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Commit b1547d31 authored by Josh Poimboeuf's avatar Josh Poimboeuf Committed by Ingo Molnar
Browse files

objtool: Detect falling through to the next function 



There are several cases in compiled C code where a function may not
return at the end, and may instead fall through to the next function.

That may indicate a bug in the code, or a gcc bug, or even an objtool
bug.  But in each case, objtool reports an unhelpful warning, something
like:

  drivers/scsi/qla2xxx/qla_attr.o: warning: objtool: qla2x00_get_fc_host_stats()+0x0: duplicate frame pointer save
  drivers/scsi/qla2xxx/qla_attr.o: warning: objtool: qla2x00_get_fc_host_stats()+0x0: frame pointer state mismatch

Detect this situation and print a more useful error message:

  drivers/scsi/qla2xxx/qla_attr.o: warning: objtool: qla2x00_get_host_fabric_name() falls through to next function qla2x00_get_starget_node_name()

Also add some information about this warning and its potential causes to
the documentation.

Reported-by: default avatarkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
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 Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/caa4ec6c687931db805e692d4e4bf06cd87d33e6.1460729697.git.jpoimboe@redhat.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 7e578441

tools/objtool/Documentation/stack-validation.txt

+29 −9
Original line number Diff line number Diff line
@@ -299,17 +299,37 @@ 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()



   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:



   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.



   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.



Another possible cause for errors in C code is if the Makefile removes

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

tools/objtool/builtin-check.c

+32 −14
Original line number Diff line number Diff line
@@ -54,6 +54,7 @@ struct instruction { 
	struct symbol *call_dest;

	struct instruction *jump_dest;

	struct list_head alts;

	struct symbol *func;

};



struct alternative {
@@ -66,7 +67,7 @@ struct objtool_file { 
	struct list_head insn_list;

	DECLARE_HASHTABLE(insn_hash, 16);

	struct section *rodata, *whitelist;

	bool ignore_unreachables;

	bool ignore_unreachables, c_file;

};



const char *objname;
@@ -229,7 +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;

	}
@@ -249,6 +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;
@@ -282,6 +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;
@@ -824,6 +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;
@@ -838,6 +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",
@@ -848,13 +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;
@@ -1060,12 +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;
@@ -1162,6 +1179,7 @@ int cmd_check(int argc, const char **argv) 
	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)