linux-kernel - Re: linux-next: Tree for Jun 23 (objtool (2))

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Message-ID: <alpine.LSU.2.21.2007151250390.25290@pobox.suse.cz>
Date:   Wed, 15 Jul 2020 13:11:14 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Randy Dunlap <rdunlap@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, pmladek@...e.cz,
        live-patching@...r.kernel.org
Subject: Re: linux-next: Tree for Jun 23 (objtool (2))

On Tue, 14 Jul 2020, Josh Poimboeuf wrote:

> On Tue, Jul 14, 2020 at 12:56:21PM +0200, Miroslav Benes wrote:
> > On Thu, 2 Jul 2020, Josh Poimboeuf wrote:
> > 
> > > On Tue, Jun 23, 2020 at 08:06:07AM -0700, Randy Dunlap wrote:
> > > > On 6/22/20 11:28 PM, Stephen Rothwell wrote:
> > > > > Hi all,
> > > > > 
> > > > > Changes since 20200622:
> > > > > 
> > > > 
> > > > on x86_64:
> > > > 
> > > > arch/x86/kernel/cpu/mce/core.o: warning: objtool: mce_timed_out()+0x24: unreachable instruction
> > > > kernel/exit.o: warning: objtool: __x64_sys_exit_group()+0x14: unreachable instruction
> > > > 
> > > > Full randconfig file is attached.
> > > 
> > > More livepatch...
> > 
> > Correct.
> > 
> > Both are known and I thought Josh had fixes queued somewhere for both, but 
> > my memory fails me quite often. See below.
> 
> I did have fixes for some of them in a stash somewhere, but I never
> finished them because I decided it's a GCC bug.

Same here.
 
> > However, I think it is time to decide how to approach this whole saga. It 
> > seems that there are not so many places in the kernel in need of 
> > __noreturn annotation in the end and as jikos argued at least some of 
> > those should be fixed regardless.
> 
> I would agree that global functions like do_group_exit() deserve a
> __noreturn annotation, though it should be in the header file.  But
> static functions shouldn't need it.

Agreed. I'll post the patches for global functions eventually, but see 
below first.

> > Josh, should I prepare proper patches and submit them to relevant
> > maintainers to see where this path is going?
> 
> If that's how you want to handle it, ok, but it doesn't seem right to
> me, for the static functions at least.
> 
> > It would be much better to fix it in GCC, but it has been like banging 
> > one's head against a wall so far. Josh, you wanted to create a bug 
> > for GCC in this respect in the past? Has that happened?
> 
> I didn't open a bug, but I could, if you think that would help.  I
> haven't had a lot of success with GCC bugs in the past.

Understood.

> > If I remember correctly, we discussed briefly a possibility to cope with 
> > that in objtool, but no solution was presented.
> 
> That would also feel like a GCC workaround and might impede objtool's
> ability to find bugs like this one, and possibly more serious bugs.
> 
> > Removing -flive-patching is also a possibility. I don't like it much, but 
> > we discussed it with Petr M. a couple of months ago and it might be a way 
> > too.
> 
> -flive-patching has many problems which I outlined before.  None of them
> have been addressed.  I still feel the same way, that it should be
> reverted until it's ready.  Otherwise it's a drain on upstream.
> 
> Also, if the GCC developers won't acknowledge this bug then it doesn't
> give me confidence in their ability to keep the feature working as
> optimizations are added or changed.

I must admit that I've started to share the sentiment recently. And it is 
probably the main reason for changing my mind about the whole thing.

> I still think a potential alternative exists: objtool could be used as a
> simple tree-wide object diff tool by generating a checksum for each
> function.  Then the patch can be applied and built to see exactly which
> functions have changed, based on the changed checksums.  In which case
> this feature would no longer be needed anyway, would you agree?

Yes.

> I also think that could be a first step for converging our patch
> creation processes.

Yes again.

Petr, would you agree to revert -flive-patching due to reasons above? Is 
there anything you want to add?

Miroslav