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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>
Subject: [PATCH 1/2] objtool: improve rare switch jump table pattern detection
Date: Thu, 13 Oct 2016 16:22:52 -0500	[thread overview]
Message-ID: <8a9ed68ae1780e8d3963e4ee13f2f257fe3a3c33.1476393584.git.jpoimboe@redhat.com> (raw)
In-Reply-To: <cover.1476393584.git.jpoimboe@redhat.com>

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@arndb.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 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)
-- 
2.7.4

  reply	other threads:[~2016-10-13 22:17 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-10-13 21:22 [PATCH 0/2] objtool: a couple of minor fixes Josh Poimboeuf
2016-10-13 21:22 ` Josh Poimboeuf [this message]
2016-10-16 11:18   ` [tip:core/urgent] objtool: Improve rare switch jump table pattern detection tip-bot for Josh Poimboeuf
2016-10-13 21:22 ` [PATCH 2/2] objtool: skip all "unreachable instruction" warnings for gcov kernels Josh Poimboeuf
2016-10-16 11:18   ` [tip:core/urgent] objtool: Skip " tip-bot for Josh Poimboeuf
2016-10-26  7:58 ` [PATCH 0/2] objtool: a couple of minor fixes Arnd Bergmann
2016-10-26  9:16   ` Arnd Bergmann
2016-10-26 12:37   ` Josh Poimboeuf
2016-10-26 12:43     ` Josh Poimboeuf
2016-10-26 13:18       ` Josh Poimboeuf
2016-10-26 15:34   ` [PATCH] objtool: fix rare switch jump table pattern detection Josh Poimboeuf
2016-10-26 16:03     ` Arnd Bergmann
2016-10-26 16:45       ` Josh Poimboeuf
2016-10-27  7:37     ` [tip:core/urgent] objtool: Fix " tip-bot for Josh Poimboeuf

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