[PATCH] objtool/x86: Fixup frame-pointer vs rethunk

[Peter Zijlstra]

On Wed, Aug 16, 2023 at 01:31:52PM -0700, Josh Poimboeuf wrote:
> On Wed, Aug 16, 2023 at 01:59:21PM +0200, Peter Zijlstra wrote:
> > Turns out I forgot to build with FRAME_POINTER=y, that still gives:
> > 
> > vmlinux.o: warning: objtool: srso_untrain_ret+0xd: call without frame pointer save/setup
> > 
> > the below seems to cure this.
> 
> LGTM

OK, with Changelog below.

---
Subject: objtool/x86: Fixup frame-pointer vs rethunk
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Wed, 16 Aug 2023 13:59:21 +0200

For stack-validation of a frame-pointer build, objtool validates that
every CALL instructions is preceded by a frame-setup. The new SRSO
return thunks violate this with their RSB stuffing trickery.

Extend the __fentry__ exception to also cover the embedded_insn case
used for this. This cures:

vmlinux.o: warning: objtool: srso_untrain_ret+0xd: call without frame pointer save/setup

Fixes: 4ae68b26c3ab ("objtool/x86: Fix SRSO mess")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 tools/objtool/check.c |   17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2630,12 +2630,17 @@ static int decode_sections(struct objtoo
 	return 0;
 }
 
-static bool is_fentry_call(struct instruction *insn)
+static bool is_special_call(struct instruction *insn)
 {
-	if (insn->type == INSN_CALL &&
-	    insn_call_dest(insn) &&
-	    insn_call_dest(insn)->fentry)
-		return true;
+	if (insn->type == INSN_CALL) {
+		struct symbol *dest = insn_call_dest(insn);
+
+		if (!dest)
+			return false;
+
+		if (dest->fentry || dest->embedded_insn)
+			return true;
+	}
 
 	return false;
 }
@@ -3636,7 +3641,7 @@ static int validate_branch(struct objtoo
 			if (ret)
 				return ret;
 
-			if (opts.stackval && func && !is_fentry_call(insn) &&
+			if (opts.stackval && func && !is_special_call(insn) &&
 			    !has_valid_stack_frame(&state)) {
 				WARN_INSN(insn, "call without frame pointer save/setup");
 				return 1;