objtool: Refactor jump table code

Now that C jump tables are supported, call them "jump tables" instead of
"switch tables". Also rename some other variables, add comments, and
simplify the code flow a bit.

Signed-off-by: Josh Poimboeuf <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Tested-by: Nick Desaulniers <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

tools/objtool/check.c

···
 			 * However this code can't completely replace the
 			 * read_symbols() code because this doesn't detect the
 			 * case where the parent function's only reference to a
-			 * subfunction is through a switch table.
+			 * subfunction is through a jump table.
 			 */
 			if (!strstr(insn->func->name, ".cold.") &&
 			    strstr(insn->jump_dest->func->name, ".cold.")) {
···
 	return ret;
 }
 
-static int add_switch_table(struct objtool_file *file, struct instruction *insn,
+static int add_jump_table(struct objtool_file *file, struct instruction *insn,
 			    struct rela *table, struct rela *next_table)
 {
 	struct rela *rela = table;
-	struct instruction *alt_insn;
+	struct instruction *dest_insn;
 	struct alternative *alt;
 	struct symbol *pfunc = insn->func->pfunc;
 	unsigned int prev_offset = 0;
 
-	list_for_each_entry_from(rela, &table->rela_sec->rela_list, list) {
+	/*
+	 * Each @rela is a switch table relocation which points to the target
+	 * instruction.
+	 */
+	list_for_each_entry_from(rela, &table->sec->rela_list, list) {
 		if (rela == next_table)
 			break;
 
-		/* Make sure the switch table entries are consecutive: */
+		/* Make sure the table entries are consecutive: */
 		if (prev_offset && rela->offset != prev_offset + 8)
 			break;
 
···
 		    rela->addend == pfunc->offset)
 			break;
 
-		alt_insn = find_insn(file, rela->sym->sec, rela->addend);
-		if (!alt_insn)
+		dest_insn = find_insn(file, rela->sym->sec, rela->addend);
+		if (!dest_insn)
 			break;
 
-		/* Make sure the jmp dest is in the function or subfunction: */
-		if (alt_insn->func->pfunc != pfunc)
+		/* Make sure the destination is in the same function: */
+		if (dest_insn->func->pfunc != pfunc)
 			break;
 
 		alt = malloc(sizeof(*alt));
···
 			return -1;
 		}
 
-		alt->insn = alt_insn;
+		alt->insn = dest_insn;
 		list_add_tail(&alt->list, &insn->alts);
 		prev_offset = rela->offset;
 	}
···
 }
 
 /*
- * find_switch_table() - Given a dynamic jump, find the switch jump table in
+ * find_jump_table() - Given a dynamic jump, find the switch jump table in
  * .rodata associated with it.
  *
  * There are 3 basic patterns:
···
  *
  *    NOTE: RETPOLINE made it harder still to decode dynamic jumps.
  */
-static struct rela *find_switch_table(struct objtool_file *file,
+static struct rela *find_jump_table(struct objtool_file *file,
 				      struct symbol *func,
 				      struct instruction *insn)
 {
-	struct rela *text_rela, *rodata_rela;
+	struct rela *text_rela, *table_rela;
 	struct instruction *orig_insn = insn;
-	struct section *rodata_sec;
+	struct section *table_sec;
 	unsigned long table_offset;
 
 	/*
···
 			continue;
 
 		table_offset = text_rela->addend;
-		rodata_sec = text_rela->sym->sec;
+		table_sec = text_rela->sym->sec;
 
 		if (text_rela->type == R_X86_64_PC32)
 			table_offset += 4;
···
 		 * need to be placed in the C_JUMP_TABLE_SECTION section.  They
 		 * have symbols associated with them.
 		 */
-		if (find_symbol_containing(rodata_sec, table_offset) &&
-		    strcmp(rodata_sec->name, C_JUMP_TABLE_SECTION))
+		if (find_symbol_containing(table_sec, table_offset) &&
+		    strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
 			continue;
 
