linux-kernel - [PATCH v4 01/13] objtool: Remove CFI save/restore special case

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Date:   Wed, 25 Mar 2020 18:45:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     tglx@...utronix.de, jpoimboe@...hat.com
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, peterz@...radead.org,
        mhiramat@...nel.org, mbenes@...e.cz
Subject: [PATCH v4 01/13] objtool: Remove CFI save/restore special case

There is a special case in the UNWIND_HINT_RESTORE code. When, upon
looking for the UNWIND_HINT_SAVE instruction to restore from, it finds
the instruction hasn't been visited yet, it normally issues a WARN,
except when this HINT_SAVE instruction is the first instruction of
this branch.

This special case is of dubious correctness and is certainly unused
(verified with an allmodconfig build), the two sites that employ
UNWIND_HINT_SAVE/RESTORE (sync_core() and ftrace_regs_caller()) have
the SAVE on unconditional instructions at the start of the function.
It is therefore impossible for the save_insn not to have been visited
when we do hit the RESTORE.

Remove this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 tools/objtool/check.c |   15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2040,15 +2040,14 @@ static int validate_return(struct symbol
  * tools/objtool/Documentation/stack-validation.txt.
  */
 static int validate_branch(struct objtool_file *file, struct symbol *func,
-			   struct instruction *first, struct insn_state state)
+			   struct instruction *insn, struct insn_state state)
 {
+	struct instruction *next_insn;
 	struct alternative *alt;
-	struct instruction *insn, *next_insn;
 	struct section *sec;
 	u8 visited;
 	int ret;
 
-	insn = first;
 	sec = insn->sec;
 
 	if (insn->alt_group && list_empty(&insn->alts)) {
@@ -2101,16 +2100,6 @@ static int validate_branch(struct objtoo
 				}
 
 				if (!save_insn->visited) {
-					/*
-					 * Oops, no state to copy yet.
-					 * Hopefully we can reach this
-					 * instruction from another branch
-					 * after the save insn has been
-					 * visited.
-					 */
-					if (insn == first)
-						return 0;
-
 					WARN_FUNC("objtool isn't smart enough to handle this CFI save/restore combo",
 						  sec, insn->offset);
 					return 1;