Subject: [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values

From: Jiri Olsa <[email-masked]>
Date: Thu, 21 Jan 2021 21:22:02 +0100

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_symbols function.

Signed-off-by: Jiri Olsa <[email-masked]>
---
 btf_encoder.c | 36 ++++++++++++++++++++++++++++++++----
 elf_symtab.c  | 39 ++++++++++++++++++++++++++++++++++++++-
 elf_symtab.h  |  2 ++
 3 files changed, 72 insertions(+), 5 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 5557c9efd365..6e6f22c438ce 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -585,12 +585,13 @@ 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,
+			   Elf32_Word 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 &&
@@ -598,9 +599,36 @@ static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl)
 		fl->mcount_stop = sym->st_value;
 }
 
+static 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_getsym(syms, id, sym))
+		return false;
+
+	*sym_sec_idx = sym->st_shndx;
+
+	if (sym->st_shndx == SHN_XINDEX) {
+		if (!syms_sec_idx_table)
+			return false;
+		if (!gelf_getsymshndx(syms, syms_sec_idx_table,
+				      id, sym, sym_sec_idx))
+			return false;
+	}
+
+	return true;
+}
+
+#define elf_symtab__for_each_symbol_index(symtab, id, sym, sym_sec_idx)		\
+	for (id = 0, elf_sym__get(symtab->syms, symtab->syms_sec_idx_table,	\
+				  id, &sym, &sym_sec_idx);			\
+	     id < symtab->nr_syms;						\
+	     id++, elf_sym__get(symtab->syms, symtab->syms_sec_idx_table,	\
+				id, &sym, &sym_sec_idx))
+
 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 +636,12 @@ 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) {
+	elf_symtab__for_each_symbol_index(btfe->symtab, core_id, sym, sym_sec_idx) {
 		if (collect_percpu_vars && collect_percpu_var(btfe, &sym))
 			return -1;
 		if (collect_function(btfe, &sym))
 			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..fad5e0c0ba3c 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;
@@ -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..2e05ca98158b 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;
 };
 
-- 
2.27.0