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check.c
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// SPDX-License-Identifier: GPL-2.0-or-later
/*
* Copyright (C) 2015-2017 Josh Poimboeuf <jpoimboe@redhat.com>
*/
#include <string.h>
#include <stdlib.h>
#include "builtin.h"
#include "check.h"
#include "elf.h"
#include "special.h"
#include "arch.h"
#include "warn.h"
#include <linux/hashtable.h>
#include <linux/kernel.h>
#define FAKE_JUMP_OFFSET -1
#define C_JUMP_TABLE_SECTION ".rodata..c_jump_table"
struct alternative {
struct list_head list;
struct instruction *insn;
bool skip_orig;
};
const char *objname;
struct cfi_state initial_func_cfi;
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 || &next->list == &file->insn_list || next->sec != insn->sec)
return NULL;
return next;
}
static struct instruction *next_insn_same_func(struct objtool_file *file,
struct instruction *insn)
{
struct instruction *next = list_next_entry(insn, list);
struct symbol *func = insn->func;
if (!func)
return NULL;
if (&next->list != &file->insn_list && next->func == func)
return next;
/* Check if we're already in the subfunction: */
if (func == func->cfunc)
return NULL;
/* Move to the subfunction: */
return find_insn(file, func->cfunc->sec, func->cfunc->offset);
}
#define func_for_each_insn_all(file, func, insn) \
for (insn = find_insn(file, func->sec, func->offset); \
insn; \
insn = next_insn_same_func(file, insn))
#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))
#define sec_for_each_insn_continue(file, insn) \
for (insn = next_insn_same_sec(file, insn); insn; \
insn = next_insn_same_sec(file, insn))
static bool is_sibling_call(struct instruction *insn)
{
/* An indirect jump is either a sibling call or a jump to a table. */
if (insn->type == INSN_JUMP_DYNAMIC)
return list_empty(&insn->alts);
if (insn->type != INSN_JUMP_CONDITIONAL &&
insn->type != INSN_JUMP_UNCONDITIONAL)
return false;
/* add_jump_destinations() sets insn->call_dest for sibling calls. */
return !!insn->call_dest;
}
/*
* 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.
*/
static bool __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",
"__reiserfs_panic",
"lbug_with_loc",
"fortify_panic",
"usercopy_abort",
"machine_real_restart",
"rewind_stack_do_exit",
};
if (!func)
return false;
if (func->bind == STB_WEAK)
return false;
if (func->bind == STB_GLOBAL)
for (i = 0; i < ARRAY_SIZE(global_noreturns); i++)
if (!strcmp(func->name, global_noreturns[i]))
return true;
if (!func->len)
return false;
insn = find_insn(file, func->sec, func->offset);
if (!insn->func)
return false;
func_for_each_insn_all(file, func, insn) {
empty = false;
if (insn->type == INSN_RETURN)
return false;
}
if (empty)
return false;
/*
* 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_all(file, func, insn) {
if (is_sibling_call(insn)) {
struct instruction *dest = insn->jump_dest;
if (!dest)
/* sibling call to another file */
return false;
/* local sibling call */
if (recursion == 5) {
/*
* Infinite recursion: two functions have
* sibling calls to each other. This is a very
* rare case. It means they aren't dead ends.
