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 <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Jiri Slaby <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Mike Galbraith <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/orc_types.h

···
+/*
+ * Copyright (C) 2017 Josh Poimboeuf <[e-mail]>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef _ORC_TYPES_H
+#define _ORC_TYPES_H
+
+#include <linux/types.h>
+#include <linux/compiler.h>
+
+/*
+ * The ORC_REG_* registers are base registers which are used to find other
+ * registers on the stack.
+ *
+ * ORC_REG_PREV_SP, also known as DWARF Call Frame Address (CFA), is the
+ * address of the previous frame: the caller's SP before it called the current
+ * function.
+ *
+ * ORC_REG_UNDEFINED means the corresponding register's value didn't change in
+ * the current frame.
+ *
+ * The most commonly used base registers are SP and BP -- which the previous SP
+ * is usually based on -- and PREV_SP and UNDEFINED -- which the previous BP is
+ * usually based on.
+ *
+ * The rest of the base registers are needed for special cases like entry code
+ * and GCC realigned stacks.
+ */
+#define ORC_REG_UNDEFINED		0
+#define ORC_REG_PREV_SP			1
+#define ORC_REG_DX			2
+#define ORC_REG_DI			3
+#define ORC_REG_BP			4
+#define ORC_REG_SP			5
+#define ORC_REG_R10			6
+#define ORC_REG_R13			7
+#define ORC_REG_BP_INDIRECT		8
+#define ORC_REG_SP_INDIRECT		9
+#define ORC_REG_MAX			15
+
+/*
+ * ORC_TYPE_CALL: Indicates that sp_reg+sp_offset resolves to PREV_SP (the
+ * caller's SP right before it made the call).  Used for all callable
+ * functions, i.e. all C code and all callable asm functions.
+ *
+ * ORC_TYPE_REGS: Used in entry code to indicate that sp_reg+sp_offset points
+ * to a fully populated pt_regs from a syscall, interrupt, or exception.
+ *
+ * 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
+ * CFI, simplified for ease of access by the in-kernel unwinder.  It tells the
+ * unwinder how to find the previous SP and BP (and sometimes entry regs) on
+ * the stack for a given code address.  Each instance of the struct corresponds
+ * to one or more code locations.
+ */
+struct orc_entry {
+	s16		sp_offset;
+	s16		bp_offset;
+	unsigned	sp_reg:4;
+	unsigned	bp_reg:4;
+	unsigned	type:2;
+};
+
+/*
+ * 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 */

