Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Arnd Bergmann]

On Fri, Mar 19, 2021 at 1:25 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> On Thu, Mar 18, 2021 at 09:41:54AM +0100, Arnd Bergmann wrote:
> > On Wed, Mar 17, 2021 at 5:18 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > > On Wed, Mar 17, 2021 at 02:37:57PM +0000, Catalin Marinas wrote:
> > > > On Thu, Feb 25, 2021 at 12:20:56PM +0100, Arnd Bergmann wrote:
> > > > > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> > > > > index bad2b9eaab22..926cdb597a45 100644
> > > > > --- a/arch/arm64/kernel/vmlinux.lds.S
> > > > > +++ b/arch/arm64/kernel/vmlinux.lds.S
> > > > > @@ -217,7 +217,7 @@ SECTIONS
> > > > >             INIT_CALLS
> > > > >             CON_INITCALL
> > > > >             INIT_RAM_FS
> > > > > -           *(.init.altinstructions .init.bss .init.bss.*)  /* from the EFI stub */
> > > > > +           *(.init.altinstructions .init.data.* .init.bss .init.bss.*)     /* from the EFI stub */
> > > >
> > > > INIT_DATA already covers .init.data and .init.data.*, so I don't think
> > > > we need this change.
> > >
> > > Ah, INIT_DATA only covers init.data.* (so no dot in front). The above
> > > is needed for the EFI stub.
> >
> > I wonder if that is just a typo in INIT_DATA. Nico introduced it as part of
> > 266ff2a8f51f ("kbuild: Fix asm-generic/vmlinux.lds.h for
> > LD_DEAD_CODE_DATA_ELIMINATION"), so perhaps that should have
> > been .init.data.* instead.
>
> I think it was the other Nicholas ;) (with an 'h'). The vmlinux.lds.h
> change indeed looks like a typo (it's been around since 4.18).

Right, my mistake.

> > It looks like pointer authentication gone wrong, which ended up
> > with dereferencing the broken pointer in x22, and it explains why
> > it only happens with -cpu max. Presumably this also only happens
> > on secondary CPUs, so maybe the bit that initializes PAC on
> > secondary CPUs got discarded?
>
> I seems that the whole alternative instructions section is gone, so any
> run-time code patching that the kernel does won't work. The kernel boots
> with the diff below but I'm not convinced we don't miss anything else.
> In some cases you get a linker warning about gc sections but not in this
> case. Maybe we need some more asserts to ensure that certain sections
> are not empty.
>
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 11909782ee3e..036cc59033d3 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -203,7 +203,7 @@ SECTIONS
>         . = ALIGN(4);
>         .altinstructions : {
>                 __alt_instructions = .;
> -               *(.altinstructions)
> +               KEEP(*(.altinstructions))
>                 __alt_instructions_end = .;
>         }
>
> Do we need a KEEP(.init.altinstructions) as well?

I would guess so. Whatever causes the .altinstructions to get dropped
presumably also leads to the same happening to .init.altinstructions.

Ideally each use of altinstructions would cause a reference to a
particular symbol so that one gets kept, while any .altinstructions
for unused functions get discarded.

        Arnd