linux-kernel - [PATCH 16/20] objtool: Rewrite alt->skip

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Date:   Thu, 07 Mar 2019 12:45:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     torvalds@...ux-foundation.org, tglx@...utronix.de, hpa@...or.com,
        julien.thierry@....com, will.deacon@....com, luto@...capital.net,
        mingo@...nel.org, catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        dvyukov@...gle.com, rostedt@...dmis.org
Subject: [PATCH 16/20] objtool: Rewrite alt->skip_orig

Really skip the original instruction flow, instead of letting it
continue with NOPs.

Since the alternative code flow already continues after the original
instructions, only the alt-original is skipped.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 tools/objtool/check.c |   16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -31,6 +31,7 @@
 struct alternative {
 	struct list_head list;
 	struct instruction *insn;
+	bool skip_orig;
 };
 
 const char *objname;
@@ -623,9 +624,6 @@ static int add_call_destinations(struct
  *    conditionally jumps to the _end_ of the entry.  We have to modify these
  *    jumps' destinations to point back to .text rather than the end of the
  *    entry in .altinstr_replacement.
- *
- * 4. It has been requested that we don't validate the !POPCNT feature path
- *    which is a "very very small percentage of machines".
  */
 static int handle_group_alt(struct objtool_file *file,
 			    struct special_alt *special_alt,
@@ -641,9 +639,6 @@ static int handle_group_alt(struct objto
 		if (insn->offset >= special_alt->orig_off + special_alt->orig_len)
 			break;
 
-		if (special_alt->skip_orig)
-			insn->type = INSN_NOP;
-
 		insn->alt_group = true;
 		last_orig_insn = insn;
 	}
@@ -808,6 +803,7 @@ static int add_special_section_alts(stru
 		}
 
 		alt->insn = new_insn;
+		alt->skip_orig = special_alt->skip_orig;
 		list_add_tail(&alt->list, &orig_insn->alts);
 
 		list_del(&special_alt->list);
@@ -1883,7 +1879,12 @@ static int validate_branch(struct objtoo
 		insn->visited = true;
 
 		if (!insn->ignore_alts) {
+			bool skip_orig = false;
+
 			list_for_each_entry(alt, &insn->alts, list) {
+				if (alt->skip_orig)
+					skip_orig = true;
+
 				ret = validate_branch(file, alt->insn, state);
 				if (ret) {
 					if (backtrace)
@@ -1891,6 +1892,9 @@ static int validate_branch(struct objtoo
 					return ret;
 				}
 			}
+
+			if (skip_orig)
+				return 0;
 		}
 
 		switch (insn->type) {