objtool: Provide elf_write_{insn,reloc}()

This provides infrastructure to rewrite instructions; this is
immediately useful for helping out with KCOV-vs-noinstr, but will
also come in handy for a bunch of variable sized jump-label patches
that are still on ice.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

tools/objtool/elf.c

@@ -529,8 +529,9 @@ static int read_relas(struct elf *elf)
 			rela->addend = rela->rela.r_addend;
 			rela->offset = rela->rela.r_offset;
 			symndx = GELF_R_SYM(rela->rela.r_info);
-			rela->sym = find_symbol_by_index(elf, symndx);
 			rela->sec = sec;
+			rela->idx = i;
+			rela->sym = find_symbol_by_index(elf, symndx);
 			if (!rela->sym) {
 				WARN("can't find rela entry symbol %d for %s",
 				     symndx, sec->name);
@@ -784,6 +785,43 @@ int elf_rebuild_rela_section(struct elf *elf, struct section *sec)
 	return 0;
 }
 
+int elf_write_insn(struct elf *elf, struct section *sec,
+		   unsigned long offset, unsigned int len,
+		   const char *insn)
+{
+	Elf_Data *data = sec->data;
+
+	if (data->d_type != ELF_T_BYTE || data->d_off) {
+		WARN("write to unexpected data for section: %s", sec->name);
+		return -1;
+	}
+
+	memcpy(data->d_buf + offset, insn, len);
+	elf_flagdata(data, ELF_C_SET, ELF_F_DIRTY);
+
+	elf->changed = true;
+
+	return 0;
+}
+
+int elf_write_rela(struct elf *elf, struct rela *rela)
+{
+	struct section *sec = rela->sec;
+
+	rela->rela.r_info = GELF_R_INFO(rela->sym->idx, rela->type);
+	rela->rela.r_addend = rela->addend;
+	rela->rela.r_offset = rela->offset;
+
+	if (!gelf_update_rela(sec->data, rela->idx, &rela->rela)) {
+		WARN_ELF("gelf_update_rela");
+		return -1;
+	}
+
+	elf->changed = true;
+
+	return 0;
+}
+
 int elf_write(struct elf *elf)
 {
 	struct section *sec;

tools/objtool/elf.h

@@ -64,9 +64,10 @@ struct rela {
 	GElf_Rela rela;
 	struct section *sec;
 	struct symbol *sym;
-	unsigned int type;
 	unsigned long offset;
+	unsigned int type;
 	int addend;
+	int idx;
 	bool jump_table_start;
 };
 
@@ -119,6 +120,10 @@ struct elf *elf_open_read(const char *name, int flags);
 struct section *elf_create_section(struct elf *elf, const char *name, size_t entsize, int nr);
 struct section *elf_create_rela_section(struct elf *elf, struct section *base);
 void elf_add_rela(struct elf *elf, struct rela *rela);
+int elf_write_insn(struct elf *elf, struct section *sec,
+		   unsigned long offset, unsigned int len,
+		   const char *insn);
+int elf_write_rela(struct elf *elf, struct rela *rela);
 int elf_write(struct elf *elf);
 void elf_close(struct elf *elf);