Date:	Fri, 19 Nov 2010 16:43:50 -0200
From:	Arnaldo Carvalho de Melo <acme@...radead.org>
To:	Dave Martin <dave.martin@...aro.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH v3] perf symbols: fix symbol offset breakage with
	separated debug

Em Tue, Nov 16, 2010 at 01:35:56PM +0000, Dave Martin escreveu:
> +	for (scn = NULL; (scn = elf_nextscn(elf, scn)); ) {
> +		size_t j;
> +
> +		/*
> +		 * Just assuming we get section 0, 1, ... in sequence may lead
> +		 * to wrong section indices.  Check the index explicitly:
> +		 */
> +		j = elf_ndxscn(scn);
> +		assert(j < ehdr->e_shnum);


please repost after removing the assert calls, this is a library, we
better return NULL here, i.e. goto error, than stopping the whole
session that otherwiser could continue with the other symtabs producing
useful output.

Use pr_err if you think its warranted.

Thanks,

- Arnaldo

> +		if (j > max_index)
> +			max_index = j;
> +
> +		if (!gelf_getshdr(scn, &shdrs[j]))
> +			goto error;
> +	}
> +
> +	*count = max_index + 1;
> +	return shdrs;
> +
> +error:
> +	free(shdrs);
> +	return NULL;
> +}
> +
> +/**
> + * Check that the section headers @shdrs reflect accurately the file data
> + * layout of the image that was loaded during perf record.  This is generally
> + * not true for separated debug images generated with e.g.,
> + * objcopy --only-keep-debug.
> + *
> + * We identify invalid files by checking for non-empty sections which are
> + * declared as having no file data (SHT_NOBITS) but are not writable.
> + *
> + * @shdrs: the full set of section headers, as loaded by elf_get_all_shdrs().
> + * @count: the number of headers present in @shdrs.
> + *
> + * Returns 1 for valid headers, 0 otherwise.
> + */
> +static int elf_check_shdrs_valid(GElf_Shdr const *shdrs, unsigned count)
> +{
> +	unsigned i;
> +
> +	for (i = 0; i < count; i++) {
> +		if (shdrs[i].sh_type == SHT_NOBITS &&
> +		    !(shdrs[i].sh_flags & SHF_WRITE) &&
> +		    shdrs[i].sh_size != 0)
> +			return 0;
> +	}
> +
> +	return 1;
> +}
> +
> +/*
> + * Notes:
> + *
> + * If saved_shdrs is non-NULL, the section headers will be read if found, and
> + * will be used for address fixups.  saved_shdrs_count must also be non-NULL in
> + * this case.  This may be needed for separated debug images, since the section
> + * headers and symbols may need to come from separate images in that case.
> + *
> + * Note: irrespective of whether this function returns successfully,
> + * *saved_shdrs may get initialised if saved_shdrs is non-NULL.  It is the
> + * caller's responsibility to free() it when non longer needed.
> + *
> + * If want_symtab == 1, this function will only load symbols from .symtab
> + * sections.  Otherwise (want_symtab == 0), .dynsym or .symtab symbols are
> + * loaded.  This feature is used by dso__load() to search for the best image
> + * to load.
> + */
>  static int dso__load_sym(struct dso *self, struct map *map, const char *name,
>  			 int fd, symbol_filter_t filter, int kmodule,
> +			 GElf_Shdr **saved_shdrs, unsigned *saved_shdrs_count,
>  			 int want_symtab)
>  {
>  	struct kmap *kmap = self->kernel ? map__kmap(map) : NULL;
> @@ -1012,6 +1111,9 @@ static int dso__load_sym(struct dso *self, struct map *map, const char *name,
>  	int nr = 0;
>  	size_t opdidx = 0;
>  
> +	if (saved_shdrs != NULL)
> +		assert(saved_shdrs_count != NULL);
> +
>  	elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
>  	if (elf == NULL) {
>  		pr_debug("%s: cannot read %s ELF file.\n", __func__, name);
> @@ -1035,6 +1137,34 @@ static int dso__load_sym(struct dso *self, struct map *map, const char *name,
>  			goto out_elf_end;
>  	}
>  
> +	/*
> +	 * Copy all section headers from the image if requested and if not
> +	 * already loaded.
> +	 */
> +	if (saved_shdrs != NULL && *saved_shdrs == NULL) {
> +		GElf_Shdr *shdrs;
> +		unsigned count;
> +
> +		shdrs = elf_get_all_shdrs(elf, &ehdr, &count);
> +		if (shdrs == NULL)
> +			goto out_elf_end;
> +
> +		/*
> +		 * Only keep the headers if they reflect the actual run-time
> +		 * image's file layout:
> +		 */
> +		if (elf_check_shdrs_valid(shdrs, count)) {
> +			*saved_shdrs = shdrs;
> +			*saved_shdrs_count = count;
> +		} else
> +			free(shdrs);
> +	}
> +
> +	/* If no genuine ELF headers are available yet, give up: we can't
> +	 * adjust symbols correctly in that case: */
> +	if (saved_shdrs != NULL && *saved_shdrs == NULL)
> +		goto out_elf_end;
> +
