path: root/tools/objtool/builtin-check.c

/*
 * Copyright (C) 2015 Josh Poimboeuf <jpoimboe@redhat.com>
 *
 * 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/>.
 */

/*
 * objtool check:
 *
 * This command analyzes every .o file and ensures the validity of its stack
 * trace metadata.  It enforces a set of rules on asm code and C inline
 * assembly code so that stack traces can be reliable.
 *
 * For more information, see tools/objtool/Documentation/stack-validation.txt.
 */

#include <string.h>
#include <stdlib.h>
#include <subcmd/parse-options.h>

#include "builtin.h"
#include "elf.h"
#include "special.h"
#include "arch.h"
#include "warn.h"

#include <linux/hashtable.h>

#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))

#define STATE_FP_SAVED		0x1
#define STATE_FP_SETUP		0x2
#define STATE_FENTRY		0x4

struct instruction {
	struct list_head list;
	struct hlist_node hash;
	struct section *sec;
	unsigned long offset;
	unsigned int len, state;
	unsigned char type;
	unsigned long immediate;
	bool alt_group, visited;
	struct symbol *call_dest;
	struct instruction *jump_dest;
	struct list_head alts;
	struct symbol *func;
};

struct alternative {
	struct list_head list;
	struct instruction *insn;
};

struct objtool_file {
	struct elf *elf;
	struct list_head insn_list;
	DECLARE_HASHTABLE(insn_hash, 16);
	struct section *rodata, *whitelist;
	bool ignore_unreachables, c_file;
};

const char *objname;
static bool nofp;

static struct instruction *find_insn(struct objtool_file *file,
				     struct section *sec, unsigned long offset)
{
	struct instruction *insn;

	hash_for_each_possible(file->insn_hash, insn, hash, offset)
		if (insn->sec == sec && insn->offset == offset)
			return insn;

	return NULL;
}

static struct instruction *next_insn_same_sec(struct objtool_file *file,
					      struct instruction *insn)
{
	struct instruction *next = list_next_entry(insn, list);

	if (&next->list == &file->insn_list || next->sec != insn->sec)
		return NULL;

	return next;
}

static bool gcov_enabled(struct objtool_file *file)
{
	struct section *sec;
	struct symbol *sym;

	list_for_each_entry(sec, &file->elf->sections, list)
		list_for_each_entry(sym, &sec->symbol_list, list)
			if (!strncmp(sym->name, "__gcov_.", 8))
				return true;

	return false;
}

#define for_each_insn(file, insn)					\
	list_for_each_entry(insn, &file->insn_list, list)

#define func_for_each_insn(file, func, insn)				\
	for (insn = find_insn(file, func->sec, func->offset);		\
	     insn && &insn->list != &file->insn_list &&			\
		insn->sec == func->sec &&				\
		insn->offset < func->offset + func->len;		\
	     insn = list_next_entry(insn, list))

#define func_for_each_insn_continue_reverse(file, func, insn)		\
	for (insn = list_prev_entry(insn, list);			\
	     &insn->list != &file->insn_list &&				\
		insn->sec == func->sec && insn->offset >= func->offset;	\
	     insn = list_prev_entry(insn, list))

#define sec_for_each_insn_from(file, insn)				\
	for (; insn; insn = next_insn_same_sec(file, insn))


/*
 * Check if the function has been manually whitelisted with the
 * STACK_FRAME_NON_STANDARD macro, or if it should be automatically whitelisted
 * due to its use of a context switching instruction.
 */
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)
		list_for_each_entry(rela, &file->whitelist->rela->rela_list, list) {
			if (rela->sym->type == STT_SECTION &&
			    rela->sym->sec == func->sec &&
			    rela->addend == func->offset)
				return true;
			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;
}

/*
 * This checks to see if the given function is a "noreturn" function.
 *
 * For global functions which are outside the scope of this object file, we
 * have to keep a manual list of them.
 *
 * For local functions, we have to detect them manually by simply looking for
 * the lack of a return instruction.
 *
 * Returns:
 *  -1: error
 *   0: no dead end
 *   1: dead end
 */
static int __dead_end_function(struct objtool_file *file, struct symbol *func,
			       int recursion)
{
	int i;
	struct instruction *insn;
	bool empty = true;

