For very large ELF objects (with many sections), we could
get special value SHN_XINDEX (65535) for symbol's st_shndx.
This patch is adding code to detect the optional extended
section index table and use it to resolve symbol's section
index.
Adding elf_symtab__for_each_symbol_index macro that returns
symbol's section index and usign it in collect functions.
Tested by running pahole on kernel compiled with:
make KCFLAGS="-ffunction-sections -fdata-sections" -j$(nproc) vmlinux
and ensure FUNC records are generated and match normal
build (without above KCFLAGS).
Also bpf selftest passed and generated kernel BTF,
is same as without the patch.
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
btf_encoder.c | 33 +++++++++++++++++----------------
elf_symtab.c | 41 +++++++++++++++++++++++++++++++++++++++--
elf_symtab.h | 29 +++++++++++++++++++++++++++++
3 files changed, 85 insertions(+), 18 deletions(-)
diff --git a/btf_encoder.c b/btf_encoder.c
index 5557c9efd365..b124ec20a689 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -63,13 +63,13 @@ static void delete_functions(void)
#define max(x, y) ((x) < (y) ? (y) : (x))
#endif
-static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
+static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
+ size_t sym_sec_idx)
{
struct elf_function *new;
static GElf_Shdr sh;
- static int last_idx;
+ static size_t last_idx;
const char *name;
- int idx;
if (elf_sym__type(sym) != STT_FUNC)
return 0;
@@ -90,12 +90,10 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
functions = new;
}
- idx = elf_sym__section(sym);
-
- if (idx != last_idx) {
- if (!elf_section_by_idx(btfe->elf, &sh, idx))
+ if (sym_sec_idx != last_idx) {
+ if (!elf_section_by_idx(btfe->elf, &sh, sym_sec_idx))
return 0;
- last_idx = idx;
+ last_idx = sym_sec_idx;
}
functions[functions_cnt].name = name;
@@ -542,14 +540,15 @@ static bool percpu_var_exists(uint64_t addr, uint32_t *sz, const char **name)
return true;
}
-static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
+static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym,
+ size_t sym_sec_idx)
{
const char *sym_name;
uint64_t addr;
uint32_t size;
/* compare a symbol's shndx to determine if it's a percpu variable */
- if (elf_sym__section(sym) != btfe->percpu_shndx)
+ if (sym_sec_idx != btfe->percpu_shndx)
return 0;
if (elf_sym__type(sym) != STT_OBJECT)
return 0;
@@ -585,12 +584,13 @@ static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
return 0;
}
-static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl)
+static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl,
+ size_t sym_sec_idx)
{
if (!fl->mcount_start &&
!strcmp("__start_mcount_loc", elf_sym__name(sym, btfe->symtab))) {
fl->mcount_start = sym->st_value;
- fl->mcount_sec_idx = sym->st_shndx;
+ fl->mcount_sec_idx = sym_sec_idx;
}
if (!fl->mcount_stop &&
@@ -601,6 +601,7 @@ static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl)
static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
{
struct funcs_layout fl = { };
+ Elf32_Word sym_sec_idx;
uint32_t core_id;
GElf_Sym sym;
@@ -608,12 +609,12 @@ static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
percpu_var_cnt = 0;
/* search within symtab for percpu variables */
- elf_symtab__for_each_symbol(btfe->symtab, core_id, sym) {
- if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
+ elf_symtab__for_each_symbol_index(btfe->symtab, core_id, sym, sym_sec_idx) {
+ if (collect_percpu_vars && collect_percpu_var(btfe, &sym, sym_sec_idx))
return -1;
- if (collect_function(btfe, &sym))
+ if (collect_function(btfe, &sym, sym_sec_idx))
return -1;
- collect_symbol(&sym, &fl);
+ collect_symbol(&sym, &fl, sym_sec_idx);
}
if (collect_percpu_vars) {
diff --git a/elf_symtab.c b/elf_symtab.c
index 741990ea3ed9..77c5dc423c56 100644
--- a/elf_symtab.c
+++ b/elf_symtab.c
@@ -17,11 +17,13 @@
struct elf_symtab *elf_symtab__new(const char *name, Elf *elf, GElf_Ehdr *ehdr)
{
+ size_t symtab_index;
+
if (name == NULL)
name = ".symtab";
GElf_Shdr shdr;
- Elf_Scn *sec = elf_section_by_name(elf, ehdr, &shdr, name, NULL);
+ Elf_Scn *sec = elf_section_by_name(elf, ehdr, &shdr, name, &symtab_index);
if (sec == NULL)
return NULL;
@@ -29,7 +31,7 @@ struct elf_symtab *elf_symtab__new(const char *name, Elf *elf, GElf_Ehdr *ehdr)
if (gelf_getshdr(sec, &shdr) == NULL)
return NULL;
- struct elf_symtab *symtab = malloc(sizeof(*symtab));
+ struct elf_symtab *symtab = zalloc(sizeof(*symtab));
if (symtab == NULL)
return NULL;
@@ -41,6 +43,12 @@ struct elf_symtab *elf_symtab__new(const char *name, Elf *elf, GElf_Ehdr *ehdr)
if (symtab->syms == NULL)
goto out_free_name;
+ /*
+ * This returns extended section index table's
+ * section index, if it exists.
