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.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 61 insertions, 23 deletions

diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
index 5a95896105bc..55a60d331f47 100644
--- a/tools/objtool/Documentation/stack-validation.txt
+++ b/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()
 
-   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.
diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
index 157a0f96d64d..e8a1e69eb92c 100644
--- a/tools/objtool/builtin-check.c
+++ b/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)