	/*
	 * Unfortunately these have to be hard coded because the noreturn
	 * attribute isn't provided in ELF data.
	 */
	static const char * const global_noreturns[] = {
		"__stack_chk_fail",
		"panic",
		"do_exit",
		"do_task_dead",
		"__module_put_and_exit",
		"complete_and_exit",
		"kvm_spurious_fault",
		"__reiserfs_panic",
		"lbug_with_loc"
	};

	if (func->bind == STB_WEAK)
		return 0;

	if (func->bind == STB_GLOBAL)
		for (i = 0; i < ARRAY_SIZE(global_noreturns); i++)
			if (!strcmp(func->name, global_noreturns[i]))
				return 1;

	if (!func->sec)
		return 0;

	func_for_each_insn(file, func, insn) {
		empty = false;

		if (insn->type == INSN_RETURN)
			return 0;
	}

	if (empty)
		return 0;

	/*
	 * A function can have a sibling call instead of a return.  In that
	 * case, the function's dead-end status depends on whether the target
	 * of the sibling call returns.
	 */
	func_for_each_insn(file, func, insn) {
		if (insn->sec != func->sec ||
		    insn->offset >= func->offset + func->len)
			break;

		if (insn->type == INSN_JUMP_UNCONDITIONAL) {
			struct instruction *dest = insn->jump_dest;
			struct symbol *dest_func;

			if (!dest)
				/* sibling call to another file */
				return 0;

			if (dest->sec != func->sec ||
			    dest->offset < func->offset ||
			    dest->offset >= func->offset + func->len) {
				/* local sibling call */
				dest_func = find_symbol_by_offset(dest->sec,
								  dest->offset);
				if (!dest_func)
					continue;

				if (recursion == 5) {
					WARN_FUNC("infinite recursion (objtool bug!)",
						  dest->sec, dest->offset);
					return -1;
				}

				return __dead_end_function(file, dest_func,
							   recursion + 1);
			}
		}

		if (insn->type == INSN_JUMP_DYNAMIC && list_empty(&insn->alts))
			/* sibling call */
			return 0;
	}

	return 1;
}

static int dead_end_function(struct objtool_file *file, struct symbol *func)
{
	return __dead_end_function(file, func, 0);
}

/*
 * Call the arch-specific instruction decoder for all the instructions and add
 * them to the global instruction list.
 */
static int decode_instructions(struct objtool_file *file)
{
	struct section *sec;
	struct symbol *func;
	unsigned long offset;
	struct instruction *insn;
	int ret;

	list_for_each_entry(sec, &file->elf->sections, list) {

		if (!(sec->sh.sh_flags & SHF_EXECINSTR))
			continue;

		for (offset = 0; offset < sec->len; offset += insn->len) {
			insn = malloc(sizeof(*insn));
			memset(insn, 0, sizeof(*insn));

			INIT_LIST_HEAD(&insn->alts);
			insn->sec = sec;
			insn->offset = offset;

			ret = arch_decode_instruction(file->elf, sec, offset,
						      sec->len - offset,
						      &insn->len, &insn->type,
						      &insn->immediate);
			if (ret)
				return ret;

			if (!insn->type || insn->type > INSN_LAST) {
				WARN_FUNC("invalid instruction type %d",
					  insn->sec, insn->offset, insn->type);
				return -1;
			}

			hash_add(file->insn_hash, &insn->hash, insn->offset);
			list_add_tail(&insn->list, &file->insn_list);
		}

		list_for_each_entry(func, &sec->symbol_list, list) {
			if (func->type != STT_FUNC)
				continue;

			if (!find_insn(file, sec, func->offset)) {
				WARN("%s(): can't find starting instruction",
				     func->name);
				return -1;
			}

			func_for_each_insn(file, func, insn)
				if (!insn->func)
					insn->func = func;
		}
	}

	return 0;
}

/*
 * Warnings shouldn't be reported for ignored functions.
 */
static void add_ignores(struct objtool_file *file)
{
	struct instruction *insn;
	struct section *sec;
	struct symbol *func;

