objtool, x86: Add facility for asm code to provide unwind hints

Some asm (and inline asm) code does special things to the stack which
objtool can't understand.  (Nor can GCC or GNU assembler, for that
matter.)  In such cases we need a facility for the code to provide
annotations, so the unwinder can unwind through it.

This provides such a facility, in the form of unwind hints.  They're
similar to the GNU assembler .cfi* directives, but they give more
information, and are needed in far fewer places, because objtool can
fill in the blanks by following branches and adjusting the stack pointer
for pushes and pops.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: live-patching@vger.kernel.org
Link: http://lkml.kernel.org/r/0f5f3c9104fca559ff4088bece1d14ae3bca52d5.1499786555.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

4 files changed, 207 insertions, 13 deletions

diff --git a/tools/objtool/Makefile b/tools/objtool/Makefile
index 0e2765e243c0..3a6425fefc43 100644
--- a/tools/objtool/Makefile
+++ b/tools/objtool/Makefile
@@ -52,6 +52,9 @@ $(OBJTOOL): $(LIBSUBCMD) $(OBJTOOL_IN)
 	diff -I'^#include' arch/x86/insn/inat.h ../../arch/x86/include/asm/inat.h >/dev/null && \
 	diff -I'^#include' arch/x86/insn/inat_types.h ../../arch/x86/include/asm/inat_types.h >/dev/null) \
 	|| echo "warning: objtool: x86 instruction decoder differs from kernel" >&2 )) || true
+	@(test -d ../../kernel -a -d ../../tools -a -d ../objtool && (( \
+	diff ../../arch/x86/include/asm/orc_types.h orc_types.h >/dev/null) \
+	|| echo "warning: objtool: orc_types.h differs from kernel" >&2 )) || true
 	$(QUIET_LINK)$(CC) $(OBJTOOL_IN) $(LDFLAGS) -o $@
 
 
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index cb57c526ba17..368275de5f23 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -100,7 +100,6 @@ static bool gcov_enabled(struct objtool_file *file)
 static bool ignore_func(struct objtool_file *file, struct symbol *func)
 {
 	struct rela *rela;
-	struct instruction *insn;
 
 	/* check for STACK_FRAME_NON_STANDARD */
 	if (file->whitelist && file->whitelist->rela)
@@ -113,11 +112,6 @@ static bool ignore_func(struct objtool_file *file, struct symbol *func)
 				return true;
 		}
 
-	/* check if it has a context switching instruction */
-	func_for_each_insn(file, func, insn)
-		if (insn->type == INSN_CONTEXT_SWITCH)
-			return true;
-
 	return false;
 }
 
@@ -879,6 +873,99 @@ static int add_switch_table_alts(struct objtool_file *file)
 	return 0;
 }
 
