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

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Date:   Fri, 23 Sep 2022 17:56:51 +0800
From:   Youling Tang <tangyouling@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>
Cc:     Jinyang He <hejinyang@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>,
        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 09/22/2022 07:19 PM, Xi Ruoyao wrote:
> On Thu, 2022-09-08 at 16:01 +0800, Youling Tang wrote:
>> After adding KBUILD_CFLAGS_KERNEL += -mdirect-extern-access, the
>> kernel
>> will not generate .got, .plt and .got.plt sections (in the new
>> toolchain), we should unexpectedly detect that the kernel has these
>> sections, maybe add similar patch [1] to detect, x86_64 has the same
>> operation.
>>
>> But when adding LDFLAGS_vmlinux += -pie (or -shared), there will be
>> .got, .plt and .got.plt sections generated, I don't know how the
>> toolchain handles it :(?
>
> Hi Youling,
>
> I've pulled your code and make some adjustments for it:
>
> https://github.com/xry111/linux/commits/xry111/la-dev/pie
>
> One adjustment is for FDT removal, another uses a static-PIE style
> LDFLAGS as the kernel is more "similar" to a static PIE than a shared
> library. In userspace, a static PIE is linked with [1]:
>
>  -static -pie --no-dynamic-linker -z text
>
> [1]: https://gcc.gnu.org/r13-2728
>
> But we have to use "-z notext" for vmlinux. I'm not an expert on kernel
> hacking, and I guess it's because the kernel doesn't really care the RWX
> permission of itself (I heard this during some discussion about a W/X
> page warning added in Binutils-2.39 which is triggered for vmlinux).
>
> With "-static -pie --no-dynamic-linker -z notext" (and GCC trunk &
> Binutils trunk), .plt and .got.plt are gone.
>
> .got is still there but it only contains one entry (8 bytes).  AFAIK
> this entry (`_GLOBAL_OFFSET_TABLE_[0]`) is set to the link-time address
> of _DYNAMIC for a userspace static PIE [2], but vmlinux does not need it
> at all.  We can tell the linker to discard it IIUC.

Yes, only .got contains one entry (8 bytes) after using LDFLAGS_vmlinux 
as above, thanks for your help.

Youling.

>
> [2]:
> https://maskray.me/blog/2021-08-29-all-about-global-offset-table#global_offset_table_0
>
> I've boot the kernel successfully and it seems KASLR is in-effect:
>
> $ sudo cat /proc/kallsyms | grep ' _printk$'
> 90000000023b28f4 T _printk
> $ grep ' _printk$' /boot/System.map-6.0.0-rc6-pie+
> 90000000009828f4 T _printk
>