linux-kernel - [PATCH v2 11/14] objtool: Add elf_create_undef

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Date:   Thu, 18 Mar 2021 18:11:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, jpoimboe@...hat.com, jgross@...e.com,
        mbenes@...e.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org
Subject: [PATCH v2 11/14] objtool: Add elf_create_undef_symbol()

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@...radead.org>
---
 tools/objtool/elf.c                 |   63 ++++++++++++++++++++++++++++++++++++
 tools/objtool/include/objtool/elf.h |    1 
 2 files changed, 64 insertions(+)

--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -724,6 +724,69 @@ static int elf_strtab_concat(struct elf
 	return len;
 }
 
+struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
+{
+	struct section *symtab;
+	struct symbol *sym;
+	Elf_Data *data;
+	Elf_Scn *s;
+
+	sym = malloc(sizeof(*sym));
+	if (!sym) {
+		perror("malloc");
+		return NULL;
+	}
+	memset(sym, 0, sizeof(*sym));
+
+	sym->name = strdup(name);
+
+	sym->sym.st_name = elf_strtab_concat(elf, sym->name, NULL);
+	if (sym->sym.st_name == -1)
+		return NULL;
+
+	sym->sym.st_info = 0x10; /* STB_GLOBAL, STT_NOTYPE */
+	// st_other 0
+	// st_shndx 0
+	// st_value 0
+	// st_size 0
+
+	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;
+
+	sym->sec = find_section_by_index(elf, 0);
+
+	if (!elf_symbol_add(elf, sym)) {
+		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
@@ -136,6 +136,7 @@ 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);