+static int read_unwind_hints(struct objtool_file *file)
+{
+	struct section *sec, *relasec;
+	struct rela *rela;
+	struct unwind_hint *hint;
+	struct instruction *insn;
+	struct cfi_reg *cfa;
+	int i;
+
+	sec = find_section_by_name(file->elf, ".discard.unwind_hints");
+	if (!sec)
+		return 0;
+
+	relasec = sec->rela;
+	if (!relasec) {
+		WARN("missing .rela.discard.unwind_hints section");
+		return -1;
+	}
+
+	if (sec->len % sizeof(struct unwind_hint)) {
+		WARN("struct unwind_hint size mismatch");
+		return -1;
+	}
+
+	file->hints = true;
+
+	for (i = 0; i < sec->len / sizeof(struct unwind_hint); i++) {
+		hint = (struct unwind_hint *)sec->data->d_buf + i;
+
+		rela = find_rela_by_dest(sec, i * sizeof(*hint));
+		if (!rela) {
+			WARN("can't find rela for unwind_hints[%d]", i);
+			return -1;
+		}
+
+		insn = find_insn(file, rela->sym->sec, rela->addend);
+		if (!insn) {
+			WARN("can't find insn for unwind_hints[%d]", i);
+			return -1;
+		}
+
+		cfa = &insn->state.cfa;
+
+		if (hint->type == UNWIND_HINT_TYPE_SAVE) {
+			insn->save = true;
+			continue;
+
+		} else if (hint->type == UNWIND_HINT_TYPE_RESTORE) {
+			insn->restore = true;
+			insn->hint = true;
+			continue;
+		}
+
+		insn->hint = true;
+
+		switch (hint->sp_reg) {
+		case ORC_REG_UNDEFINED:
+			cfa->base = CFI_UNDEFINED;
+			break;
+		case ORC_REG_SP:
+			cfa->base = CFI_SP;
+			break;
+		case ORC_REG_BP:
+			cfa->base = CFI_BP;
+			break;
+		case ORC_REG_SP_INDIRECT:
+			cfa->base = CFI_SP_INDIRECT;
+			break;
+		case ORC_REG_R10:
+			cfa->base = CFI_R10;
+			break;
+		case ORC_REG_R13:
+			cfa->base = CFI_R13;
+			break;
+		case ORC_REG_DI:
+			cfa->base = CFI_DI;
+			break;
+		case ORC_REG_DX:
+			cfa->base = CFI_DX;
+			break;
+		default:
+			WARN_FUNC("unsupported unwind_hint sp base reg %d",
+				  insn->sec, insn->offset, hint->sp_reg);
+			return -1;
+		}
+
+		cfa->offset = hint->sp_offset;
+		insn->state.type = hint->type;
+	}
+
+	return 0;
+}
+
 static int decode_sections(struct objtool_file *file)
 {
 	int ret;
@@ -909,6 +996,10 @@ static int decode_sections(struct objtool_file *file)
 	if (ret)
 		return ret;
 
+	ret = read_unwind_hints(file);
+	if (ret)
+		return ret;
+
 	return 0;
 }
 
@@ -1382,7 +1473,7 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 			   struct insn_state state)
 {
 	struct alternative *alt;
-	struct instruction *insn;
+	struct instruction *insn, *next_insn;
 	struct section *sec;
 	struct symbol *func = NULL;
 	int ret;
@@ -1397,6 +1488,8 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 	}
 
 	while (1) {
+		next_insn = next_insn_same_sec(file, insn);
+
 		if (file->c_file && insn->func) {
 			if (func && func != insn->func) {
 				WARN("%s() falls through to next function %s()",
@@ -1414,13 +1507,54 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 		}
 
 		if (insn->visited) {
-			if (!!insn_state_match(insn, &state))
+			if (!insn->hint && !insn_state_match(insn, &state))
 				return 1;
 
 			return 0;
 		}
 
-		insn->state = state;
+		if (insn->hint) {
+			if (insn->restore) {
+				struct instruction *save_insn, *i;
+
+				i = insn;
+				save_insn = NULL;
+				func_for_each_insn_continue_reverse(file, func, i) {
+					if (i->save) {
+						save_insn = i;
+						break;
+					}
+				}
+
+				if (!save_insn) {
+					WARN_FUNC("no corresponding CFI save for CFI restore",
+						  sec, insn->offset);
+					return 1;
+				}
+
+				if (!save_insn->visited) {
+					/*
+					 * Oops, no state to copy yet.
+					 * Hopefully we can reach this
+					 * instruction from another branch
+					 * after the save insn has been
+					 * visited.
+					 */
+					if (insn == first)
+						return 0;
+
+					WARN_FUNC("objtool isn't smart enough to handle this CFI save/restore combo",
+						  sec, insn->offset);
+					return 1;
+				}
+
+				insn->state = save_insn->state;
+			}
+
+			state = insn->state;
+
+		} else
+			insn->state = state;
 
 		insn->visited = true;
 
@@ -1497,6 +1631,14 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 
 			return 0;
 
+		case INSN_CONTEXT_SWITCH:
+			if (func && (!next_insn || !next_insn->hint)) {
+				WARN_FUNC("unsupported instruction in callable function",
+					  sec, insn->offset);
+				return 1;
+			}
+			return 0;
+
 		case INSN_STACK:
 			if (update_insn_state(insn, &state))
 				return -1;
@@ -1510,7 +1652,7 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 		if (insn->dead_end)
 			return 0;
 
-		insn = next_insn_same_sec(file, insn);
+		insn = next_insn;
 		if (!insn) {
 			WARN("%s: unexpected end of section", sec->name);
 			return 1;
@@ -1520,6 +1662,27 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 	return 0;
 }
 
