Date:	Fri, 1 Jul 2016 15:56:49 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Masami Hiramatsu <mhiramat@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Hemant Kumar <hemant@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
	Brendan Gregg <brendan.d.gregg@...il.com>
Subject: Re: [PATCH perf/core v13 04/15] perf/sdt: ELF support for SDT

Em Fri, Jul 01, 2016 at 05:03:46PM +0900, Masami Hiramatsu escreveu:
> From: Hemant Kumar <hemant@...ux.vnet.ibm.com>
> 
> 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.

Breaks the build on older systems:


[root@...et perf]# dm
centos:5: FAIL
  CC       /tmp/build/perf/util/symbol-elf.o
cc1: warnings being treated as errors
util/symbol-elf.c: In function 'construct_sdt_notes_list':
util/symbol-elf.c:1927: warning: implicit declaration of function 'elf_getshdrstrndx'
util/symbol-elf.c:1927: warning: nested extern declaration of 'elf_getshdrstrndx'
mv: cannot stat `/tmp/build/perf/util/.symbol-elf.o.tmp': No such file or directory
make[3]: *** [/tmp/build/perf/util/symbol-elf.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [util] Error 2
make[1]: *** [/tmp/build/perf/libperf-in.o] Error 2
make: *** [install-bin] Error 2
make: Leaving directory `/git/linux/tools/perf'
centos:6: Ok
centos:7: Ok
debian:experimental: Ok
debian:7: Ok
debian:8: Ok
fedora:21: Ok
fedora:22: Ok
fedora:23: Ok
fedora:24: Ok
opensuse:13.2: Ok
opensuse:42.1: Ok
ubuntu:14.04.4: Ok
ubuntu:15.10: Ok
ubuntu:16.04: Ok
ubuntu:12.04.5: Ok

---------

Needs a feature detection test, I will try to contribute that.

