Re: [PATCH] Replace __toc_start + 0x8000 with .TOC.

On Sat, Mar 6, 2021 at 10:25 PM Segher Boessenkool
[off-list ref] wrote:

Hi!

On Sat, Mar 06, 2021 at 09:14:33PM -0800, Fangrui Song wrote:

quoted

TOC relocations are like GOT relocations on other architectures.
However, unlike other architectures, GNU ld's ppc64 port defines .TOC.
relative to the .got output section instead of the linker synthesized
.got input section. LLD defines .TOC. as the .got input section plus
0x8000. When CONFIG_PPC_OF_BOOT_TRAMPOLINE=y,
arch/powerpc/kernel/prom_init.o is built, and LLD computed .TOC. can be
different from __toc_start defined by the linker script.

Simplify kernel_toc_addr with asm label .TOC. so that we can get rid of
__toc_start.

With this change, powernv_defconfig with CONFIG_PPC_OF_BOOT_TRAMPOLINE=y
is bootable with LLD. There is still an untriaged issue with Alexey's
configuration.

Do you have any explanation why this *does* work, while the original
doesn't?  Some explanation that says *what* is wrong.  To me it doesn't
look like the kernel script is.


Segher

The kernel code probably wants to access .TOC. (the TOC base symbol)
via __toc_start+0x8000.
If the kernel understood TOC base is different from the linker
understood TOC base (.TOC.), there should be a problem.
By using .TOC. in the kernel code, the two concepts are guaranteed to match.