linux-kernel - Re: two more objtool warnings: lib/ubsan.o and fs/fs

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Message-ID: <20170728170447.jclf2vafqy5i2hk3@treble>
Date:   Fri, 28 Jul 2017 12:04:47 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: two more objtool warnings: lib/ubsan.o and fs/fs_pin.o

On Fri, Jul 28, 2017 at 01:25:27PM +0200, Arnd Bergmann wrote:
> Hi Josh,
> 
> I ran into two more warnings with the two patches you sent me in private,
> using gcc-7.1.1:
> 
> lib/ubsan.o: warning: objtool: val_to_string.constprop.7()+0x97: leave
> instruction with modified stack frame
> .config: https://pastebin.com/dl/g04Dwdwx
> 
> fs/fs_pin.o: warning: objtool: pin_kill()+0x139: stack state mismatch:
> cfa1=7+88 cfa2=7+96
> .config: https://pastebin.com/dl/6gK6MadB
> 
> I assume you are faster at analysing than me, so I have not tried to get
> to the bottom of these at all. Let me know if you need help reproducing
> the warnings. It's also possible that they are fixed by some other patches
> that you already sent but that are not in linux-next yet.

Thanks.  The first one is a relatively easy fix in objtool's drap
register handling.  I have a working patch for it.

The second one is a little trickier.

Instead of:

  lea	0x8(%rsp), %rsp

GCC is doing:

  lea    0x8(%rsp), %rcx
  mov    %rcx, %rsp

I don't see why GCC would do that.  We should probably ask some GCC
folks.  Objtool isn't built to track register values, so it may be a
little tricky to deal with that pattern correctly if we can't convince
GCC to fix it.

Anyway, I'm going to be on vacation next week, but I'll investigate
further when I get back.

-- 
Josh