perf resolves symbols to wrong offsets when debug info is separated from the lib/executable. This patch is based on Dave Martin's initial work first published at http://lists.linaro.org/pipermail/linaro-dev/2010-August/000421.html This patch loads the ELF section headers from a separate file if necessary, to avoid getting confused by the different section file offsets seen in debug images. Invalid section headers are detected by checking for the presence of non-writable SHT_NOBITS sections, which don't make sense under normal circumstances. In particular, this allows symbols in ET_EXEC images to get fixed up correctly in the presence of separated debug images. Signed-off-by: Prashanth Nageshappa <prashanth@xxxxxxxxxxxxxxxxxx> Cc: Dave Martin <dave.martin@xxxxxxxxxx> --- tools/perf/util/symbol.c | 163 +++++++++++++++++++++++++++++++++++++++++++++- 1 files changed, 159 insertions(+), 4 deletions(-) diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c index caaf75a..d32db9a 100644 --- a/tools/perf/util/symbol.c +++ b/tools/perf/util/symbol.c @@ -1,3 +1,4 @@ +#include <assert.h> #include <dirent.h> #include <errno.h> #include <stdlib.h> @@ -1158,8 +1159,107 @@ static size_t elf_addr_to_index(Elf *elf, GElf_Addr addr) return -1; } +/** + * Read all section headers, copying them into a separate array so they survive + * elf_end. + * + * @elf: the libelf instance to operate on. + * @ehdr: the elf header: this must already have been read with gelf_getehdr(). + * @count: the number of headers read is assigned to *count on successful + * return. count must not be NULL. + * + * Returns a pointer to the allocated headers, which should be deallocated with + * free() when no longer needed. + */ +static GElf_Shdr *elf_get_all_shdrs(Elf *elf, GElf_Ehdr const *ehdr, + unsigned *count) +{ + GElf_Shdr *shdrs; + Elf_Scn *scn; + unsigned max_index = 0; + unsigned i; + + shdrs = malloc(ehdr->e_shnum * sizeof *shdrs); + if (!shdrs) + return NULL; + + for (i = 0; i < ehdr->e_shnum; i++) + shdrs[i].sh_type = SHT_NULL; + + 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); + + 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 *dso, 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 = dso->kernel ? map__kmap(map) : NULL; @@ -1178,6 +1278,9 @@ static int dso__load_sym(struct dso *dso, 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); @@ -1200,6 +1303,36 @@ static int dso__load_sym(struct dso *dso, 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) @@ -1344,12 +1477,25 @@ static int dso__load_sym(struct dso *dso, 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: %#" PRIx64 " " "sh_addr: %#" PRIx64 " sh_offset: %#" PRIx64 "\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 @@ -1590,6 +1736,8 @@ int dso__load(struct dso *dso, 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(dso, map->type); @@ -1692,6 +1840,7 @@ restart: continue; ret = dso__load_sym(dso, map, name, fd, filter, 0, + &saved_shdrs, &saved_shdrs_count, want_symtab); close(fd); @@ -1713,14 +1862,19 @@ restart: /* * 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, + * provided we saw some valid section headers: */ - if (ret <= 0 && want_symtab) { + if (ret <= 0 && want_symtab && saved_shdrs != NULL) { want_symtab = 0; goto restart; } free(name); + + if (saved_shdrs) + free(saved_shdrs); + if (ret < 0 && strstr(dso->name, " (deleted)") != NULL) return 0; return ret; @@ -1989,7 +2143,8 @@ int dso__load_vmlinux(struct dso *dso, struct map *map, dso__set_long_name(dso, (char *)vmlinux); dso__set_loaded(dso, map->type); - err = dso__load_sym(dso, map, symfs_vmlinux, fd, filter, 0, 0); + err = dso__load_sym(dso, map, symfs_vmlinux, fd, filter, 0, + NULL, NULL, 0); close(fd); if (err > 0) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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