[PATCH v3 2/2] arm64: efi: Account for the EFI runtime stack in stack unwinder

[Nathan Chancellor]

Hi Ard,

On Wed, Jan 11, 2023 at 09:45:32AM +0100, Ard Biesheuvel wrote:
> On Tue, 10 Jan 2023 at 21:48, Nathan Chancellor <nathan at kernel.org> wrote:
> > On Fri, Jan 06, 2023 at 06:47:03PM +0100, Ard Biesheuvel wrote:
> > > The EFI runtime services run from a dedicated stack now, and so the
> > > stack unwinder needs to be informed about this.
> > >
> > > Acked-by: Mark Rutland <mark.rutland at arm.com>
> > > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> >
> > Apologies if this has been reported and/or fixed already, I searched
> > lore and did not find anything but I just bisected a QEMU boot hang [1]
> > that we see in the ClangBuiltLinux CI with Fedora's configuration [2] to
> > this change in next-20220110 as commit a7334dc70496 ("arm64: efi:
> > Account for the EFI runtime stack in stack unwinder").
> >
> 
> Thanks for the report. This is due to an oversight on my part: we
> removed a spin_is_locked() check, and the lock in question can only be
> in the locked state when EFI runtime services are enabled to begin
> with.
> 
> Without the lock check, we may end up dereferencing the uninitialized
> efi_rt_stack_top on non-EFI boots.
> 
> I've fixed this up in the EFI fixes tree, so the issue should
> disappear once -next is updated. (We just missed 20230111
> unfortunately)

Thank you for the quick response! That issue appears to be fixed.

Unfortunately, I am still seeing a hang while booting via EFI on either
bare metal or KVM when CONFIG_DEBUG_PREEMPT is enabled (Fedora's rawhide
config appears to enable several debugging options), so it appears I was
seeing two distinct issues :/ defconfig + CONFIG_DEBUG_PREEMPT=y is
enough for me to reproduce this problem.

I see

  [    0.015382] Remapping and enabling EFI services.

as the last line in the console (via earlycon) with the bad kernel and
nothing after it (I assume we deadlock somewhere or hit a BUG_ON()?), vs

  [    0.015191] Remapping and enabling EFI services.
  [    0.016725] smp: Bringing up secondary CPUs ...

on the good kernel, followed by a normal boot.

Sorry for not noticing this sooner! It should be pretty easy to
reproduce but if there is any other information I can provide, I am more
than happy to do so.

Cheers,
Nathan