	list_for_each_entry(sec, &file->elf->sections, list) {
		list_for_each_entry(func, &sec->symbol_list, list) {
			if (func->type != STT_FUNC)
				continue;

			if (!ignore_func(file, func))
				continue;

			func_for_each_insn(file, func, insn)
				insn->visited = true;
		}
	}
}

/*
 * Find the destination instructions for all jumps.
 */
static int add_jump_destinations(struct objtool_file *file)
{
	struct instruction *insn;
	struct rela *rela;
	struct section *dest_sec;
	unsigned long dest_off;

	for_each_insn(file, insn) {
		if (insn->type != INSN_JUMP_CONDITIONAL &&
		    insn->type != INSN_JUMP_UNCONDITIONAL)
			continue;

		/* skip ignores */
		if (insn->visited)
			continue;

		rela = find_rela_by_dest_range(insn->sec, insn->offset,
					       insn->len);
		if (!rela) {
			dest_sec = insn->sec;
			dest_off = insn->offset + insn->len + insn->immediate;
		} else if (rela->sym->type == STT_SECTION) {
			dest_sec = rela->sym->sec;
			dest_off = rela->addend + 4;
		} else if (rela->sym->sec->idx) {
			dest_sec = rela->sym->sec;
			dest_off = rela->sym->sym.st_value + rela->addend + 4;
		} else {
			/* sibling call */
			insn->jump_dest = 0;
			continue;
		}

		insn->jump_dest = find_insn(file, dest_sec, dest_off);
		if (!insn->jump_dest) {

			/*
			 * This is a special case where an alt instruction
			 * jumps past the end of the section.  These are
			 * handled later in handle_group_alt().
			 */
			if (!strcmp(insn->sec->name, ".altinstr_replacement"))
				continue;

			WARN_FUNC("can't find jump dest instruction at %s+0x%lx",
				  insn->sec, insn->offset, dest_sec->name,
				  dest_off);
			return -1;
		}
	}

	return 0;
}

/*
 * Find the destination instructions for all calls.
 */
static int add_call_destinations(struct objtool_file *file)
{
	struct instruction *insn;
	unsigned long dest_off;
	struct rela *rela;

	for_each_insn(file, insn) {
		if (insn->type != INSN_CALL)
			continue;

		rela = find_rela_by_dest_range(insn->sec, insn->offset,
					       insn->len);
		if (!rela) {
			dest_off = insn->offset + insn->len + insn->immediate;
			insn->call_dest = find_symbol_by_offset(insn->sec,
								dest_off);
			if (!insn->call_dest) {
				WARN_FUNC("can't find call dest symbol at offset 0x%lx",
					  insn->sec, insn->offset, dest_off);
				return -1;
			}
		} else if (rela->sym->type == STT_SECTION) {
			insn->call_dest = find_symbol_by_offset(rela->sym->sec,
								rela->addend+4);
			if (!insn->call_dest ||
			    insn->call_dest->type != STT_FUNC) {
				WARN_FUNC("can't find call dest symbol at %s+0x%x",
					  insn->sec, insn->offset,
					  rela->sym->sec->name,
					  rela->addend + 4);
				return -1;
			}
		} else
			insn->call_dest = rela->sym;
	}

	return 0;
}

/*
 * The .alternatives section requires some extra special care, over and above
 * what other special sections require:
 *
 * 1. Because alternatives are patched in-place, we need to insert a fake jump
 *    instruction at the end so that validate_branch() skips all the original
 *    replaced instructions when validating the new instruction path.
 *
 * 2. An added wrinkle is that the new instruction length might be zero.  In
 *    that case the old instructions are replaced with noops.  We simulate that
 *    by creating a fake jump as the only new instruction.
 *
 * 3. In some cases, the alternative section includes an instruction which
 *    conditionally jumps to the _end_ of the entry.  We have to modify these
 *    jumps' destinations to point back to .text rather than the end of the
 *    entry in .altinstr_replacement.
 *
 * 4. It has been requested that we don't validate the !POPCNT feature path
 *    which is a "very very small percentage of machines".
 */
static int handle_group_alt(struct objtool_file *file,
			    struct special_alt *special_alt,
			    struct instruction *orig_insn,
			    struct instruction **new_insn)
{
	struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump;
	unsigned long dest_off;