*/
return false;
}
return __dead_end_function(file, dest->func, recursion+1);
}
}
return true;
}
static bool dead_end_function(struct objtool_file *file, struct symbol *func)
{
return __dead_end_function(file, func, 0);
}
static void clear_insn_state(struct insn_state *state)
{
int i;
memset(state, 0, sizeof(*state));
state->cfa.base = CFI_UNDEFINED;
for (i = 0; i < CFI_NUM_REGS; i++) {
state->regs[i].base = CFI_UNDEFINED;
state->vals[i].base = CFI_UNDEFINED;
}
state->drap_reg = CFI_UNDEFINED;
state->drap_offset = -1;
}
/*
* 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;
for_each_sec(file, sec) {
if (!(sec->sh.sh_flags & SHF_EXECINSTR))
continue;
if (strcmp(sec->name, ".altinstr_replacement") &&
strcmp(sec->name, ".altinstr_aux") &&
strncmp(sec->name, ".discard.", 9))
sec->text = true;
for (offset = 0; offset < sec->len; offset += insn->len) {
insn = malloc(sizeof(*insn));
if (!insn) {
WARN("malloc failed");
return -1;
}
memset(insn, 0, sizeof(*insn));
INIT_LIST_HEAD(&insn->alts);
clear_insn_state(&insn->state);
insn->sec = sec;
insn->offset = offset;
ret = arch_decode_instruction(file->elf, sec, offset,
sec->len - offset,
&insn->len, &insn->type,
&insn->immediate,
&insn->stack_op);
if (ret)
goto err;
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 || func->alias != 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)
insn->func = func;
}
}
return 0;
err:
free(insn);
return ret;
}
/*
* Mark "ud2" instructions and manually annotated dead ends.
*/
static int add_dead_ends(struct objtool_file *file)
{
struct section *sec;
struct rela *rela;
struct instruction *insn;
bool found;
/*
* By default, "ud2" is a dead end unless otherwise annotated, because
* GCC 7 inserts it for certain divide-by-zero cases.
*/
for_each_insn(file, insn)
if (insn->type == INSN_BUG)
insn->dead_end = true;
/*
* Check for manually annotated dead ends.
*/
sec = find_section_by_name(file->elf, ".rela.discard.unreachable");
if (!sec)
goto reachable;
list_for_each_entry(rela, &sec->rela_list, list) {
if (rela->sym->type != STT_SECTION) {
WARN("unexpected relocation symbol type in %s", sec->name);
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;
}
reachable:
/*
* These manually annotated reachable checks are needed for GCC 4.4,
* where the Linux unreachable() macro isn't supported. In that case
* GCC doesn't know the "ud2" is fatal, so it generates code as if it's
* not a dead end.
*/
sec = find_section_by_name(file->elf, ".rela.discard.reachable");
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 %s", sec->name);
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 reachable insn at %s+0x%x",
rela->sym->sec->name, rela->addend);
return -1;
}
} else {
WARN("can't find reachable insn at %s+0x%x",
rela->sym->sec->name, rela->addend);
return -1;
}
insn->dead_end = false;
}
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;
struct rela *rela;
sec = find_section_by_name(file->elf, ".rela.discard.func_stack_frame_non_standard");
if (!sec)
return;
list_for_each_entry(rela, &sec->rela_list, list) {
switch (rela->sym->type) {
case STT_FUNC:
func = rela->sym;
break;
case STT_SECTION:
func = find_symbol_by_offset(rela->sym->sec, rela->addend);
if (!func || func->type != STT_FUNC)
continue;
break;
default:
WARN("unexpected relocation symbol type in %s: %d", sec->name, rela->sym->type);
continue;
}
func_for_each_insn_all(file, func, insn)
insn->ignore = true;
}
}
/*
* This is a whitelist of functions that is allowed to be called with AC set.
* The list is meant to be minimal and only contains compiler instrumentation
* ABI and a few functions used to implement *_{to,from}_user() functions.
*
* These functions must not directly change AC, but may PUSHF/POPF.
