objtool: Improve detection of BUG() and other dead ends

The BUG() macro's use of __builtin_unreachable() via the unreachable()
macro tells gcc that the instruction is a dead end, and that it's safe
to assume the current code path will not execute past the previous
instruction.

On x86, the BUG() macro is implemented with the 'ud2' instruction.  When
objtool's branch analysis sees that instruction, it knows the current
code path has come to a dead end.

Peter Zijlstra has been working on a patch to change the WARN macros to
use 'ud2'.  That patch will break objtool's assumption that 'ud2' is
always a dead end.

Generally it's best for objtool to avoid making those kinds of
assumptions anyway.  The more ignorant it is of kernel code internals,
the better.

So create a more generic way for objtool to detect dead ends by adding
an annotation to the unreachable() macro.  The annotation stores a
pointer to the end of the unreachable code path in an '__unreachable'
section.  Objtool can read that section to find the dead ends.
Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/41a6d33971462ebd944a1c60ad4bf5be86c17b77.1487712920.git.jpoimboe@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/vmlinux.lds.S

@@ -346,6 +346,7 @@ SECTIONS
 	/DISCARD/ : {
 		*(.eh_frame)
 		*(__func_stack_frame_non_standard)
+		*(__unreachable)
 	}
 }
 

include/linux/compiler-gcc.h

@@ -195,6 +195,17 @@
 #endif
 #endif
 
+#ifdef CONFIG_STACK_VALIDATION
+#define annotate_unreachable() ({					\
+	asm("1:\t\n"							\
+	    ".pushsection __unreachable, \"a\"\t\n"			\
+	    ".long 1b\t\n"						\
+	    ".popsection\t\n");						\
+})
+#else
+#define annotate_unreachable()
+#endif
+
 /*
  * Mark a position in code as unreachable.  This can be used to
  * suppress control flow warnings after asm blocks that transfer
@@ -204,7 +215,7 @@
  * this in the preprocessor, but we can live with this because they're
  * unreleased.  Really, we need to have autoconf for the kernel.
  */
-#define unreachable() __builtin_unreachable()
+#define unreachable() annotate_unreachable(); __builtin_unreachable()
 
 /* Mark a function definition as prohibited from being cloned. */
 #define __noclone	__attribute__((__noclone__, __optimize__("no-tracer")))

tools/objtool/arch.h

@@ -31,9 +31,8 @@
 #define INSN_CALL_DYNAMIC	8
 #define INSN_RETURN		9
 #define INSN_CONTEXT_SWITCH	10
-#define INSN_BUG		11
-#define INSN_NOP		12
-#define INSN_OTHER		13
+#define INSN_NOP		11
+#define INSN_OTHER		12
 #define INSN_LAST		INSN_OTHER
 
 int arch_decode_instruction(struct elf *elf, struct section *sec,

tools/objtool/arch/x86/decode.c

@@ -118,9 +118,6 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
 			 op2 == 0x35)
 			/* sysenter, sysret */
 			*type = INSN_CONTEXT_SWITCH;
-		else if (op2 == 0x0b || op2 == 0xb9)
-			/* ud2 */
-			*type = INSN_BUG;
 		else if (op2 == 0x0d || op2 == 0x1f)
 			/* nopl/nopw */
 			*type = INSN_NOP;

tools/objtool/builtin-check.c

@@ -51,7 +51,7 @@ struct instruction {
 	unsigned int len, state;
 	unsigned char type;
 	unsigned long immediate;
-	bool alt_group, visited;
+	bool alt_group, visited, dead_end;
 	struct symbol *call_dest;
 	struct instruction *jump_dest;
 	struct list_head alts;
@@ -329,6 +329,54 @@ static int decode_instructions(struct objtool_file *file)
 	return 0;
 }
 
+/*
+ * Find all uses of the unreachable() macro, which are code path dead ends.
+ */
+static int add_dead_ends(struct objtool_file *file)
+{
+	struct section *sec;
+	struct rela *rela;
+	struct instruction *insn;
+	bool found;
+
+	sec = find_section_by_name(file->elf, ".rela__unreachable");
+	if (!sec)
+		return 0;
+
+	list_for_each_entry(rela, &sec->rela_list, list) {
+		if (rela->sym->type != STT_SECTION) {
+			WARN("unexpected relocation symbol type in .rela__unreachable");
+			return -1;
+		}
+		insn = find_insn(file, rela->sym->sec, rela->addend);
+		if (insn)
+			insn = list_prev_entry(insn, list);
+		else if (rela->addend == rela->sym->sec->len) {
+			found = false;
+			list_for_each_entry_reverse(insn, &file->insn_list, list) {
+				if (insn->sec == rela->sym->sec) {
+					found = true;
+					break;
+				}
+			}
+
+			if (!found) {
+				WARN("can't find unreachable insn at %s+0x%x",
+				     rela->sym->sec->name, rela->addend);
+				return -1;
+			}
+		} else {
+			WARN("can't find unreachable insn at %s+0x%x",
+			     rela->sym->sec->name, rela->addend);
+			return -1;
+		}
+
+		insn->dead_end = true;
+	}
+
+	return 0;
+}
+
 /*
  * Warnings shouldn't be reported for ignored functions.
  */
@@ -843,6 +891,10 @@ static int decode_sections(struct objtool_file *file)
 	if (ret)
 		return ret;
 
+	ret = add_dead_ends(file);
+	if (ret)
+		return ret;
+
 	add_ignores(file);
 
 	ret = add_jump_destinations(file);
@@ -1037,13 +1089,13 @@ static int validate_branch(struct objtool_file *file,
 
 			return 0;
 
-		case INSN_BUG:
-			return 0;
-
 		default:
 			break;
 		}
 
+		if (insn->dead_end)
+			return 0;
+
 		insn = next_insn_same_sec(file, insn);
 		if (!insn) {
 			WARN("%s: unexpected end of section", sec->name);