linux-kernel - Re: This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations)

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Date:	Mon, 18 Apr 2016 08:39:32 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Thomas Graf <tgraf@...g.ch>,
	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-scsi <linux-scsi@...r.kernel.org>, jamborm@....gnu.org
Subject: Re: This patch triggers a bad gcc bug (was Re: [PATCH] force
 inlining of some byteswap operations)

On Sat, Apr 16, 2016 at 11:03:32AM +0200, Ingo Molnar wrote:
> 
> * Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > > I don't think we know yet if there's a reliable way to turn the bug off.
> > > 
> > > Also, according to the gcc guys, this bug won't always result in a
> > > truncated function, and may sometimes just make some inline function
> > > call sites disappear:
> > > 
> > >   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646#c14
> > > 
> > > though I haven't been able to confirm that experimentally.  But if it's
> > > true, that means that objtool won't be able to detect all cases of the
> > > bug and some function calls may just silently disappear!
> > > 
> > > There's a lot of activity in the bug now, so hopefully they'll be able
> > > to tell us soon if there's a reliable way to avoid it and/or detect it.
> > > 
> > > BTW, Denys posted a workaround patch for the qla2xxxx code:
> > > 
> > >   https://lkml.kernel.org/r/1460716583-15673-1-git-send-email-dvlasenk@redhat.com
> > 
> > Martin Jambor wrote a succinct summary of the conditions needed for this
> > bug:
> > 
> >   "This bug can occur when an inlineable function containing a call to
> >   __builtin_constant_p, which checks a parameter or a value it
> >   references and a (possibly indirect) caller of the function actually
> >   passes a constant, but stores it using a type of a different size."
> > 
> > So to prevent it from happening elsewhere in the kernel, it sounds like
> > we'd have to either remove all uses of __builtin_constant_p() or disable
> > inlining completely.
> > 
> > There's also no reliable way to detect the bug has occurred, though
> > objtool will detect it in cases when the function gets truncated.
> 
> So it appears to me that due to the hard to detect nature of the GCC bug the fix 
> will probably be backported by them, so I think we should be fine with relying on 
> objtool to detect weird code sequences in the kernel, and should work around 
> specific instances of the bug.

I agree.  So how should we work around the bug in this case?  There have
been several suggestions:

- change wwn_to_u64() to __always_inline

- change qla2x00_get_host_fabric_name() to skip the unnecessary call to
  wwn_to_u64()

- revert one of the two commits:
  bc27fb68aaad ("include/uapi/linux/byteorder, swab: force inlining of some byteswap operations")
  ef3fb2422ffe ("scsi: fc: use get/put_unaligned64 for wwn access")


-- 
Josh