Re: [PATCH v4] perf symbols: fix symbol offset breakage withseparated debug
From: Dave Martin
Date: Mon Dec 06 2010 - 12:50:07 EST
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@xxxxxxxxxx> 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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