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Subject[PATCH v4] perf symbols: fix symbol offset breakage with separated debug
Date
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.

Applies to 2.6.37-rc3

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.
---
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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