[PATCH 2/2] objtool: Fix function fallthrough detection

[Josh Poimboeuf]

When a function falls through to the next function due to a compiler
bug, objtool prints some obscure warnings.  For example:

  drivers/regulator/core.o: warning: objtool: regulator_count_voltages()+0x95: return with modified stack frame
  drivers/regulator/core.o: warning: objtool: regulator_count_voltages()+0x0: stack state mismatch: cfa1=7+32 cfa2=7+8

Instead it should be printing:

  drivers/regulator/core.o: warning: objtool: regulator_supply_is_couple() falls through to next function regulator_count_voltages()

This used to work, but was broken by the following commit:

  13810435b9a7 ("objtool: Support GCC 8's cold subfunctions").

The padding nops at the end of a function aren't actually part of the
function, as defined by the symbol table.  So the 'func' variable in
validate_branch() is getting cleared to NULL when a padding nop is
encountered, breaking the fallthrough detection.

If the current instruction doesn't have a function associated with it,
just consider it to be part of the previously detected function by not
overwriting the previous value of 'func'.

Fixes: 13810435b9a7 ("objtool: Support GCC 8's cold subfunctions")
Reported-by: kbuild test robot <lkp@xxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 tools/objtool/check.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 90226791df6b..7325d89ccad9 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1959,7 +1959,8 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 			return 1;
 		}
 
-		func = insn->func ? insn->func->pfunc : NULL;
+		if (insn->func)
+			func = insn->func->pfunc;
 
 		if (func && insn->ignore) {
 			WARN_FUNC("BUG: why am I validating an ignored function?",
-- 
2.17.2