	last_orig_insn = NULL;
	insn = orig_insn;
	sec_for_each_insn_from(file, insn) {
		if (insn->offset >= special_alt->orig_off + special_alt->orig_len)
			break;

		if (special_alt->skip_orig)
			insn->type = INSN_NOP;

		insn->alt_group = true;
		last_orig_insn = insn;
	}

	if (!next_insn_same_sec(file, last_orig_insn)) {
		WARN("%s: don't know how to handle alternatives at end of section",
		     special_alt->orig_sec->name);
		return -1;
	}

	fake_jump = malloc(sizeof(*fake_jump));
	if (!fake_jump) {
		WARN("malloc failed");
		return -1;
	}
	memset(fake_jump, 0, sizeof(*fake_jump));
	INIT_LIST_HEAD(&fake_jump->alts);
	fake_jump->sec = special_alt->new_sec;
	fake_jump->offset = -1;
	fake_jump->type = INSN_JUMP_UNCONDITIONAL;
	fake_jump->jump_dest = list_next_entry(last_orig_insn, list);

	if (!special_alt->new_len) {
		*new_insn = fake_jump;
		return 0;
	}

	last_new_insn = NULL;
	insn = *new_insn;
	sec_for_each_insn_from(file, insn) {
		if (insn->offset >= special_alt->new_off + special_alt->new_len)
			break;

		last_new_insn = insn;

		if (insn->type != INSN_JUMP_CONDITIONAL &&
		    insn->type != INSN_JUMP_UNCONDITIONAL)
			continue;

		if (!insn->immediate)
			continue;

		dest_off = insn->offset + insn->len + insn->immediate;
		if (dest_off == special_alt->new_off + special_alt->new_len)
			insn->jump_dest = fake_jump;

		if (!insn->jump_dest) {
			WARN_FUNC("can't find alternative jump destination",
				  insn->sec, insn->offset);
			return -1;
		}
	}

	if (!last_new_insn) {
		WARN_FUNC("can't find last new alternative instruction",
			  special_alt->new_sec, special_alt->new_off);
		return -1;
	}

	list_add(&fake_jump->list, &last_new_insn->list);

	return 0;
}

/*
 * A jump table entry can either convert a nop to a jump or a jump to a nop.
 * If the original instruction is a jump, make the alt entry an effective nop
 * by just skipping the original instruction.
 */
static int handle_jump_alt(struct objtool_file *file,
			   struct special_alt *special_alt,
			   struct instruction *orig_insn,
			   struct instruction **new_insn)
{
	if (orig_insn->type == INSN_NOP)
		return 0;

	if (orig_insn->type != INSN_JUMP_UNCONDITIONAL) {
		WARN_FUNC("unsupported instruction at jump label",
			  orig_insn->sec, orig_insn->offset);
		return -1;
	}

	*new_insn = list_next_entry(orig_insn, list);
	return 0;
}

/*
 * Read all the special sections which have alternate instructions which can be
 * patched in or redirected to at runtime.  Each instruction having alternate
 * instruction(s) has them added to its insn->alts list, which will be
 * traversed in validate_branch().
 */
static int add_special_section_alts(struct objtool_file *file)
{
	struct list_head special_alts;
	struct instruction *orig_insn, *new_insn;
	struct special_alt *special_alt, *tmp;
	struct alternative *alt;
	int ret;

	ret = special_get_alts(file->elf, &special_alts);
	if (ret)
		return ret;

	list_for_each_entry_safe(special_alt, tmp, &special_alts, list) {
		alt = malloc(sizeof(*alt));
		if (!alt) {
			WARN("malloc failed");
			ret = -1;
			goto out;
		}

		orig_insn = find_insn(file, special_alt->orig_sec,
				      special_alt->orig_off);
		if (!orig_insn) {
			WARN_FUNC("special: can't find orig instruction",
				  special_alt->orig_sec, special_alt->orig_off);
			ret = -1;
			goto out;
		}