-		rodata_rela = find_rela_by_dest(rodata_sec, table_offset);
-		if (rodata_rela) {
-			/*
-			 * Use of RIP-relative switch jumps is quite rare, and
-			 * indicates a rare GCC quirk/bug which can leave dead
-			 * code behind.
-			 */
-			if (text_rela->type == R_X86_64_PC32)
-				file->ignore_unreachables = true;
+		/* Each table entry has a rela associated with it. */
+		table_rela = find_rela_by_dest(table_sec, table_offset);
+		if (!table_rela)
+			continue;
 
-			return rodata_rela;
-		}
+		/*
+		 * Use of RIP-relative switch jumps is quite rare, and
+		 * indicates a rare GCC quirk/bug which can leave dead code
+		 * behind.
+		 */
+		if (text_rela->type == R_X86_64_PC32)
+			file->ignore_unreachables = true;
+
+		return table_rela;
 	}
 
 	return NULL;
 }
 
 
-static int add_func_switch_tables(struct objtool_file *file,
+static int add_func_jump_tables(struct objtool_file *file,
 				  struct symbol *func)
 {
 	struct instruction *insn, *last = NULL, *prev_jump = NULL;
···
 
 		/*
 		 * Store back-pointers for unconditional forward jumps such
-		 * that find_switch_table() can back-track using those and
+		 * that find_jump_table() can back-track using those and
 		 * avoid some potentially confusing code.
 		 */
 		if (insn->type == INSN_JUMP_UNCONDITIONAL && insn->jump_dest &&
···
 		if (insn->type != INSN_JUMP_DYNAMIC)
 			continue;
 
-		rela = find_switch_table(file, func, insn);
+		rela = find_jump_table(file, func, insn);
 		if (!rela)
 			continue;
 
 		/*
-		 * We found a switch table, but we don't know yet how big it
+		 * We found a jump table, but we don't know yet how big it
 		 * is.  Don't add it until we reach the end of the function or
-		 * the beginning of another switch table in the same function.
+		 * the beginning of another jump table in the same function.
 		 */
 		if (prev_jump) {
-			ret = add_switch_table(file, prev_jump, prev_rela, rela);
+			ret = add_jump_table(file, prev_jump, prev_rela, rela);
 			if (ret)
 				return ret;
 		}
···
 	}
 
 	if (prev_jump) {
-		ret = add_switch_table(file, prev_jump, prev_rela, NULL);
+		ret = add_jump_table(file, prev_jump, prev_rela, NULL);
 		if (ret)
 			return ret;
 	}
···
  * section which contains a list of addresses within the function to jump to.
  * This finds these jump tables and adds them to the insn->alts lists.
  */
-static int add_switch_table_alts(struct objtool_file *file)
+static int add_jump_table_alts(struct objtool_file *file)
 {
 	struct section *sec;
 	struct symbol *func;
···
 			if (func->type != STT_FUNC)
 				continue;
 
-			ret = add_func_switch_tables(file, func);
+			ret = add_func_jump_tables(file, func);
 			if (ret)
 				return ret;
 		}
···
 	if (ret)
 		return ret;
 
-	ret = add_switch_table_alts(file);
+	ret = add_jump_table_alts(file);
 	if (ret)
 		return ret;
 

tools/objtool/elf.c

···
 			rela->offset = rela->rela.r_offset;
 			symndx = GELF_R_SYM(rela->rela.r_info);
 			rela->sym = find_symbol_by_index(elf, symndx);
-			rela->rela_sec = sec;
+			rela->sec = sec;
 			if (!rela->sym) {
 				WARN("can't find rela entry symbol %d for %s",
 				     symndx, sec->name);

tools/objtool/elf.h

···
 	struct list_head list;
 	struct hlist_node hash;
 	GElf_Rela rela;
-	struct section *rela_sec;
+	struct section *sec;
 	struct symbol *sym;
 	unsigned int type;
 	unsigned long offset;