objtool: Allow arch code to discover jump table size

In preparation for adding support for annotated jump tables, where
ELF relocations and symbols are used to describe the locations of jump
tables in the executable, refactor the jump table discovery logic so the
table size can be returned from arch_find_switch_table().

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20241011170847.334429-12-ardb+git@google.com

Author: ardbiesheuvel

tools/objtool/arch/loongarch/special.c

@@ -9,7 +9,8 @@ bool arch_support_alt_relocation(struct special_alt *special_alt,
 }
 
 struct reloc *arch_find_switch_table(struct objtool_file *file,
-				     struct instruction *insn)
+				     struct instruction *insn,
+				     unsigned long *table_size)
 {
 	return NULL;
 }

tools/objtool/arch/powerpc/special.c

@@ -13,7 +13,8 @@ bool arch_support_alt_relocation(struct special_alt *special_alt,
 }
 
 struct reloc *arch_find_switch_table(struct objtool_file *file,
-				    struct instruction *insn)
+				     struct instruction *insn,
+				     unsigned long *table_size)
 {
 	exit(-1);
 }

tools/objtool/arch/x86/special.c

@@ -109,7 +109,8 @@ bool arch_support_alt_relocation(struct special_alt *special_alt,
  *    NOTE: MITIGATION_RETPOLINE made it harder still to decode dynamic jumps.
  */
 struct reloc *arch_find_switch_table(struct objtool_file *file,
-				    struct instruction *insn)
+				     struct instruction *insn,
+				     unsigned long *table_size)
 {
 	struct reloc  *text_reloc, *rodata_reloc;
 	struct section *table_sec;
@@ -158,5 +159,6 @@ struct reloc *arch_find_switch_table(struct objtool_file *file,
 	if (reloc_type(text_reloc) == R_X86_64_PC32)
 		file->ignore_unreachables = true;
 
+	*table_size = 0;
 	return rodata_reloc;
 }

tools/objtool/check.c

@@ -150,6 +150,15 @@ static inline struct reloc *insn_jump_table(struct instruction *insn)
 	return NULL;
 }
 
+static inline unsigned long insn_jump_table_size(struct instruction *insn)
+{
+	if (insn->type == INSN_JUMP_DYNAMIC ||
+	    insn->type == INSN_CALL_DYNAMIC)
+		return insn->_jump_table_size;
+
+	return 0;
+}
+
 static bool is_jump_table_jump(struct instruction *insn)
 {
 	struct alt_group *alt_group = insn->alt_group;
@@ -1937,6 +1946,7 @@ static int add_special_section_alts(struct objtool_file *file)
 static int add_jump_table(struct objtool_file *file, struct instruction *insn,
 			  struct reloc *next_table)
 {
+	unsigned long table_size = insn_jump_table_size(insn);
 	struct symbol *pfunc = insn_func(insn)->pfunc;
 	struct reloc *table = insn_jump_table(insn);
 	struct instruction *dest_insn;
@@ -1951,6 +1961,8 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
 	for_each_reloc_from(table->sec, reloc) {
 
 		/* Check for the end of the table: */
+		if (table_size && reloc_offset(reloc) - reloc_offset(table) >= table_size)
+			break;
 		if (reloc != table && reloc == next_table)
 			break;
 
@@ -1995,12 +2007,12 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
  * find_jump_table() - Given a dynamic jump, find the switch jump table
  * associated with it.
  */
-static struct reloc *find_jump_table(struct objtool_file *file,
-				      struct symbol *func,
-				      struct instruction *insn)
+static void find_jump_table(struct objtool_file *file, struct symbol *func,
+			    struct instruction *insn)
 {
 	struct reloc *table_reloc;
 	struct instruction *dest_insn, *orig_insn = insn;
+	unsigned long table_size;
 
 	/*
 	 * Backward search using the @first_jump_src links, these help avoid
@@ -2021,17 +2033,17 @@ static struct reloc *find_jump_table(struct objtool_file *file,
 		     insn->jump_dest->offset > orig_insn->offset))
 		    break;
 
-		table_reloc = arch_find_switch_table(file, insn);
+		table_reloc = arch_find_switch_table(file, insn, &table_size);
 		if (!table_reloc)
 			continue;
 		dest_insn = find_insn(file, table_reloc->sym->sec, reloc_addend(table_reloc));
 		if (!dest_insn || !insn_func(dest_insn) || insn_func(dest_insn)->pfunc != func)
 			continue;
 
-		return table_reloc;
+		orig_insn->_jump_table = table_reloc;
+		orig_insn->_jump_table_size = table_size;
+		break;
 	}
-
-	return NULL;
 }
 
 /*
@@ -2042,7 +2054,6 @@ static void mark_func_jump_tables(struct objtool_file *file,
 				    struct symbol *func)
 {
 	struct instruction *insn, *last = NULL;
-	struct reloc *reloc;
 
 	func_for_each_insn(file, func, insn) {
 		if (!last)
@@ -2065,9 +2076,7 @@ static void mark_func_jump_tables(struct objtool_file *file,
 		if (insn->type != INSN_JUMP_DYNAMIC)
 			continue;
 
-		reloc = find_jump_table(file, func, insn);
-		if (reloc)
-			insn->_jump_table = reloc;
+		find_jump_table(file, func, insn);
 	}
 }
 

tools/objtool/include/objtool/check.h

@@ -71,7 +71,10 @@ struct instruction {
 	struct instruction *first_jump_src;
 	union {
 		struct symbol *_call_dest;
-		struct reloc *_jump_table;
+		struct {
+			struct reloc *_jump_table;
+			unsigned long _jump_table_size;
+		};
 	};
 	struct alternative *alts;
 	struct symbol *sym;

tools/objtool/include/objtool/special.h

@@ -38,5 +38,6 @@ bool arch_support_alt_relocation(struct special_alt *special_alt,
 				 struct instruction *insn,
 				 struct reloc *reloc);
 struct reloc *arch_find_switch_table(struct objtool_file *file,
-				    struct instruction *insn);
+				     struct instruction *insn,
+				     unsigned long *table_size);
 #endif /* _SPECIAL_H */