+static int validate_unwind_hints(struct objtool_file *file)
+{
+	struct instruction *insn;
+	int ret, warnings = 0;
+	struct insn_state state;
+
+	if (!file->hints)
+		return 0;
+
+	clear_insn_state(&state);
+
+	for_each_insn(file, insn) {
+		if (insn->hint && !insn->visited) {
+			ret = validate_branch(file, insn, state);
+			warnings += ret;
+		}
+	}
+
+	return warnings;
+}
+
 static bool is_kasan_insn(struct instruction *insn)
 {
 	return (insn->type == INSN_CALL &&
@@ -1665,8 +1828,9 @@ int check(const char *_objname, bool _nofp, bool orc)
 	hash_init(file.insn_hash);
 	file.whitelist = find_section_by_name(file.elf, ".discard.func_stack_frame_non_standard");
 	file.rodata = find_section_by_name(file.elf, ".rodata");
-	file.ignore_unreachables = false;
 	file.c_file = find_section_by_name(file.elf, ".comment");
+	file.ignore_unreachables = false;
+	file.hints = false;
 
 	arch_initial_func_cfi_state(&initial_func_cfi);
 
@@ -1683,6 +1847,11 @@ int check(const char *_objname, bool _nofp, bool orc)
 		goto out;
 	warnings += ret;
 
+	ret = validate_unwind_hints(&file);
+	if (ret < 0)
+		goto out;
+	warnings += ret;
+
 	if (!warnings) {
 		ret = validate_reachable_instructions(&file);
 		if (ret < 0)
diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index 046874bbe226..ac3d4b13f17b 100644
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -43,7 +43,7 @@ struct instruction {
 	unsigned int len;
 	unsigned char type;
 	unsigned long immediate;
-	bool alt_group, visited, dead_end, ignore;
+	bool alt_group, visited, dead_end, ignore, hint, save, restore;
 	struct symbol *call_dest;
 	struct instruction *jump_dest;
 	struct list_head alts;
@@ -58,7 +58,7 @@ struct objtool_file {
 	struct list_head insn_list;
 	DECLARE_HASHTABLE(insn_hash, 16);
 	struct section *rodata, *whitelist;
-	bool ignore_unreachables, c_file;
+	bool ignore_unreachables, c_file, hints;
 };
 
 int check(const char *objname, bool nofp, bool orc);
diff --git a/tools/objtool/orc_types.h b/tools/objtool/orc_types.h
index fc5cf6cffd9a..9c9dc579bd7d 100644
--- a/tools/objtool/orc_types.h
+++ b/tools/objtool/orc_types.h
@@ -61,11 +61,19 @@
  *
  * ORC_TYPE_REGS_IRET: Used in entry code to indicate that sp_reg+sp_offset
  * points to the iret return frame.
+ *
+ * The UNWIND_HINT macros are used only for the unwind_hint struct.  They
+ * aren't used in struct orc_entry due to size and complexity constraints.
+ * Objtool converts them to real types when it converts the hints to orc
+ * entries.
  */
 #define ORC_TYPE_CALL			0
 #define ORC_TYPE_REGS			1
 #define ORC_TYPE_REGS_IRET		2
+#define UNWIND_HINT_TYPE_SAVE		3
+#define UNWIND_HINT_TYPE_RESTORE	4
 
+#ifndef __ASSEMBLY__
 /*
  * This struct is more or less a vastly simplified version of the DWARF Call
  * Frame Information standard.  It contains only the necessary parts of DWARF
@@ -82,4 +90,18 @@ struct orc_entry {
 	unsigned	type:2;
 } __packed;
 
+/*
+ * This struct is used by asm and inline asm code to manually annotate the
+ * location of registers on the stack for the ORC unwinder.
+ *
+ * Type can be either ORC_TYPE_* or UNWIND_HINT_TYPE_*.
+ */
+struct unwind_hint {
+	u32		ip;
+	s16		sp_offset;
+	u8		sp_reg;
+	u8		type;
+};
+#endif /* __ASSEMBLY__ */
+
 #endif /* _ORC_TYPES_H */