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: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Josh 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.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/Documentation/stack-validation.txt

@@ -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()
+
+   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.
+   Otherwise the stack frame may not get created before the call.
 
-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.
+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.

tools/objtool/builtin-check.c

@@ -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)