Re: [PATCH v2] x86: Avoid relocation information in final vmlinux

[Petr Pavlu]

On 11/23/22 16:54, Borislav Petkov wrote:
> On Wed, Nov 23, 2022 at 04:43:20PM +0100, Petr Pavlu wrote:
>> The only post-link analysis tool in this case should be arch/x86/tools/relocs.
>> It produces a vmlinux.relocs file which is appended to vmlinux.bin. This is
>> all internal to the Linux build. I'm not aware of any external tooling, such
>> as kernel debuggers, that would require this relocation information in
>> vmlinux.
> 
> It would be good to know why --emit-relocs was added in the first place
> - that might give us a hint. Lemme talk to Micha.

My understanding is that option --emit-relocs is needed to provide relocation
data for KASLR support. Produced relocation sections in vmlinux get processed
by arch/x86/tools/relocs which creates vmlinux.relocs with only relocation
data. This file is appended to vmlinux.bin. Upon boot, function
extract_kernel() decompresses the kernel at the target address and relocates
it using the vmlinux.relocs data.

>> The size command used in your example includes only allocatable code, data and
>> bss sections. It does not show size of any relocation sections.
> 
> This:
> 
> 361M vmlinux.before
> 361M vmlinux.after
> 
> is simple ls output.
> 
> Maybe I need something else enabled in my .config which would show this
> significant difference *and* *explain* it.

Option CONFIG_RANDOMIZE_BASE=y needs to be enabled. Switching it on should
automatically select also CONFIG_X86_NEED_RELOCS=y which is what actually
enables use of --emit-relocs in arch/x86/Makefile.

Petr