linux-kernel - Re: [PATCH 2/2] x86/unwind: Make CONFIG_UNWINDER

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Date:   Mon, 19 Mar 2018 16:22:55 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, peterz@...radead.org, hpa@...or.com,
        tglx@...utronix.de
Subject: Re: [PATCH 2/2] x86/unwind: Make CONFIG_UNWINDER_ORC=y the default
 in kconfig for 64-bit

El Mon, Mar 19, 2018 at 04:20:53PM -0500 Josh Poimboeuf ha dit:

> On Mon, Mar 19, 2018 at 01:31:30PM -0700, Matthias Kaehlcke wrote:
> > > The ORC unwinder relies on objtool, which reverse engineers the compiled
> > > code.  This is objtool's first exposure to clang, so I'm not at all
> > > surprised if it's getting confused.
> > > 
> > > Send me one of the .o files and I can take a quick look to see how bad
> > > it is, but I'm guessing it's going to be a lot of work to make objtool
> > > compatible with clang (and unfortunately I won't have the bandwidth to
> > > work on that in the near term.)
> > > 
> > > In the meantime I'd recommend that you use frame pointers (and
> > > CONFIG_STACK_VALIDATION=n) for clang-compiled kernels.
> > 
> > Thanks for your assessment!
> > 
> > dvo_ch7017.o is attached.
> 
> Here's a (surprisingly easy) fix for this particular issue, though I'd
> be shocked if there weren't a bunch more issues lurking elsewhere.  Let
> me know how it goes.

Thanks for having a look, this fixes the vast majority of warnings in
a defconfig build!

The remaining warnings are:

arch/x86/mm/pti.o: warning: objtool: pti_init() falls through to next
function pti_user_pagetable_walk_pmd()
s/debugfs/file.o: warning: objtool: full_proxy_llseek() falls through to next function full_proxy_read()
fs/debugfs/file.o: warning: objtool: full_proxy_read() falls through to next function full_proxy_write()
fs/debugfs/file.o: warning: objtool: full_proxy_write() falls through to next function full_proxy_poll()
fs/debugfs/file.o: warning: objtool: full_proxy_poll() falls through to next function full_proxy_unlocked_ioctl()
fs/debugfs/file.o: warning: objtool: full_proxy_unlocked_ioctl() falls
through to next function fops_u8_open()

In all these functions it's an 'early' return that 'causes' the
warning.

Obviously I don't expect you to spend large amounts of time
investigating this, but should there be a similarily easy fix it would
be certainly welcome :)

> BTW, one thing I noticed in the .o file is that most of the functions'
> stacks are aligned to 16 bytes.  It might be worth checking if the clang
> -mstack-alignment=8 option is getting set, and if so, if it's working
> properly.  Otherwise, with aligned stacks, the frame pointer is forced,
> which defeats most of the benefits of ORC.

Thanks for pointing this out. I verified that -mstack-alignment=8 is
set. Do the "and $0xfffffffffffffff0,%rsp" instructions indicate the
stack alignment of 16 bytes? These are also present in the object file
generated by gcc.

Download attachment "pti.o" of type "application/x-object" (9176 bytes)

Download attachment "file.o" of type "application/x-object" (65376 bytes)