[RFC][PATCH 4/6] objtool: Add elf_create_undef_symbol()

[Peter Zijlstra]

Allow objtool to create undefined symbols; this allows creating
relocations to symbols not currently in the symbol table.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 tools/objtool/elf.c                 |  180 +++++++++++++++++++++++++++---------
 tools/objtool/include/objtool/elf.h |    1 
 2 files changed, 139 insertions(+), 42 deletions(-)

--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -367,12 +367,60 @@ static int read_sections(struct elf *elf
 	return 0;
 }
 
+static bool elf_symbol_add(struct elf *elf, struct symbol *sym, Elf32_Word shndx)
+{
+	struct list_head *entry;
+	struct rb_node *pnode;
+
+	sym->type = GELF_ST_TYPE(sym->sym.st_info);
+	sym->bind = GELF_ST_BIND(sym->sym.st_info);
+
+	if ((sym->sym.st_shndx > SHN_UNDEF &&
+	     sym->sym.st_shndx < SHN_LORESERVE) ||
+	    (shndx != SHN_XINDEX && sym->sym.st_shndx == SHN_XINDEX)) {
+		if (sym->sym.st_shndx != SHN_XINDEX)
+			shndx = sym->sym.st_shndx;
+
+		sym->sec = find_section_by_index(elf, shndx);
+		if (!sym->sec) {
+			WARN("couldn't find section for symbol %s",
+			     sym->name);
+			return false;
+		}
+		if (sym->type == STT_SECTION) {
+			sym->name = sym->sec->name;
+			sym->sec->sym = sym;
+		}
+	} else
+		sym->sec = find_section_by_index(elf, 0);
+
+	sym->offset = sym->sym.st_value;
+	sym->len = sym->sym.st_size;
+
+	rb_add(&sym->sec->symbol_tree, &sym->node, symbol_to_offset);
+	pnode = rb_prev(&sym->node);
+	if (pnode)
+		entry = &rb_entry(pnode, struct symbol, node)->list;
+	else
+		entry = &sym->sec->symbol_list;
+	list_add(&sym->list, entry);
+	elf_hash_add(elf->symbol_hash, &sym->hash, sym->idx);
+	elf_hash_add(elf->symbol_name_hash, &sym->name_hash, str_hash(sym->name));
+
+	/*
+	 * Don't store empty STT_NOTYPE symbols in the rbtree.  They
+	 * can exist within a function, confusing the sorting.
+	 */
+	if (!sym->len)
+		rb_erase(&sym->node, &sym->sec->symbol_tree);
+
+	return true;
+}
+
 static int read_symbols(struct elf *elf)
 {
 	struct section *symtab, *symtab_shndx, *sec;
 	struct symbol *sym, *pfunc;
-	struct list_head *entry;
-	struct rb_node *pnode;
 	int symbols_nr, i;
 	char *coldstr;
 	Elf_Data *shndx_data = NULL;
@@ -417,47 +465,11 @@ static int read_symbols(struct elf *elf)
 			goto err;
 		}
 
-		sym->type = GELF_ST_TYPE(sym->sym.st_info);
-		sym->bind = GELF_ST_BIND(sym->sym.st_info);
+		if (!shndx_data)
+			shndx = SHN_XINDEX;
 