- Arnaldo

 
> A typical note entry in ".note.stapsdt" section is as follows :
> 
> 
>                                  |--nhdr.n_namesz--|
>                 ------------------------------------
>                 |      nhdr      |     "stapsdt"   |
>         -----   |----------------------------------|
>          |      |  <location>       <base_address> |
>          |      |  <semaphore>                     |
> nhdr.n_descsize |  "provider_name"   "note_name"   |
>          |      |   <args>                         |
>         -----   |----------------------------------|
>                 |      nhdr      |     "stapsdt"   |
>                 |...
> 
> The above shows an excerpt from the section ".note.stapsdt".
> 'nhdr' is a structure which has the note name size (n_namesz), note
> description size (n_desc_sz) and note type (n_type). So, in order to
> parse the note note info, we need nhdr to tell us where to start from.
> As can be seen from <sys/sdt.h>, the name of the SDT notes given is "stapsdt".
> But this is not the identifier of the note.
> After that, we go to description of the note to find out its location, the
> address of the ".stapsdt.base" section and the semaphore address.
> Then, we find the provider name and the SDT marker name and then follow the
> arguments.
> 
> Signed-off-by: Hemant Kumar <hemant@...ux.vnet.ibm.com>
> Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org>
> Acked-by: Namhyung Kim <namhyung@...nel.org>
> ---
>  tools/perf/util/symbol-elf.c |  252 ++++++++++++++++++++++++++++++++++++++++++
>  tools/perf/util/symbol.h     |   22 ++++
>  2 files changed, 274 insertions(+)
> 
> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> index 87a297d..e74ce17 100644
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -1781,6 +1781,258 @@ void kcore_extract__delete(struct kcore_extract *kce)
>  	unlink(kce->extract_filename);
>  }
>  
> +/**
> + * populate_sdt_note : Parse raw data and identify SDT note
> + * @elf: elf of the opened file
> + * @data: raw data of a section with description offset applied
> + * @len: note description size
> + * @type: type of the note
> + * @sdt_notes: List to add the SDT note
> + *
> + * Responsible for parsing the @data in section .note.stapsdt in @elf and
> + * if its an SDT note, it appends to @sdt_notes list.
> + */
> +static int populate_sdt_note(Elf **elf, const char *data, size_t len,
> +			     struct list_head *sdt_notes)
> +{
> +	const char *provider, *name;
> +	struct sdt_note *tmp = NULL;
> +	GElf_Ehdr ehdr;
> +	GElf_Addr base_off = 0;
> +	GElf_Shdr shdr;
> +	int ret = -EINVAL;
> +
> +	union {
> +		Elf64_Addr a64[NR_ADDR];
> +		Elf32_Addr a32[NR_ADDR];
> +	} buf;
> +
> +	Elf_Data dst = {
> +		.d_buf = &buf, .d_type = ELF_T_ADDR, .d_version = EV_CURRENT,
> +		.d_size = gelf_fsize((*elf), ELF_T_ADDR, NR_ADDR, 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
> +	};
> +
> +	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) {
> +		pr_err("gelf_xlatetom : %s\n", elf_errmsg(-1));
> +		goto out_free_note;
> +	}
> +
> +	/* 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;
> +	}
> +
> +	if (gelf_getclass(*elf) == ELFCLASS32) {
> +		memcpy(&tmp->addr, &buf, 3 * sizeof(Elf32_Addr));
> +		tmp->bit32 = true;
> +	} else {
> +		memcpy(&tmp->addr, &buf, 3 * sizeof(Elf64_Addr));
> +		tmp->bit32 = false;
> +	}
> +
> +	if (!gelf_getehdr(*elf, &ehdr)) {
> +		pr_debug("%s : cannot get elf header.\n", __func__);
> +		ret = -EBADF;
> +		goto out_free_name;
> +	}
> +
> +	/* Adjust the prelink effect :
> +	 * 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) {
> +			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];
> +		}
> +	}
> +
> +	list_add_tail(&tmp->note_list, sdt_notes);
> +	return 0;
> +
> +out_free_name:
> +	free(tmp->name);
> +out_free_prov:
> +	free(tmp->provider);
> +out_free_note:
> +	free(tmp);
> +out_err:
> +	return ret;
> +}
> +
> +/**
> + * construct_sdt_notes_list : constructs a list of SDT notes
> + * @elf : elf to look into
> + * @sdt_notes : empty list_head
> + *
> + * Scans the sections in 'elf' for the section
> + * .note.stapsdt. It, then calls populate_sdt_note to find
> + * out the SDT events and populates the 'sdt_notes'.
> + */
> +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)) {
> +		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))) {
> +			/* Check the type of the note */
> +			if (nhdr.n_type != SDT_NOTE_TYPE)
> +				goto out_ret;
> +
> +			ret = populate_sdt_note(&elf, ((data->d_buf) + desc_off),
> +						nhdr.n_descsz, sdt_notes);
> +			if (ret < 0)
> +				goto out_ret;
> +		}
> +	}
> +	if (list_empty(sdt_notes))
> +		ret = -ENOENT;
> +
> +out_ret:
> +	return ret;
> +}
> +
> +/**
> + * get_sdt_note_list : Wrapper to construct a list of sdt notes
> + * @head : empty list_head
> + * @target : file to find SDT notes from
> + *
> + * 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, PERF_ELF_C_READ_MMAP, NULL);
> +	if (!elf) {
> +		ret = -EBADF;
> +		goto out_close;
> +	}
> +	ret = construct_sdt_notes_list(elf, head);
> +	elf_end(elf);
> +out_close:
> +	close(fd);
> +	return ret;
> +}
> +
> +/**
> + * cleanup_sdt_note_list : free the sdt notes' list
> + * @sdt_notes: sdt notes' list
> + *
> + * Free up the SDT notes in @sdt_notes.
> + * Returns the number of SDT notes free'd.
> + */
> +int cleanup_sdt_note_list(struct list_head *sdt_notes)
> +{
> +	struct sdt_note *tmp, *pos;
> +	int nr_free = 0;
> +
> +	list_for_each_entry_safe(pos, tmp, sdt_notes, note_list) {
> +		list_del(&pos->note_list);
> +		free(pos->name);
> +		free(pos->provider);
> +		free(pos);
> +		nr_free++;
> +	}
> +	return nr_free;
> +}
> +
> +/**
> + * sdt_notes__get_count: Counts the number of sdt events
> + * @start: list_head to sdt_notes list
> + *
> + * Returns the number of SDT notes in a list
> + */
> +int sdt_notes__get_count(struct list_head *start)
> +{
> +	struct sdt_note *sdt_ptr;
> +	int count = 0;
> +
> +	list_for_each_entry(sdt_ptr, start, note_list)
> +		count++;
> +	return count;
> +}
> +
>  void symbol__elf_init(void)
>  {
>  	elf_version(EV_CURRENT);
> diff --git a/tools/perf/util/symbol.h b/tools/perf/util/symbol.h
> index b10d558..699f7cb 100644
> --- a/tools/perf/util/symbol.h
> +++ b/tools/perf/util/symbol.h
> @@ -342,4 +342,26 @@ void arch__sym_update(struct symbol *s, GElf_Sym *sym);
>  
>  int arch__choose_best_symbol(struct symbol *syma, struct symbol *symb);
>  
> +/* structure containing an SDT note's info */
> +struct sdt_note {
> +	char *name;			/* name of the note*/
> +	char *provider;			/* provider name */
> +	bool bit32;			/* whether the location is 32 bits? */
> +	union {				/* location, base and semaphore addrs */
> +		Elf64_Addr a64[3];
> +		Elf32_Addr a32[3];
> +	} addr;
> +	struct list_head note_list;	/* SDT notes' list */
> +};
> +
> +int get_sdt_note_list(struct list_head *head, const char *target);
> +int cleanup_sdt_note_list(struct list_head *sdt_notes);
> +int sdt_notes__get_count(struct list_head *start);
> +
> +#define SDT_BASE_SCN ".stapsdt.base"
> +#define SDT_NOTE_SCN  ".note.stapsdt"
> +#define SDT_NOTE_TYPE 3
> +#define SDT_NOTE_NAME "stapsdt"
> +#define NR_ADDR 3
> +
>  #endif /* __PERF_SYMBOL */

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