*/
static const char *uaccess_safe_builtin[] = {
/* KASAN */
"kasan_report",
"check_memory_region",
/* KASAN out-of-line */
"__asan_loadN_noabort",
"__asan_load1_noabort",
"__asan_load2_noabort",
"__asan_load4_noabort",
"__asan_load8_noabort",
"__asan_load16_noabort",
"__asan_storeN_noabort",
"__asan_store1_noabort",
"__asan_store2_noabort",
"__asan_store4_noabort",
"__asan_store8_noabort",
"__asan_store16_noabort",
/* KASAN in-line */
"__asan_report_load_n_noabort",
"__asan_report_load1_noabort",
"__asan_report_load2_noabort",
"__asan_report_load4_noabort",
"__asan_report_load8_noabort",
"__asan_report_load16_noabort",
"__asan_report_store_n_noabort",
"__asan_report_store1_noabort",
"__asan_report_store2_noabort",
"__asan_report_store4_noabort",
"__asan_report_store8_noabort",
"__asan_report_store16_noabort",
/* KCOV */
"write_comp_data",
"__sanitizer_cov_trace_pc",
"__sanitizer_cov_trace_const_cmp1",
"__sanitizer_cov_trace_const_cmp2",
"__sanitizer_cov_trace_const_cmp4",
"__sanitizer_cov_trace_const_cmp8",
"__sanitizer_cov_trace_cmp1",
"__sanitizer_cov_trace_cmp2",
"__sanitizer_cov_trace_cmp4",
"__sanitizer_cov_trace_cmp8",
/* UBSAN */
"ubsan_type_mismatch_common",
"__ubsan_handle_type_mismatch",
"__ubsan_handle_type_mismatch_v1",
"__ubsan_handle_shift_out_of_bounds",
/* misc */
"csum_partial_copy_generic",
"__memcpy_mcsafe",
"mcsafe_handle_tail",
"ftrace_likely_update", /* CONFIG_TRACE_BRANCH_PROFILING */
NULL
};
static void add_uaccess_safe(struct objtool_file *file)
{
struct symbol *func;
const char **name;
if (!uaccess)
return;
for (name = uaccess_safe_builtin; *name; name++) {
func = find_symbol_by_name(file->elf, *name);
if (!func)
continue;
func->uaccess_safe = true;
}
}
/*
* FIXME: For now, just ignore any alternatives which add retpolines. This is
* a temporary hack, as it doesn't allow ORC to unwind from inside a retpoline.
* But it at least allows objtool to understand the control flow *around* the
* retpoline.
*/
static int add_ignore_alternatives(struct objtool_file *file)
{
struct section *sec;
struct rela *rela;
struct instruction *insn;
sec = find_section_by_name(file->elf, ".rela.discard.ignore_alts");
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 %s", sec->name);
return -1;
}
insn = find_insn(file, rela->sym->sec, rela->addend);
if (!insn) {
WARN("bad .discard.ignore_alts entry");
return -1;
}
insn->ignore_alts = true;
}
return 0;
}
/*
* 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;
if (insn->ignore || insn->offset == FAKE_JUMP_OFFSET)
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 if (strstr(rela->sym->name, "_indirect_thunk_")) {
/*
* Retpoline jumps are really dynamic jumps in
* disguise, so convert them accordingly.
*/
if (insn->type == INSN_JUMP_UNCONDITIONAL)
insn->type = INSN_JUMP_DYNAMIC;
else
insn->type = INSN_JUMP_DYNAMIC_CONDITIONAL;
insn->retpoline_safe = true;
continue;
} else {
/* external sibling call */
insn->call_dest = rela->sym;
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;
}
/*
* Cross-function jump.
*/
if (insn->func && insn->jump_dest->func &&
insn->func != insn->jump_dest->func) {
/*
* For GCC 8+, create parent/child links for any cold
* subfunctions. This is _mostly_ redundant with a
* similar initialization in read_symbols().
*
* If a function has aliases, we want the *first* such
* function in the symbol table to be the subfunction's
* parent. In that case we overwrite the
* initialization done in read_symbols().
*
* However this code can't completely replace the
* read_symbols() code because this doesn't detect the
* case where the parent function's only reference to a
* subfunction is through a jump table.