		new_insn = NULL;
		if (!special_alt->group || special_alt->new_len) {
			new_insn = find_insn(file, special_alt->new_sec,
					     special_alt->new_off);
			if (!new_insn) {
				WARN_FUNC("special: can't find new instruction",
					  special_alt->new_sec,
					  special_alt->new_off);
				ret = -1;
				goto out;
			}
		}

		if (special_alt->group) {
			ret = handle_group_alt(file, special_alt, orig_insn,
					       &new_insn);
			if (ret)
				goto out;
		} else if (special_alt->jump_or_nop) {
			ret = handle_jump_alt(file, special_alt, orig_insn,
					      &new_insn);
			if (ret)
				goto out;
		}

		alt->insn = new_insn;
		list_add_tail(&alt->list, &orig_insn->alts);

		list_del(&special_alt->list);
		free(special_alt);
	}

out:
	return ret;
}

static int add_switch_table(struct objtool_file *file, struct symbol *func,
			    struct instruction *insn, struct rela *table,
			    struct rela *next_table)
{
	struct rela *rela = table;
	struct instruction *alt_insn;
	struct alternative *alt;

	list_for_each_entry_from(rela, &file->rodata->rela->rela_list, list) {
		if (rela == next_table)
			break;

		if (rela->sym->sec != insn->sec ||
		    rela->addend <= func->offset ||
		    rela->addend >= func->offset + func->len)
			break;

		alt_insn = find_insn(file, insn->sec, rela->addend);
		if (!alt_insn) {
			WARN("%s: can't find instruction at %s+0x%x",
			     file->rodata->rela->name, insn->sec->name,
			     rela->addend);
			return -1;
		}

		alt = malloc(sizeof(*alt));
		if (!alt) {
			WARN("malloc failed");
			return -1;
		}

		alt->insn = alt_insn;
		list_add_tail(&alt->list, &insn->alts);
	}

	return 0;
}

/*
 * find_switch_table() - Given a dynamic jump, find the switch jump table in
 * .rodata associated with it.
 *
 * There are 3 basic patterns:
 *
 * 1. jmpq *[rodata addr](,%reg,8)
 *
 *    This is the most common case by far.  It jumps to an address in a simple
 *    jump table which is stored in .rodata.
 *
 * 2. jmpq *[rodata addr](%rip)
 *
 *    This is caused by a rare GCC quirk, currently only seen in three driver
 *    functions in the kernel, only with certain obscure non-distro configs.
 *
 *    As part of an optimization, GCC makes a copy of an existing switch jump
 *    table, modifies it, and then hard-codes the jump (albeit with an indirect
 *    jump) to use a single entry in the table.  The rest of the jump table and
 *    some of its jump targets remain as dead code.
 *
 *    In such a case we can just crudely ignore all unreachable instruction
 *    warnings for the entire object file.  Ideally we would just ignore them
 *    for the function, but that would require redesigning the code quite a
 *    bit.  And honestly that's just not worth doing: unreachable instruction
 *    warnings are of questionable value anyway, and this is such a rare issue.
 *
 * 3. mov [rodata addr],%reg1
 *    ... some instructions ...
 *    jmpq *(%reg1,%reg2,8)
 *
 *    This is a fairly uncommon pattern which is new for GCC 6.  As of this
 *    writing, there are 11 occurrences of it in the allmodconfig kernel.
 *
 *    TODO: Once we have DWARF CFI and smarter instruction decoding logic,
 *    ensure the same register is used in the mov and jump instructions.
 */
static struct rela *find_switch_table(struct objtool_file *file,
				      struct symbol *func,
				      struct instruction *insn)
{
	struct rela *text_rela, *rodata_rela;
	struct instruction *orig_insn = insn;

	text_rela = find_rela_by_dest_range(insn->sec, insn->offset, insn->len);
	if (text_rela && text_rela->sym == file->rodata->sym) {
		/* case 1 */
		rodata_rela = find_rela_by_dest(file->rodata,
						text_rela->addend);
		if (rodata_rela)
			return rodata_rela;

