 0x0.
    >
    > This format should help when probing will be implemented. The adjusted address
    > from the required entry can be directly used in probing if the build_id matches.
    >
    > To use this feature, invoke :
    > # perf list sdt --scan
    >
    > "--scan" should be used for the first time and whenever there is any change in
    > the files containing the SDT markers.
    >
    > And then use :
    > # perf list sdt
    >
    > This displays a list of SDT markers (read from the cache file) present in most of
    > the dsos and other binaries.

    The default action of perf list (with no argument) prints all available
    events on the system - so it should also print SDT markers IMHO.

    >
    > However, not all the SDT markers are present in the cache file. Only the ELFs
    > present in the directories in PATH variable are looked into.
    > An individual file argument can be given to "perf list" to find out the SDT markers
    > present in that file. Usage is as below :
    >
    > # perf list sdt /home/hemant/tmp
    >
    > /home/hemant/tmp:
    > %user : foo
    > %user : bar
    >
    > Signed-off-by : hkshaw@linux.vnet.ibm.com
    > ---

    [SNIP]
    > +/*
    > + * Finds out the libraries present in a system as shown by the command
    > + * "ldconfig --print-cache". Uses "=>" and '/' to find out the start of a
    > + * dso path.
    > + */
    > +static void parse_lib_name(char *str, char *path)
    > +{
    > + char *ptr, *q, *r;
    > + char c = '=';
    > +
    > + while (str != NULL) {
    > + /* look for "=>" and then '/' */
    > + ptr = strchr(str, c);
    > + if (ptr && (*(ptr + 1) == '>')) {

    Hmm.. why not searching "=>" by strstr() then?


    > + ptr++;
    > + q = strchr(ptr, '/');
    > + if (!q)
    > + return;
    > + r = strchr(ptr, '\n');
    > + *r = '\0';
    > + strcpy(path, q);
    > + return;
    > + } else if (ptr == NULL) {
    > + return;
    > + } else {
    > + str = ptr + 1;
    > + continue;
    > + }
    > + }
    > +}

    [SNIP]
    > + /*
    > + * Take a line from the o/p of ldconfig and
    > + * parse it to find a library path
    > + */
    > + while (getline(&line, &len, fp) != -1) {
    > + memset(path, '\0', PATH_MAX);
    > + parse_lib_name(line, path);
    > + if (strcmp(path, ""))

    I think it'd better changing the parse_lib_name to return a value
    instead of checking whether the path is an empty string.


    > + append_path(path, &lib_list.list);
    > + }
    > + /* line and fp no more required */
    > + free(line);
    > + pclose(fp);
    > +
    > + /* Find and write the SDT markers in the perf-sdt cache*/
    > + flush_sdt_list(&lib_list.list, cache);
    > +
    > + cleanup_path_list(&lib_list.list);
    > + return;
    > +}

    [SNIP]
    > +static int populate_sdt_note(Elf **elf, const char *data, size_t len, int type,
    > + struct sdt_note **note)
    > +{
    > + const char *provider, *name;
    > + struct sdt_note *tmp = NULL;
    > + GElf_Ehdr ehdr;
    > + GElf_Addr base_off = 0;
    > + GElf_Shdr shdr;
    > + int ret = -1;
    > + int i;
    > +
    > + union {
    > + Elf64_Addr a64[3];
    > + Elf32_Addr a32[3];
    > + } buf;
    > +
    > + Elf_Data dst = {
    > + .d_buf = &buf, .d_type = ELF_T_ADDR, .d_version = EV_CURRENT,
    > + .d_size = gelf_fsize((*elf), ELF_T_ADDR, 3, EV_CURRENT),
    > + .d_off = 0, .d_align = 0
    > + };
    > + Elf_Data src = {
    > + .d_buf = (void *) data, .d_type = ELF_T_ADDR,
    > + .d_version = EV_CURRENT, .d_size = dst.d_size, .d_off = 0,
    > + .d_align = 0
    > + };
    > +
    > + /* Check the type of each of the notes */
    > + if (type != SDT_NOTE_TYPE)
    > + goto out_err;
    > +
    > + tmp = (struct sdt_note *)calloc(1, sizeof(struct sdt_note));
    > + if (!tmp) {
    > + ret = -ENOMEM;
    > + goto out_err;
    > + }
    > +
    > + INIT_LIST_HEAD(&tmp->note_list);
    > +
    > + if (len < dst.d_size + 3)
    > + goto out_free_note;
    > +
    > + /* Translation from file representation to memory representation */
    > + if (gelf_xlatetom(*elf, &dst, &src,
    > + elf_getident(*elf, NULL)[EI_DATA]) == NULL)

