linux-kernel - Re: [PATCH 1/3] LoongArch: tools: Add relocs tool support

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Date:   Thu, 8 Sep 2022 10:44:42 +0800
From:   Youling Tang <tangyouling@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>, Jinyang He <hejinyang@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     linux-kernel@...r.kernel.org, loongarch@...ts.linux.dev,
        Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>
Subject: Re: [PATCH 1/3] LoongArch: tools: Add relocs tool support

Hi, Ruoyao

On 09/07/2022 06:19 PM, Youling Tang wrote:
> Hi, Ruoyao
>
> On 09/06/2022 01:17 PM, Xi Ruoyao wrote:
>> On Tue, 2022-09-06 at 10:16 +0800, Youling Tang wrote:
>>
>>> Switch to relative exception tables:
>>>
>>> https://github.com/tangyouling/linux/commit/6525b8da
>>> https://github.com/tangyouling/linux/commit/b6ac0827
>>>
>>> Will switch to the relative exception tables after applying the above
>>> two patches. So there is no need to relocate the exception table
>>> (remove relocate_exception_table).
>>>
>>> Now we can remove the relocation of la.abs , got and ex_table, but
>>> still need to relocate LARCH_64. Is there anything else that needs to
>>> be modified to eliminate this relocation?
>>
>> You may see the RISC-V patch as a reference:
>>
>> https://lore.kernel.org/all/20211009171259.2515351-2-alexandre.ghiti@canonical.com/
>>
>>
>> Basically, make the linker to generate R_*_RELATIVE instead of R_*_64
>> for pointers.  And, perform R_*_RELATIVE relocation loading the kernel.
>>
>> Something problematic IMO: RISC-V uses "-shared" to trick the linker to
>> generate R_*_RELATIVE but I consider it ugly (if the kernel is a shared
>> library, my grandma will be a wagon!)  I prefer "-pie -static", but our
>> Glibc does not have static-pie support for now.  It won't really affect
>> the kernel (we are -nostdlib), but we cannot learn how to handle
>> R_*_RELATIVE in static pie from Glibc then.
>>
>
> After applying all the patches in the link [1], the implementation of
> the relocs tool can now be removed :).
>
> commit: 244fb0971a ... ad45233ef6
>
> [1] https://github.com/tangyouling/loongarch-next/commits/loongarch-next

The previous LDFLAGS_vmlinux has not changed,
LDFLAGS_vmlinux  += -G0 -static -n -nostdlib

When enabling CONFIG_RELOCATABLE to generate the PIE kernel, link
vmliunx with different flags:

1）LDFLAGS_vmlinux   += -shared -Bsymbolic -z notext -z norelro
This way is similar to arm64.

The following warnings appear when linking when using the old toolchain
(no warnings for the new toolchain):

kernel/kallsyms.o(.text+0x4): warning: R_LARCH_SOP_PUSH_PCREL against
[undefweak] `kallsyms_offsets':
Undefweak need to be resolved dynamically, but PLT stub doesn't
represent.

2）LDFLAGS_vmlinux  += -pie
No warnings appear, it looks fine together.

I looked at some commits for arm64, at first arm64 used -pie -shared
(direct link fails in LoongArch), Removed -pie after link [1] and only
used -shared.

[1] 
https://github.com/torvalds/linux/commit/3bbd3db86470c701091fb1d67f1fab6621debf50

Youling.

>
> Youling.
>