		/* case 2 */
		rodata_rela = find_rela_by_dest(file->rodata,
						text_rela->addend + 4);
		if (!rodata_rela)
			return NULL;
		file->ignore_unreachables = true;
		return rodata_rela;
	}

	/* case 3 */
	func_for_each_insn_continue_reverse(file, func, insn) {
		if (insn->type == INSN_JUMP_DYNAMIC)
			break;

		/* allow small jumps within the range */
		if (insn->type == INSN_JUMP_UNCONDITIONAL &&
		    insn->jump_dest &&
		    (insn->jump_dest->offset <= insn->offset ||
		     insn->jump_dest->offset >= orig_insn->offset))
		    break;

		text_rela = find_rela_by_dest_range(insn->sec, insn->offset,
						    insn->len);
		if (text_rela && text_rela->sym == file->rodata->sym)
			return find_rela_by_dest(file->rodata,
						 text_rela->addend);
	}

	return NULL;
}

static int add_func_switch_tables(struct objtool_file *file,
				  struct symbol *func)
{
	struct instruction *insn, *prev_jump = NULL;
	struct rela *rela, *prev_rela = NULL;
	int ret;

	func_for_each_insn(file, func, insn) {
		if (insn->type != INSN_JUMP_DYNAMIC)
			continue;

		rela = find_switch_table(file, func, insn);
		if (!rela)
			continue;

		/*
		 * We found a switch 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.
		 */
		if (prev_jump) {
			ret = add_switch_table(file, func, prev_jump, prev_rela,
					       rela);
			if (ret)
				return ret;
		}

		prev_jump = insn;
		prev_rela = rela;
	}

	if (prev_jump) {
		ret = add_switch_table(file, func, prev_jump, prev_rela, NULL);
		if (ret)
			return ret;
	}

	return 0;
}

/*
 * For some switch statements, gcc generates a jump table in the .rodata
 * 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)
{
	struct section *sec;
	struct symbol *func;
	int ret;

	if (!file->rodata || !file->rodata->rela)
		return 0;

	list_for_each_entry(sec, &file->elf->sections, list) {
		list_for_each_entry(func, &sec->symbol_list, list) {
			if (func->type != STT_FUNC)
				continue;

			ret = add_func_switch_tables(file, func);
			if (ret)
				return ret;
		}
	}

	return 0;
}

static int decode_sections(struct objtool_file *file)
{
	int ret;

	ret = decode_instructions(file);
	if (ret)
		return ret;

	add_ignores(file);

	ret = add_jump_destinations(file);
	if (ret)
		return ret;

	ret = add_call_destinations(file);
	if (ret)
		return ret;

	ret = add_special_section_alts(file);
	if (ret)
		return ret;

	ret = add_switch_table_alts(file);
	if (ret)
		return ret;

	return 0;
}

static bool is_fentry_call(struct instruction *insn)
{
	if (insn->type == INSN_CALL &&
	    insn->call_dest->type == STT_NOTYPE &&
	    !strcmp(insn->call_dest->name, "__fentry__"))
		return true;

	return false;
}

static bool has_modified_stack_frame(struct instruction *insn)
{
	return (insn->state & STATE_FP_SAVED) ||
	       (insn->state & STATE_FP_SETUP);
}

static bool has_valid_stack_frame(struct instruction *insn)
{
	return (insn->state & STATE_FP_SAVED) &&
	       (insn->state & STATE_FP_SETUP);
}

static unsigned int frame_state(unsigned long state)
{
	return (state & (STATE_FP_SAVED | STATE_FP_SETUP));
}