*/
if (!strstr(insn->func->name, ".cold.") &&
strstr(insn->jump_dest->func->name, ".cold.")) {
insn->func->cfunc = insn->jump_dest->func;
insn->jump_dest->func->pfunc = insn->func;
} else if (insn->jump_dest->func->pfunc != insn->func->pfunc &&
insn->jump_dest->offset == insn->jump_dest->func->offset) {
/* internal sibling call */
insn->call_dest = insn->jump_dest->func;
}
}
}
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 && !insn->ignore) {
WARN_FUNC("unsupported intra-function call",
insn->sec, insn->offset);
if (retpoline)
WARN("If this is a retpoline, please patch it in with alternatives and annotate it with ANNOTATE_NOSPEC_ALTERNATIVE.");
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.
*/
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 = NULL;
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;
insn->alt_group = true;
last_orig_insn = insn;
}
if (next_insn_same_sec(file, last_orig_insn)) {
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);
clear_insn_state(&fake_jump->state);
fake_jump->sec = special_alt->new_sec;
fake_jump->offset = FAKE_JUMP_OFFSET;
fake_jump->type = INSN_JUMP_UNCONDITIONAL;
fake_jump->jump_dest = list_next_entry(last_orig_insn, list);
fake_jump->func = orig_insn->func;
}
if (!special_alt->new_len) {
if (!fake_jump) {
WARN("%s: empty alternative at end of section",
special_alt->orig_sec->name);
return -1;
}
*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;
insn->ignore = orig_insn->ignore_alts;
insn->func = orig_insn->func;
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) {
if (!fake_jump) {
WARN("%s: alternative jump to end of section",
special_alt->orig_sec->name);
return -1;
}
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;
}
if (fake_jump)
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) {
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 = malloc(sizeof(*alt));
if (!alt) {
WARN("malloc failed");
ret = -1;
goto out;
}
alt->insn = new_insn;
alt->skip_orig = special_alt->skip_orig;
orig_insn->ignore_alts |= special_alt->skip_alt;
list_add_tail(&alt->list, &orig_insn->alts);
list_del(&special_alt->list);
free(special_alt);
}
out:
return ret;
}
static int add_jump_table(struct objtool_file *file, struct instruction *insn,
struct rela *table)
{
struct rela *rela = table;
struct instruction *dest_insn;
struct alternative *alt;
struct symbol *pfunc = insn->func->pfunc;
unsigned int prev_offset = 0;
/*
* Each @rela is a switch table relocation which points to the target
* instruction.
*/
list_for_each_entry_from(rela, &table->sec->rela_list, list) {
/* Check for the end of the table: */
if (rela != table && rela->jump_table_start)
break;
/* Make sure the table entries are consecutive: */
if (prev_offset && rela->offset != prev_offset + 8)
break;
/* Detect function pointers from contiguous objects: */
if (rela->sym->sec == pfunc->sec &&
rela->addend == pfunc->offset)
break;
dest_insn = find_insn(file, rela->sym->sec, rela->addend);
if (!dest_insn)
break;
/* Make sure the destination is in the same function: */
if (!dest_insn->func || dest_insn->func->pfunc != pfunc)
break;
alt = malloc(sizeof(*alt));
if (!alt) {
WARN("malloc failed");
return -1;
}
alt->insn = dest_insn;
list_add_tail(&alt->list, &insn->alts);
prev_offset = rela->offset;
}
if (!prev_offset) {
WARN_FUNC("can't find switch jump table",
insn->sec, insn->offset);
return -1;
}
return 0;
}
/*
* find_jump_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.
*
* As of GCC 7 there are quite a few more of these and the 'in between' code
* is significant. Esp. with KASAN enabled some of the code between the mov
* and jmpq uses .rodata itself, which can confuse things.
*
* TODO: Once we have DWARF CFI and smarter instruction decoding logic,
* ensure the same register is used in the mov and jump instructions.
*
* NOTE: RETPOLINE made it harder still to decode dynamic jumps.
*/
static struct rela *find_jump_table(struct objtool_file *file,
struct symbol *func,
struct instruction *insn)