linux-kernel - Re: x86 entry perf unwinding failure (missing IRET

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Message-ID: <20200428152552.GD13592@hirez.programming.kicks-ass.net>
Date:   Tue, 28 Apr 2020 17:25:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        alexandre.chartre@...cle.com
Subject: Re: x86 entry perf unwinding failure (missing IRET_REGS annotation
 on stack switch?)

On Tue, Apr 28, 2020 at 09:31:57AM -0500, Josh Poimboeuf wrote:
> That's quite the monstrosity, and I still don't see the point.  I
> thought we decided to just disallow CFI changes in alternatives anyway?
> That can be done much simpler.

Something like so then ?

---
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 8443ec690051..d14d83e6edb0 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -940,6 +940,7 @@ static int handle_group_alt(struct objtool_file *file,
 
 		last_new_insn = insn;
 
+		insn->alt_group = true;
 		insn->ignore = orig_insn->ignore_alts;
 		insn->func = orig_insn->func;
 
@@ -2242,6 +2243,11 @@ static int handle_insn_ops(struct instruction *insn, struct insn_state *state)
 	list_for_each_entry(op, &insn->stack_ops, list) {
 		int res;
 
+		if (insn->alt_group) {
+			WARN_FUNC("alternative has CFI", insn->sec, insn->offset);
+			return 1;
+		}
+
 		res = update_cfi_state(insn, &state->cfi, op);
 		if (res)
 			return res;
@@ -2439,12 +2445,6 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 	sec = insn->sec;
 
-	if (insn->alt_group && list_empty(&insn->alts)) {
-		WARN_FUNC("don't know how to handle branch to middle of alternative instruction group",
-			  sec, insn->offset);
-		return 1;
-	}
-
 	while (1) {
 		next_insn = next_insn_same_sec(file, insn);
 
@@ -2494,8 +2494,16 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 				}
 			}
 
-			if (skip_orig)
+			if (skip_orig) {
+				struct instruction *prev_insn = insn;
+				sec_for_each_insn_continue(file, insn) {
+					if (!insn->alt_group)
+						break;
+					if (!insn->visited)
+						insn->cfi = prev_insn->cfi;
+				}
 				return 0;
+			}
 		}
 
 		if (handle_insn_ops(insn, &state))