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Merge branch 'core-urgent-for-linus' of git://git.kernel.org/pub/scm/…
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…linux/kernel/git/tip/tip

Pull objtool fixes from Ingo Molnar:
 "A handful of objtool fixes: two improvements to how warnings are
  printed plus a false positive warning fix, and build environment fix"

* 'core-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  objtool: Fix Makefile to properly see if libelf is supported
  objtool: Detect falling through to the next function
  objtool: Add workaround for GCC switch jump table bug
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Linus Torvalds committed Apr 23, 2016
2 parents 68dc08b + c2bb9e3 commit 6527efb
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Showing 3 changed files with 103 additions and 35 deletions.
3 changes: 2 additions & 1 deletion Makefile
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Original file line number Diff line number Diff line change
Expand Up @@ -1008,7 +1008,8 @@ prepare0: archprepare FORCE
prepare: prepare0 prepare-objtool

ifdef CONFIG_STACK_VALIDATION
has_libelf := $(shell echo "int main() {}" | $(HOSTCC) -xc -o /dev/null -lelf - &> /dev/null && echo 1 || echo 0)
has_libelf := $(call try-run,\
echo "int main() {}" | $(HOSTCC) -xc -o /dev/null -lelf -,1,0)
ifeq ($(has_libelf),1)
objtool_target := tools/objtool FORCE
else
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38 changes: 29 additions & 9 deletions tools/objtool/Documentation/stack-validation.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -299,18 +299,38 @@ they mean, and suggestions for how to fix them.
Errors in .c files
------------------

If you're getting an objtool error in a compiled .c file, chances are
the file uses an asm() statement which has a "call" instruction. An
asm() statement with a call instruction must declare the use of the
stack pointer in its output operand. For example, on x86_64:
1. c_file.o: warning: objtool: funcA() falls through to next function funcB()

register void *__sp asm("rsp");
asm volatile("call func" : "+r" (__sp));
This means that funcA() doesn't end with a return instruction or an
unconditional jump, and that objtool has determined that the function
can fall through into the next function. There could be different
reasons for this:

Otherwise the stack frame may not get created before the call.
1) funcA()'s last instruction is a call to a "noreturn" function like
panic(). In this case the noreturn function needs to be added to
objtool's hard-coded global_noreturns array. Feel free to bug the
objtool maintainer, or you can submit a patch.

Another possible cause for errors in C code is if the Makefile removes
-fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options.
2) funcA() uses the unreachable() annotation in a section of code
that is actually reachable.

3) If funcA() calls an inline function, the object code for funcA()
might be corrupt due to a gcc bug. For more details, see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646

2. If you're getting any other objtool error in a compiled .c file, it
may be because the file uses an asm() statement which has a "call"
instruction. An asm() statement with a call instruction must declare
the use of the stack pointer in its output operand. For example, on
x86_64:

register void *__sp asm("rsp");
asm volatile("call func" : "+r" (__sp));

Otherwise the stack frame may not get created before the call.

3. Another possible cause for errors in C code is if the Makefile removes
-fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options.

Also see the above section for .S file errors for more information what
the individual error messages mean.
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97 changes: 72 additions & 25 deletions tools/objtool/builtin-check.c
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Original file line number Diff line number Diff line change
Expand Up @@ -54,6 +54,7 @@ struct instruction {
struct symbol *call_dest;
struct instruction *jump_dest;
struct list_head alts;
struct symbol *func;
};

struct alternative {
Expand All @@ -66,6 +67,7 @@ struct objtool_file {
struct list_head insn_list;
DECLARE_HASHTABLE(insn_hash, 16);
struct section *rodata, *whitelist;
bool ignore_unreachables, c_file;
};

const char *objname;
Expand Down Expand Up @@ -228,7 +230,7 @@ static int __dead_end_function(struct objtool_file *file, struct symbol *func,
}
}

if (insn->type == INSN_JUMP_DYNAMIC)
if (insn->type == INSN_JUMP_DYNAMIC && list_empty(&insn->alts))
/* sibling call */
return 0;
}
Expand All @@ -248,6 +250,7 @@ static int dead_end_function(struct objtool_file *file, struct symbol *func)
static int decode_instructions(struct objtool_file *file)
{
struct section *sec;
struct symbol *func;
unsigned long offset;
struct instruction *insn;
int ret;
Expand Down Expand Up @@ -281,6 +284,21 @@ static int decode_instructions(struct objtool_file *file)
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;
Expand Down Expand Up @@ -664,13 +682,40 @@ static int add_func_switch_tables(struct objtool_file *file,
text_rela->addend);

/*
* TODO: Document where this is needed, or get rid of it.
*
* rare case: jmpq *[addr](%rip)
*
* This check is for 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.
*
* kbuild reports:
* - https://lkml.kernel.org/r/201603231906.LWcVUpxm%25fengguang.wu@intel.com
* - https://lkml.kernel.org/r/201603271114.K9i45biy%25fengguang.wu@intel.com
* - https://lkml.kernel.org/r/201603291058.zuJ6ben1%25fengguang.wu@intel.com
*
* gcc bug:
* - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70604
*/
if (!rodata_rela)
if (!rodata_rela) {
rodata_rela = find_rela_by_dest(file->rodata,
text_rela->addend + 4);
if (rodata_rela)
file->ignore_unreachables = true;
}

if (!rodata_rela)
continue;
Expand Down Expand Up @@ -732,9 +777,6 @@ static int decode_sections(struct objtool_file *file)
{
int ret;

file->whitelist = find_section_by_name(file->elf, "__func_stack_frame_non_standard");
file->rodata = find_section_by_name(file->elf, ".rodata");

ret = decode_instructions(file);
if (ret)
return ret;
Expand Down Expand Up @@ -799,6 +841,7 @@ static int validate_branch(struct objtool_file *file,
struct alternative *alt;
struct instruction *insn;
struct section *sec;
struct symbol *func = NULL;
unsigned char state;
int ret;

Expand All @@ -813,6 +856,16 @@ static int validate_branch(struct objtool_file *file,
}

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",
Expand All @@ -823,13 +876,6 @@ static int validate_branch(struct objtool_file *file,
return 0;
}

/*
* Catch a rare case where a noreturn function falls through to
* the next function.
*/
if (is_fentry_call(insn) && (state & STATE_FENTRY))
return 0;

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

Expand Down Expand Up @@ -1035,12 +1081,8 @@ static int validate_functions(struct objtool_file *file)
continue;

insn = find_insn(file, sec, func->offset);
if (!insn) {
WARN("%s(): can't find starting instruction",
func->name);
warnings++;
if (!insn)
continue;
}

ret = validate_branch(file, insn, 0);
warnings += ret;
Expand All @@ -1056,13 +1098,14 @@ static int validate_functions(struct objtool_file *file)
if (insn->visited)
continue;

if (!ignore_unreachable_insn(func, insn) &&
!warnings) {
WARN_FUNC("function has unreachable instruction", insn->sec, insn->offset);
warnings++;
}

insn->visited = true;

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

WARN_FUNC("function has unreachable instruction", insn->sec, insn->offset);
warnings++;
}
}
}
Expand Down Expand Up @@ -1133,6 +1176,10 @@ int cmd_check(int argc, const char **argv)

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)
Expand Down

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