On Sat, Apr 16, 2016 at 11:03:32AM +0200, Ingo Molnar wrote:
>
> * Josh Poimboeuf <jpoim...@redhat.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-dvlas...@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
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