>  	sec = elf_section_by_name(elf, &ehdr, &shdr, ".symtab", NULL);
>  	if (sec == NULL) {
>  		if (want_symtab)
> @@ -1167,12 +1297,25 @@ static int dso__load_sym(struct dso *self, struct map *map, const char *name,
>  			goto new_symbol;
>  		}
>  
> +		/*
> +		 * Currently, symbols for shared objects and PIE executables
> +		 * (i.e., ET_DYN) do not seem to get adjusted.  This might need
> +		 * to change if file offset == virtual address is not actually
> +		 * guaranteed for these images.  ELF doesn't provide this
> +		 * guarantee natively.
> +		 */
>  		if (curr_dso->adjust_symbols) {
>  			pr_debug4("%s: adjusting symbol: st_value: %#Lx "
>  				  "sh_addr: %#Lx sh_offset: %#Lx\n", __func__,
>  				  (u64)sym.st_value, (u64)shdr.sh_addr,
>  				  (u64)shdr.sh_offset);
> -			sym.st_value -= shdr.sh_addr - shdr.sh_offset;
> +			if (saved_shdrs && *saved_shdrs &&
> +			    sym.st_shndx < *saved_shdrs_count)
> +				sym.st_value -=
> +					(*saved_shdrs)[sym.st_shndx].sh_addr -
> +					(*saved_shdrs)[sym.st_shndx].sh_offset;
> +			else
> +				sym.st_value -= shdr.sh_addr - shdr.sh_offset;
>  		}
>  		/*
>  		 * We need to figure out if the object was created from C++ sources
> @@ -1409,6 +1552,8 @@ int dso__load(struct dso *self, struct map *map, symbol_filter_t filter)
>  	struct machine *machine;
>  	const char *root_dir;
>  	int want_symtab;
> +	GElf_Shdr *saved_shdrs = NULL;
> +	unsigned saved_shdrs_count;
>  
>  	dso__set_loaded(self, map->type);
>  
> @@ -1439,13 +1584,13 @@ int dso__load(struct dso *self, struct map *map, symbol_filter_t filter)
>  	 * On the first pass, only load images if they have a full symtab.
>  	 * Failing that, do a second pass where we accept .dynsym also
>  	 */
> -	for (self->origin = DSO__ORIG_BUILD_ID_CACHE, want_symtab = 1;
> -	     self->origin != DSO__ORIG_NOT_FOUND;
> -	     self->origin++) {
> +	self->origin = DSO__ORIG_BUILD_ID_CACHE;
> +	want_symtab = 1;
> +	while (1) {
>  		switch (self->origin) {
>  		case DSO__ORIG_BUILD_ID_CACHE:
>  			if (dso__build_id_filename(self, name, size) == NULL)
> -				continue;
> +				goto continue_next;
>  			break;
>  		case DSO__ORIG_FEDORA:
>  			snprintf(name, size, "/usr/lib/debug%s.debug",
> @@ -1459,7 +1604,7 @@ int dso__load(struct dso *self, struct map *map, symbol_filter_t filter)
>  			char build_id_hex[BUILD_ID_SIZE * 2 + 1];
>  
>  			if (!self->has_build_id)
> -				continue;
> +				goto continue_next;
>  
>  			build_id__sprintf(self->build_id,
>  					  sizeof(self->build_id),
> @@ -1483,21 +1628,24 @@ int dso__load(struct dso *self, struct map *map, symbol_filter_t filter)
>  		default:
>  			/*
>  			 * If we wanted a full symtab but no image had one,
> -			 * relax our requirements and repeat the search.
> +			 * relax our requirements and repeat the search,
> +			 * providing we saw some valid section headers:
>  			 */
> -			if (want_symtab) {
> +			if (want_symtab && saved_shdrs != NULL) {
>  				want_symtab = 0;
>  				self->origin = DSO__ORIG_BUILD_ID_CACHE;
> -			} else
>  				continue;
> +			} else
> +				goto done;
>  		}
>  
>  		/* Name is now the name of the next image to try */
>  		fd = open(name, O_RDONLY);
>  		if (fd < 0)
> -			continue;
> +			goto continue_next;
>  
>  		ret = dso__load_sym(self, map, name, fd, filter, 0,
> +				    &saved_shdrs, &saved_shdrs_count,
>  				    want_symtab);
>  		close(fd);
>  
> @@ -1506,7 +1654,7 @@ int dso__load(struct dso *self, struct map *map, symbol_filter_t filter)
>  		 * info!?!?
>  		 */
>  		if (!ret)
> -			continue;
> +			goto continue_next;
>  
>  		if (ret > 0) {
>  			int nr_plt = dso__synthesize_plt_symbols(self, map, filter);
> @@ -1514,9 +1662,18 @@ int dso__load(struct dso *self, struct map *map, symbol_filter_t filter)
>  				ret += nr_plt;
>  			break;
>  		}
> +
> +continue_next:
> +		self->origin++;
> +		continue;
>  	}
>  
> +done:
>  	free(name);
> +
> +	if (saved_shdrs)
> +		free(saved_shdrs);
> +
>  	if (ret < 0 && strstr(self->name, " (deleted)") != NULL)
>  		return 0;
>  	return ret;
> @@ -1782,7 +1939,8 @@ static int dso__load_vmlinux(struct dso *self, struct map *map,
>  		return -1;
>  
>  	dso__set_loaded(self, map->type);
> -	err = dso__load_sym(self, map, vmlinux, fd, filter, 0, 0);
> +	err = dso__load_sym(self, map, vmlinux, fd, filter, 0,
> +			    NULL, NULL, 0);
>  	close(fd);
>  
>  	if (err > 0)
> -- 
> 1.7.1
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