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SubjectRe: [PATCH v4] perf symbols: fix symbol offset breakage with separated debug
From
Oops... it looks like I accidentally exported this patch with no
signed-of-by line.

Will repost shortly.

Apologies
---Dave
On Mon, Dec 6, 2010 at 5:25 PM, Dave Martin <dave.martin@linaro.org> wrote:
> v4: remove assert() calls
>
>  * Simply removed the assert() to validate section indices
>    returned by libelf: libelf should validate the ELF file
>    anyway.
>
>  * Replaced the assert() on arguments to dso__load_sym
>    with a run-time check.
>
> v3: Fixed a minor error in v2 where a check was erronueously
> done twice.
>
> Original description follows:
>
> The new 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.
>
> The image search loop is also tidied up to fix a minor bug
> which would perform the same image load attempt more than
> once in some situations.
>
> For non-separated images, the headers should get loaded from
> the same image as the symbols, in the usual way.
> ---
> KernelVersion: 2.6.37-rc3
>
>  tools/perf/util/symbol.c |  188 +++++++++++++++++++++++++++++++++++++++++++---
>  1 files changed, 176 insertions(+), 12 deletions(-)
>
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index b39f499..7d8fd34 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -992,8 +992,105 @@ 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);
> +
> +               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 +1109,17 @@ 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 && saved_shdrs_count == NULL) {
> +               /*
> +                * If you trigger this check, you're calling this function
> +                * incorrectly.  Refer to the notes above for details.
> +                */
> +               pr_debug("%s: warning: saved_shdrs_count == NULL: "
> +                        "ignoring the saved section headers.\n",
> +                        __func__);
> +               saved_shdrs = 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 +1143,34 @@ 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 +1303,25 @@ 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 +1558,8 @@ 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 +1590,13 @@ 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 +1610,7 @@ 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 +1634,24 @@ 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 +1660,7 @@ 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 +1668,18 @@ 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 +1945,8 @@ 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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