linux-kernel - [tip:core/urgent] objtool: Improve rare switch jump table pattern detection

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Date:   Sun, 16 Oct 2016 04:18:11 -0700
From:   tip-bot for Josh Poimboeuf <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     tglx@...utronix.de, torvalds@...ux-foundation.org,
        mingo@...nel.org, hpa@...or.com, jpoimboe@...hat.com,
        luto@...nel.org, arnd@...db.de, bp@...en8.de, peterz@...radead.org,
        linux-kernel@...r.kernel.org, dvlasenk@...hat.com,
        brgerst@...il.com
Subject: [tip:core/urgent] objtool: Improve rare switch jump table pattern
 detection

Commit-ID:  3732710ff6f2ce2b1b7f044937a422b717d4f953
Gitweb:     http://git.kernel.org/tip/3732710ff6f2ce2b1b7f044937a422b717d4f953
Author:     Josh Poimboeuf <jpoimboe@...hat.com>
AuthorDate: Thu, 13 Oct 2016 16:22:52 -0500
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Sun, 16 Oct 2016 09:12:35 +0200

objtool: Improve rare switch jump table pattern detection

GCC 6 added a new switch statement jump table optimization which makes
objtool's life harder.  It looks like:

  mov [rodata addr],%reg1
  ... some instructions ...
  jmpq *(%reg1,%reg2,8)

The optimization is quite rare, but objtool still needs to be able to
identify the pattern so that it can follow all possible control flow
paths related to the switch statement.

In order to detect the pattern, objtool starts from the indirect jump
and scans backwards through the function until it finds the first
instruction in the pattern.  If it encounters an unconditional jump
along the way, it stops and considers the pattern to be not found.

As it turns out, unconditional jumps can happen, as long as they are
small forward jumps within the range being scanned.

This fixes the following warnings:

  drivers/infiniband/sw/rxe/rxe_comp.o: warning: objtool: rxe_completer()+0x2f4: sibling call from callable instruction with changed frame pointer
  drivers/infiniband/sw/rxe/rxe_resp.o: warning: objtool: rxe_responder()+0x10f: sibling call from callable instruction with changed frame pointer

Reported-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/8a9ed68ae1780e8d3963e4ee13f2f257fe3a3c33.1476393584.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 tools/objtool/builtin-check.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
index 143b6cd..a00a05d 100644
--- a/tools/objtool/builtin-check.c
+++ b/tools/objtool/builtin-check.c
@@ -713,6 +713,7 @@ static struct rela *find_switch_table(struct objtool_file *file,
 				      struct instruction *insn)
 {
 	struct rela *text_rela, *rodata_rela;
+	struct instruction *orig_insn = insn;
 
 	text_rela = find_rela_by_dest_range(insn->sec, insn->offset, insn->len);
 	if (text_rela && text_rela->sym == file->rodata->sym) {
@@ -733,10 +734,16 @@ static struct rela *find_switch_table(struct objtool_file *file,
 
 	/* case 3 */
 	func_for_each_insn_continue_reverse(file, func, insn) {
-		if (insn->type == INSN_JUMP_UNCONDITIONAL ||
-		    insn->type == INSN_JUMP_DYNAMIC)
+		if (insn->type == INSN_JUMP_DYNAMIC)
 			break;
 
+		/* allow small jumps within the range */
+		if (insn->type == INSN_JUMP_UNCONDITIONAL &&
+		    insn->jump_dest &&
+		    (insn->jump_dest->offset <= insn->offset ||
+		     insn->jump_dest->offset >= orig_insn->offset))
+		    break;
+
 		text_rela = find_rela_by_dest_range(insn->sec, insn->offset,
 						    insn->len);
 		if (text_rela && text_rela->sym == file->rodata->sym)