linux-kernel - Re: drivers/target/iscsi/iscsi_target.o: warning: objtool: iscsit_handle_task_mgt_cmd()+0xbcb: function has unreachable instruction

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Message-ID: <20160323211104.akrzp4n2a32anfc3@treble.redhat.com>
Date:	Wed, 23 Mar 2016 16:11:04 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	kbuild test robot <fengguang.wu@...el.com>
Cc:	kbuild-all@...org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: drivers/target/iscsi/iscsi_target.o: warning: objtool:
 iscsit_handle_task_mgt_cmd()+0xbcb: function has unreachable instruction

On Wed, Mar 23, 2016 at 07:15:08PM +0800, kbuild test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   a24e3d414e59ac76566dedcad1ed1d319a93ec14
> commit: d4883d5d6b146fd65f762c462b2c6d4a327c7d50 objtool: Enable stack metadata validation on 64-bit x86
> date:   3 weeks ago
> config: x86_64-randconfig-x014-03231738 (attached as .config)
> reproduce:
>         git checkout d4883d5d6b146fd65f762c462b2c6d4a327c7d50
>         # save the attached .config to linux build tree
>         make ARCH=x86_64 
> 
> All warnings (new ones prefixed by >>):
> 
> >> drivers/target/iscsi/iscsi_target.o: warning: objtool: iscsit_handle_task_mgt_cmd()+0xbcb: function has unreachable instruction

Ingo,

FYI, I'm aware of this issue but I'm not yet sure about the best way to
fix it.  I'll be on vacation next week but will try to get to it in a
couple of weeks.

It only seems to manifest itself in certain randconfigs which include
CONFIG_PROFILE_ALL_BRANCHES=y.

more details:

The issue is caused by a sloppy gcc optimization.  It looks like gcc
created a jump table for a switch statement, but then decided not to use
it, and failed to clean up its mess.  So it

   10094:       ff 25 00 00 00 00       jmpq   *0x0(%rip)        # 1009a <iscsit_handle_task_mgt_cmd+0x36a>
                           10096: R_X86_64_PC32    .rodata+0x3c


There's no


-- 
Josh