+ */
+ int symtab_xindex = elf_scnshndx(sec);
+
sec = elf_getscn(elf, shdr.sh_link);
if (sec == NULL)
goto out_free_name;
@@ -49,6 +57,35 @@ struct elf_symtab *elf_symtab__new(const char *name, Elf *elf, GElf_Ehdr *ehdr)
if (symtab->symstrs == NULL)
goto out_free_name;
+ /*
+ * The .symtab section has optional extended section index
+ * table, load its data so it can be used to resolve symbol's
+ * section index.
+ **/
+ if (symtab_xindex > 0) {
+ GElf_Shdr shdr_xindex;
+ Elf_Scn *sec_xindex;
+
+ sec_xindex = elf_getscn(elf, symtab_xindex);
+ if (sec_xindex == NULL)
+ goto out_free_name;
+
+ if (gelf_getshdr(sec_xindex, &shdr_xindex) == NULL)
+ goto out_free_name;
+
+ /* Extra check to verify it's correct type */
+ if (shdr_xindex.sh_type != SHT_SYMTAB_SHNDX)
+ goto out_free_name;
+
+ /* Extra check to verify it belongs to the .symtab */
+ if (symtab_index != shdr_xindex.sh_link)
+ goto out_free_name;
+
+ symtab->syms_sec_idx_table = elf_getdata(elf_getscn(elf, symtab_xindex), NULL);
+ if (symtab->syms_sec_idx_table == NULL)
+ goto out_free_name;
+ }
+
symtab->nr_syms = shdr.sh_size / shdr.sh_entsize;
return symtab;
diff --git a/elf_symtab.h b/elf_symtab.h
index 359add69c8ab..489e2b1a3505 100644
--- a/elf_symtab.h
+++ b/elf_symtab.h
@@ -16,6 +16,8 @@ struct elf_symtab {
uint32_t nr_syms;
Elf_Data *syms;
Elf_Data *symstrs;
+ /* Data of SHT_SYMTAB_SHNDX section. */
+ Elf_Data *syms_sec_idx_table;
char *name;
};
@@ -77,6 +79,19 @@ static inline bool elf_sym__is_local_object(const GElf_Sym *sym)
sym->st_shndx != SHN_UNDEF;
}
+static inline bool
+elf_sym__get(Elf_Data *syms, Elf_Data *syms_sec_idx_table,
+ int id, GElf_Sym *sym, Elf32_Word *sym_sec_idx)
+{
+ if (!gelf_getsymshndx(syms, syms_sec_idx_table, id, sym, sym_sec_idx))
+ return false;
+
+ if (sym->st_shndx != SHN_XINDEX)
+ *sym_sec_idx = sym->st_shndx;
+
+ return true;
+}
+
/**
* elf_symtab__for_each_symbol - iterate thru all the symbols
*
@@ -89,4 +104,18 @@ static inline bool elf_sym__is_local_object(const GElf_Sym *sym)
index < symtab->nr_syms; \
index++, gelf_getsym(symtab->syms, index, &sym))
+/**
+ * elf_symtab__for_each_symbol_index - iterate through all the symbols,
+ * that takes extended symbols indexes into account
+ *
+ * @symtab: struct elf_symtab instance to iterate
+ * @index: uint32_t index
+ * @sym: GElf_Sym iterator
+ * @sym_sec_idx: symbol's index
+ */
+#define elf_symtab__for_each_symbol_index(symtab, id, sym, sym_sec_idx) \
+ for (id = 0; id < symtab->nr_syms; id++) \
+ if (elf_sym__get(symtab->syms, symtab->syms_sec_idx_table, \
+ id, &sym, &sym_sec_idx))
+
#endif /* _ELF_SYMTAB_H_ */
--
2.26.2