arch/x86/include/asm/unwind_hints.h

···
+#ifndef _ASM_X86_UNWIND_HINTS_H
+#define _ASM_X86_UNWIND_HINTS_H
+
+#include "orc_types.h"
+
+#ifdef __ASSEMBLY__
+
+/*
+ * In asm, there are two kinds of code: normal C-type callable functions and
+ * the rest.  The normal callable functions can be called by other code, and
+ * don't do anything unusual with the stack.  Such normal callable functions
+ * are annotated with the ENTRY/ENDPROC macros.  Most asm code falls in this
+ * category.  In this case, no special debugging annotations are needed because
+ * objtool can automatically generate the ORC data for the ORC unwinder to read
+ * at runtime.
+ *
+ * Anything which doesn't fall into the above category, such as syscall and
+ * interrupt handlers, tends to not be called directly by other functions, and
+ * often does unusual non-C-function-type things with the stack pointer.  Such
+ * code needs to be annotated such that objtool can understand it.  The
+ * following CFI hint macros are for this type of code.
+ *
+ * These macros provide hints to objtool about the state of the stack at each
+ * instruction.  Objtool starts from the hints and follows the code flow,
+ * making automatic CFI adjustments when it sees pushes and pops, filling out
+ * the debuginfo as necessary.  It will also warn if it sees any
+ * inconsistencies.
+ */
+.macro UNWIND_HINT sp_reg=ORC_REG_SP sp_offset=0 type=ORC_TYPE_CALL
+#ifdef CONFIG_STACK_VALIDATION
+.Lunwind_hint_ip_\@:
+	.pushsection .discard.unwind_hints
+		/* struct unwind_hint */
+		.long .Lunwind_hint_ip_\@ - .
+		.short \sp_offset
+		.byte \sp_reg
+		.byte \type
+	.popsection
+#endif
+.endm
+
+.macro UNWIND_HINT_EMPTY
+	UNWIND_HINT sp_reg=ORC_REG_UNDEFINED
+.endm
+
+.macro UNWIND_HINT_REGS base=%rsp offset=0 indirect=0 extra=1 iret=0
+	.if \base == %rsp && \indirect
+		.set sp_reg, ORC_REG_SP_INDIRECT
+	.elseif \base == %rsp
+		.set sp_reg, ORC_REG_SP
+	.elseif \base == %rbp
+		.set sp_reg, ORC_REG_BP
+	.elseif \base == %rdi
+		.set sp_reg, ORC_REG_DI
+	.elseif \base == %rdx
+		.set sp_reg, ORC_REG_DX
+	.elseif \base == %r10
+		.set sp_reg, ORC_REG_R10
+	.else
+		.error "UNWIND_HINT_REGS: bad base register"
+	.endif
+
+	.set sp_offset, \offset
+
+	.if \iret
+		.set type, ORC_TYPE_REGS_IRET
+	.elseif \extra == 0
+		.set type, ORC_TYPE_REGS_IRET
+		.set sp_offset, \offset + (16*8)
+	.else
+		.set type, ORC_TYPE_REGS
+	.endif
+
+	UNWIND_HINT sp_reg=sp_reg sp_offset=sp_offset type=type
+.endm
+
+.macro UNWIND_HINT_IRET_REGS base=%rsp offset=0
+	UNWIND_HINT_REGS base=\base offset=\offset iret=1
+.endm
+
+.macro UNWIND_HINT_FUNC sp_offset=8
+	UNWIND_HINT sp_offset=\sp_offset
+.endm
+
+#else /* !__ASSEMBLY__ */
+
+#define UNWIND_HINT(sp_reg, sp_offset, type)			\
+	"987: \n\t"						\
+	".pushsection .discard.unwind_hints\n\t"		\
+	/* struct unwind_hint */				\
+	".long 987b - .\n\t"					\
+	".short " __stringify(sp_offset) "\n\t"		\
+	".byte " __stringify(sp_reg) "\n\t"			\
+	".byte " __stringify(type) "\n\t"			\
+	".popsection\n\t"
+
+#define UNWIND_HINT_SAVE UNWIND_HINT(0, 0, UNWIND_HINT_TYPE_SAVE)
+
+#define UNWIND_HINT_RESTORE UNWIND_HINT(0, 0, UNWIND_HINT_TYPE_RESTORE)
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_X86_UNWIND_HINTS_H */

tools/objtool/Makefile

···
 	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 $@
 
 

tools/objtool/check.c

···
 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)
···
 			if (rela->sym->type == STT_FUNC && rela->sym == 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;
 }
···
 	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;
···
 		return ret;
 
 	ret = add_switch_table_alts(file);
+	if (ret)
+		return ret;
+
+	ret = read_unwind_hints(file);
 	if (ret)
 		return ret;
 
···
 			   struct insn_state state)
 {
 	struct alternative *alt;
-	struct instruction *insn;
+	struct instruction *insn, *next_insn;
 	struct section *sec;
 	struct symbol *func = NULL;
 	int ret;
···
 	}
 
 	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()",
···
 		}
 
 		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;
 
···
 
 			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;
···
 		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;
···
 	}
 
 	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)
···
 	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);
 
···
 		goto out;
 
 	ret = validate_functions(&file);
+	if (ret < 0)
+		goto out;
+	warnings += ret;
+
+	ret = validate_unwind_hints(&file);
 	if (ret < 0)
 		goto out;
 	warnings += ret;

tools/objtool/check.h

···
 	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;
···
 	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);

tools/objtool/orc_types.h

···
  *
  * 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
···
 	unsigned	bp_reg:4;
 	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 */