-		if ((sym->sym.st_shndx > SHN_UNDEF &&
-		     sym->sym.st_shndx < SHN_LORESERVE) ||
-		    (shndx_data && sym->sym.st_shndx == SHN_XINDEX)) {
-			if (sym->sym.st_shndx != SHN_XINDEX)
-				shndx = sym->sym.st_shndx;
-
-			sym->sec = find_section_by_index(elf, shndx);
-			if (!sym->sec) {
-				WARN("couldn't find section for symbol %s",
-				     sym->name);
-				goto err;
-			}
-			if (sym->type == STT_SECTION) {
-				sym->name = sym->sec->name;
-				sym->sec->sym = sym;
-			}
-		} else
-			sym->sec = find_section_by_index(elf, 0);
-
-		sym->offset = sym->sym.st_value;
-		sym->len = sym->sym.st_size;
-
-		rb_add(&sym->sec->symbol_tree, &sym->node, symbol_to_offset);
-		pnode = rb_prev(&sym->node);
-		if (pnode)
-			entry = &rb_entry(pnode, struct symbol, node)->list;
-		else
-			entry = &sym->sec->symbol_list;
-		list_add(&sym->list, entry);
-		elf_hash_add(elf->symbol_hash, &sym->hash, sym->idx);
-		elf_hash_add(elf->symbol_name_hash, &sym->name_hash, str_hash(sym->name));
-
-		/*
-		 * Don't store empty STT_NOTYPE symbols in the rbtree.  They
-		 * can exist within a function, confusing the sorting.
-		 */
-		if (!sym->len)
-			rb_erase(&sym->node, &sym->sec->symbol_tree);
+		if (!elf_symbol_add(elf, sym, shndx))
+			goto err;
 	}
 
 	if (stats)
@@ -691,6 +703,90 @@ struct elf *elf_open_read(const char *na
 	return NULL;
 }
 
+struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
+{
+	struct section *strtab, *symtab;
+	struct symbol *sym;
+	Elf_Scn *s;
+	Elf_Data *data;
+
+	sym = malloc(sizeof(*sym));
+	if (!sym) {
+		perror("malloc");
+		return NULL;
+	}
+	memset(sym, 0, sizeof(*sym));
+
+	sym->name = strdup(name);
+
+	strtab = find_section_by_name(elf, ".strtab");
+	if (!strtab) {
+		WARN("can't find .strtab");
+		return NULL;
+	}
+
+	s = elf_getscn(elf->elf, strtab->idx);
+	if (!s) {
+		WARN_ELF("elf_getscn");
+		return NULL;
+	}
+
+	data = elf_newdata(s);
+	if (!data) {
+		WARN_ELF("elf_newdata");
+		return NULL;
+	}
+
+	data->d_buf = sym->name;
+	data->d_size = strlen(sym->name) + 1;
+	data->d_align = 1;
+
+	sym->sym.st_name = strtab->len;
+	sym->sym.st_info = 0x10; /* STB_GLOBAL, STT_NOTYPE */
+	// st_other 0
+	// st_shndx 0
+	// st_value 0
+	// st_size 0
+
+	strtab->len += data->d_size;
+	strtab->changed = true;
+
+
+	symtab = find_section_by_name(elf, ".symtab");
+	if (!symtab) {
+		WARN("can't find .symtab");
+		return NULL;
+	}
+
+	s = elf_getscn(elf->elf, symtab->idx);
+	if (!s) {
+		WARN_ELF("elf_getscn");
+		return NULL;
+	}
+
+	data = elf_newdata(s);
+	if (!data) {
+		WARN_ELF("elf_newdata");
+		return NULL;
+	}
+
+	data->d_buf = &sym->sym;
+	data->d_size = sizeof(sym->sym);
+	data->d_align = 1;
+
+	sym->idx = symtab->len / sizeof(sym->sym);
+
+	symtab->len += data->d_size;
+	symtab->changed = true;
+
+	if (!elf_symbol_add(elf, sym, SHN_XINDEX)) {
+		WARN("elf_symbol_add");
+		return NULL;
+	}
+
+	return sym;
+}
+
 struct section *elf_create_section(struct elf *elf, const char *name,
 				   unsigned int sh_flags, size_t entsize, int nr)
 {
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -128,6 +128,7 @@ int elf_write_insn(struct elf *elf, stru
 		   unsigned long offset, unsigned int len,
 		   const char *insn);
 int elf_write_reloc(struct elf *elf, struct reloc *reloc);
+struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name);
 int elf_write(struct elf *elf);
 void elf_close(struct elf *elf);