/*
 * Follow the branch starting at the given instruction, and recursively follow
 * any other branches (jumps).  Meanwhile, track the frame pointer state at
 * each instruction and validate all the rules described in
 * tools/objtool/Documentation/stack-validation.txt.
 */
static int validate_branch(struct objtool_file *file,
			   struct instruction *first, unsigned char first_state)
{
	struct alternative *alt;
	struct instruction *insn;
	struct section *sec;
	struct symbol *func = NULL;
	unsigned char state;
	int ret;

	insn = first;
	sec = insn->sec;
	state = first_state;

	if (insn->alt_group && list_empty(&insn->alts)) {
		WARN_FUNC("don't know how to handle branch to middle of alternative instruction group",
			  sec, insn->offset);
		return 1;
	}

	while (1) {
		if (file->c_file && insn->func) {
			if (func && func != insn->func) {
				WARN("%s() falls through to next function %s()",
				     func->name, insn->func->name);
				return 1;
			}

			func = insn->func;
		}

		if (insn->visited) {
			if (frame_state(insn->state) != frame_state(state)) {
				WARN_FUNC("frame pointer state mismatch",
					  sec, insn->offset);
				return 1;
			}

			return 0;
		}

		insn->visited = true;
		insn->state = state;

		list_for_each_entry(alt, &insn->alts, list) {
			ret = validate_branch(file, alt->insn, state);
			if (ret)
				return 1;
		}

		switch (insn->type) {

		case INSN_FP_SAVE:
			if (!nofp) {
				if (state & STATE_FP_SAVED) {
					WARN_FUNC("duplicate frame pointer save",
						  sec, insn->offset);
					return 1;
				}
				state |= STATE_FP_SAVED;
			}
			break;

		case INSN_FP_SETUP:
			if (!nofp) {
				if (state & STATE_FP_SETUP) {
					WARN_FUNC("duplicate frame pointer setup",
						  sec, insn->offset);
					return 1;
				}
				state |= STATE_FP_SETUP;
			}
			break;

		case INSN_FP_RESTORE:
			if (!nofp) {
				if (has_valid_stack_frame(insn))
					state &= ~STATE_FP_SETUP;

				state &= ~STATE_FP_SAVED;
			}
			break;

		case INSN_RETURN:
			if (!nofp && has_modified_stack_frame(insn)) {
				WARN_FUNC("return without frame pointer restore",
					  sec, insn->offset);
				return 1;
			}
			return 0;

		case INSN_CALL:
			if (is_fentry_call(insn)) {
				state |= STATE_FENTRY;
				break;
			}

			ret = dead_end_function(file, insn->call_dest);
			if (ret == 1)
				return 0;
			if (ret == -1)
				return 1;

			/* fallthrough */
		case INSN_CALL_DYNAMIC:
			if (!nofp && !has_valid_stack_frame(insn)) {
				WARN_FUNC("call without frame pointer save/setup",
					  sec, insn->offset);
				return 1;
			}
			break;

		case INSN_JUMP_CONDITIONAL:
		case INSN_JUMP_UNCONDITIONAL:
			if (insn->jump_dest) {
				ret = validate_branch(file, insn->jump_dest,
						      state);
				if (ret)
					return 1;
			} else if (has_modified_stack_frame(insn)) {
				WARN_FUNC("sibling call from callable instruction with changed frame pointer",
					  sec, insn->offset);
				return 1;
			} /* else it's a sibling call */

			if (insn->type == INSN_JUMP_UNCONDITIONAL)
				return 0;

			break;

		case INSN_JUMP_DYNAMIC:
			if (list_empty(&insn->alts) &&
			    has_modified_stack_frame(insn)) {
				WARN_FUNC("sibling call from callable instruction with changed frame pointer",
					  sec, insn->offset);
				return 1;
			}

			return 0;

		case INSN_BUG:
			return 0;

		default:
			break;
		}

		insn = next_insn_same_sec(file, insn);
		if (!insn) {
			WARN("%s: unexpected end of section", sec->name);
			return 1;
		}
	}

	return 0;
}

static bool is_kasan_insn(struct instruction *insn)
{
	return (insn->type == INSN_CALL &&
		!strcmp(insn->call_dest->name, "__asan_handle_no_return"));
}

static bool is_ubsan_insn(struct instruction *insn)
{
	return (insn->type == INSN_CALL &&
		!strcmp(insn->call_dest->name,
			"__ubsan_handle_builtin_unreachable"));
}

static bool ignore_unreachable_insn(struct symbol *func,
				    struct instruction *insn)
{
	int i;

	if (insn->type == INSN_NOP)
		return true;