    Do we really need this xlate function? It seems elf_getdata() already
    did necessary conversions so only thing we need to do is checking its
    class and read out the addresses in a proper length, no?


    > + printf("gelf_xlatetom : %s\n", elf_errmsg(-1));
    > +
    > + /* Populate the fields of sdt_note */
    > + provider = data + dst.d_size;
    > +
    > + name = (const char *)memchr(provider, '\0', data + len - provider);
    > + if (name++ == NULL)
    > + goto out_free_note;
    > +
    > + tmp->provider = strdup(provider);
    > + if (!tmp->provider) {
    > + ret = -ENOMEM;
    > + goto out_free_note;
    > + }
    > + tmp->name = strdup(name);
    > + if (!tmp->name) {
    > + ret = -ENOMEM;
    > + goto out_free_prov;
    > + }
    > +
    > + /* Obtain the addresses */
    > + if (gelf_getclass(*elf) == ELFCLASS32) {
    > + for (i = 0; i < 3; i++)
    > + tmp->addr.a32[i] = buf.a32[i];
    > + tmp->bit32 = true;
    > + } else {
    > + for (i = 0; i < 3; i++)
    > + tmp->addr.a64[i] = buf.a64[i];
    > + tmp->bit32 = false;
    > + }
    > +
    > + /* Now Adjust the prelink effect */
    > + if (!gelf_getehdr(*elf, &ehdr)) {
    > + pr_debug("%s : cannot get elf header.\n", __func__);
    > + ret = -EBADF;
    > + goto out_free_name;
    > + }
    > +
    > + /*
    > + * Find out the .stapsdt.base section.
    > + * This scn will help us to handle prelinking (if present).
    > + * Compare the retrieved file offset of the base section with the
    > + * base address in the description of the SDT note. If its different,
    > + * then accordingly, adjust the note location.
    > + */
    > + if (elf_section_by_name(*elf, &ehdr, &shdr, SDT_BASE_SCN, NULL))
    > + base_off = shdr.sh_offset;
    > + if (base_off) {

    Why not moving this block under the above if block?


    > + if (tmp->bit32)
    > + tmp->addr.a32[0] = tmp->addr.a32[0] + base_off -
    > + tmp->addr.a32[1];
    > + else
    > + tmp->addr.a64[0] = tmp->addr.a64[0] + base_off -
    > + tmp->addr.a64[1];
    > + }
    > +
    > + *note = tmp;
    > + return 0;
    > +
    > +out_free_name:
    > + free(tmp->name);
    > +out_free_prov:
    > + free(tmp->provider);
    > +out_free_note:
    > + free(tmp);
    > +out_err:
    > + return ret;
    > +}
    > +
    > +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;
    > + struct sdt_note *tmp = NULL;
    > + int ret = 0, val = 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)) {
    > + 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))) {
    > + 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;
    > + }
    > + }
    > + }
    > + if (!sdt_notes)

    Did you mean list_empty(sdt_notes) ? :)

    Thanks,
    Namhyung


    > + Ret = -ENOENT;
    > +
    > +out_ret:
    > + return ret;
    > +}


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