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SubjectRe: 4.17.x won't boot due to "x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G"
On Tue, 3 Jul 2018, Kirill A. Shutemov wrote:
> On Tue, Jul 03, 2018 at 03:44:03PM +0300, Kirill A. Shutemov wrote:
> > On Tue, Jul 03, 2018 at 01:24:49PM +0200, Gabriel C wrote:
> > > 2018-07-01 23:32 GMT+02:00 Benjamin Gilbert <bgilbert@redhat.com>:
> > > > On Sun, Jul 01, 2018 at 05:15:59PM -0400, Benjamin Gilbert wrote:
> > > >> 4.17 kernels built with the CoreOS Container Linux toolchain and kconfig,
> > > >> up to and including 4.17.3, fail to boot on AMD64 running in (at least)
> > > >> QEMU/KVM. No messages are shown post-GRUB; the VM instantly reboots.
> > > >> Reverting commit 194a9749c73d ("x86/boot/compressed/64: Handle 5-level
> > > >> paging boot if kernel is above 4G") fixes it. I've attached our kernel
> > > >> config for reference, and am happy to test patches, provide sample QCOW
> > > >> images, etc.
> > > >
> > >
> > > Also see https://bugzilla.kernel.org/show_bug.cgi?id=200385 ,
> > >
> > > 0a1756bd2897951c03c1cb671bdfd40729ac2177 is acting up
> > > too with the same symptoms
> >
> > I tracked it down to -flto in LDFLAGS. I'll look more into this.
>
> -flto in LDFLAGS screws up this part of paging_prepare():
>
> /* Copy trampoline code in place */
> memcpy(trampoline_32bit + TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
> &trampoline_32bit_src, TRAMPOLINE_32BIT_CODE_SIZE);
>
> In particular, relocation for trampoline_32bit_src solved in the wrong
> way. Without -flto, we have rip-realtive address load:
>
> 982d30: 48 8d 35 09 cc ff ff lea -0x33f7(%rip),%rsi # 97f940 <trampoline_32bit_src>
>
> With -flto we have immediate load:
>
> 982cf0: 48 c7 c6 f0 f8 97 00 mov $0x97f8f0,%rsi
>
> It only would be okay if bootloader loads kernel at the address we compile
> it for. But it's not usually the case.
>
> As result we copy garbage into trampoline and crash when trying to execute
> it.
>
> I don't know how to solve it. As far as I know we don't support compiling
> kernel with LTO in mainline.
>
> Any suggestions?

LTO is broken. The boot code is compiled as position independent, so this
'optimization' is pure garbage.

Thanks,

tglx

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