	/*
	 * Check if this (or a subsequent) instruction is related to
	 * CONFIG_UBSAN or CONFIG_KASAN.
	 *
	 * End the search at 5 instructions to avoid going into the weeds.
	 */
	for (i = 0; i < 5; i++) {

		if (is_kasan_insn(insn) || is_ubsan_insn(insn))
			return true;

		if (insn->type == INSN_JUMP_UNCONDITIONAL && insn->jump_dest) {
			insn = insn->jump_dest;
			continue;
		}

		if (insn->offset + insn->len >= func->offset + func->len)
			break;
		insn = list_next_entry(insn, list);
	}

	return false;
}

static int validate_functions(struct objtool_file *file)
{
	struct section *sec;
	struct symbol *func;
	struct instruction *insn;
	int ret, warnings = 0;

	list_for_each_entry(sec, &file->elf->sections, list) {
		list_for_each_entry(func, &sec->symbol_list, list) {
			if (func->type != STT_FUNC)
				continue;

			insn = find_insn(file, sec, func->offset);
			if (!insn)
				continue;

			ret = validate_branch(file, insn, 0);
			warnings += ret;
		}
	}

	list_for_each_entry(sec, &file->elf->sections, list) {
		list_for_each_entry(func, &sec->symbol_list, list) {
			if (func->type != STT_FUNC)
				continue;

			func_for_each_insn(file, func, insn) {
				if (insn->visited)
					continue;

				insn->visited = true;

				if (file->ignore_unreachables || warnings ||
				    ignore_unreachable_insn(func, insn))
					continue;

				/*
				 * gcov produces a lot of unreachable
				 * instructions.  If we get an unreachable
				 * warning and the file has gcov enabled, just
				 * ignore it, and all other such warnings for
				 * the file.
				 */
				if (!file->ignore_unreachables &&
				    gcov_enabled(file)) {
					file->ignore_unreachables = true;
					continue;
				}

				WARN_FUNC("function has unreachable instruction", insn->sec, insn->offset);
				warnings++;
			}
		}
	}

	return warnings;
}

static int validate_uncallable_instructions(struct objtool_file *file)
{
	struct instruction *insn;
	int warnings = 0;

	for_each_insn(file, insn) {
		if (!insn->visited && insn->type == INSN_RETURN) {
			WARN_FUNC("return instruction outside of a callable function",
				  insn->sec, insn->offset);
			warnings++;
		}
	}

	return warnings;
}

static void cleanup(struct objtool_file *file)
{
	struct instruction *insn, *tmpinsn;
	struct alternative *alt, *tmpalt;

	list_for_each_entry_safe(insn, tmpinsn, &file->insn_list, list) {
		list_for_each_entry_safe(alt, tmpalt, &insn->alts, list) {
			list_del(&alt->list);
			free(alt);
		}
		list_del(&insn->list);
		hash_del(&insn->hash);
		free(insn);
	}
	elf_close(file->elf);
}

const char * const check_usage[] = {
	"objtool check [<options>] file.o",
	NULL,
};

int cmd_check(int argc, const char **argv)
{
	struct objtool_file file;
	int ret, warnings = 0;

	const struct option options[] = {
		OPT_BOOLEAN('f', "no-fp", &nofp, "Skip frame pointer validation"),
		OPT_END(),
	};

	argc = parse_options(argc, argv, options, check_usage, 0);

	if (argc != 1)
		usage_with_options(check_usage, options);

	objname = argv[0];

	file.elf = elf_open(objname);
	if (!file.elf) {
		fprintf(stderr, "error reading elf file %s\n", objname);
		return 1;
	}

	INIT_LIST_HEAD(&file.insn_list);
	hash_init(file.insn_hash);
	file.whitelist = find_section_by_name(file.elf, "__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");

	ret = decode_sections(&file);
	if (ret < 0)
		goto out;
	warnings += ret;

	ret = validate_functions(&file);
	if (ret < 0)
		goto out;
	warnings += ret;

	ret = validate_uncallable_instructions(&file);
	if (ret < 0)
		goto out;
	warnings += ret;

out:
	cleanup(&file);

	/* ignore warnings for now until we get all the code cleaned up */
	if (ret || warnings)
		return 0;
	return 0;
}