linux-kernel - Re: [PATCH] powerpc: Fix kernel-jump address for ppc64 wrapper boot

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Message-ID: <CAOSf1CELBh4F334LK6+zy+BPOXKcOm92QZw4g71hQ7oQNa68nw@mail.gmail.com>
Date:   Tue, 8 Jun 2021 15:26:41 +1000
From:   "Oliver O'Halloran" <oohall@...il.com>
To:     He Ying <heying24@...wei.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nathan Chancellor <nathan@...nel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powerpc: Fix kernel-jump address for ppc64 wrapper boot

On Fri, Jun 4, 2021 at 7:39 PM He Ying <heying24@...wei.com> wrote:
>
> From "64-bit PowerPC ELF Application Binary Interface Supplement 1.9",
> we know that the value of a function pointer in a language like C is
> the address of the function descriptor and the first doubleword
> of the function descriptor contains the address of the entry point
> of the function.
>
> So, when we want to jump to an address (e.g. addr) to execute for
> PPC-elf64abi, we should assign the address of addr *NOT* addr itself
> to the function pointer or system will jump to the wrong address.

How have you tested this?

IIRC the 64bit wrapper is only used for ppc64le builds. For that case
the current code is work because the LE ABI (ABIv2) doesn't use
function descriptors. I think even for a BE kernel we need the current
behaviour because the vmlinux's entry point is screwed up (i.e.
doesn't point a descriptor) and tools in the wild (probably kexec)
expect it to be screwed up.

ABIv2 (LE) reference:
https://openpowerfoundation.org/?resource_lib=64-bit-elf-v2-abi-specification-power-architecture