Re: "Bad or missing .orc_unwind table. Disabling unwinder." Clang 19 problem? 6.12.8 problem?

[Nathan Chancellor <nathan@kernel.org>]

Hi Josh,

On Mon, Jan 13, 2025 at 03:58:35PM -0800, Josh Poimboeuf wrote:
> On Mon, Jan 13, 2025 at 01:41:24PM -0800, Josh Poimboeuf wrote:
> > On Mon, Jan 13, 2025 at 06:29:10PM +0100, Klaus Kusche wrote:
> > > 
> > > Hello,
> > > 
> > > I've submitted a bug report to kernel bugzilla,
> > > but it was closed with a hint that I should contact
> > > the maintainers and LKML.
> > > 
> > > https://bugzilla.kernel.org/show_bug.cgi?id=219685
> > > 
> > > You are listed as the maintainers for x86 stack unwinding.
> > > Any ideas about that problem before I submit it to LKML?
> > > 
> > > Is it possibly related to 
> > > 
> > > https://bugzilla.kernel.org/show_bug.cgi?id=219686
> > > 
> > > which appeared at the same time?
> > > Should it go to the clang maintainers?
> > 
> > Hi Klaus,
> > 
> > Thanks for reporting it.  Indeed it's probably related to the build
> > warning:
> > 
> >   vmlinux.o: warning: objtool: start_unlink_intr+0xb7: can't find switch jump table
> > 
> > It's probably a kernel bug, possibly triggered by the new compiler.
> > I'll see if I can recreate.
> 
> [adding some folks]

Thanks for the CC, there was also an issue filed on our GitHub but I
have not had the chance to reply to it yet.

https://github.com/ClangBuiltLinux/linux/issues/2064

> Nathan, I think this is a Clang 19 issue, where one of the jump table
> entries is pointing past the end of the function.  When we saw this in
> the past I think it was due to some leftover optimization, where the
> jump table entry ended up unused so it was harmless.  Are you aware of
> any recent bugs in that area?

I am not aware of any recent bugs there but if you or Klaus have a
configuration file that triggers this, I would be happy to bisect LLVM
to see what change introduced it to give us a better understanding of
what is happening here.

> Maybe objtool should just ignore such entries?

This may be worth doing, especially in the face of the --Werror flag
that is being proposed.

Cheers,
Nathan