Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

["liuyuntao (F)" <liuyuntao12@huawei.com>]

On 2024/4/23 8:59, Wei Yang wrote:
> Hi, Yuntao
> 
> I am not that familiar with this, so got some questions below.
> 
> On Mon, Apr 22, 2024 at 06:05:56AM +0000, Yuntao Liu wrote:
>> The current x86 architecture does not yet support the
>> HAVE_LD_DEAD_CODE_DATA_ELIMINATION feature. x86 is widely used in
>> embedded scenarios, and enabling this feature would be beneficial for
>> reducing the size of the kernel image.
>>
>> In order to make this work, we keep the necessary tables by annotating
>> them with KEEP, also it requires further changes to linker script to KEEP
>> some tables and wildcard compiler generated sections into the right place.
>>
>> Enabling CONFIG_UNWINDER_ORC or CONFIG_MITIGATION_RETPOLINE will enable
>> the objtool's --orc and --retpoline parameters, which will alter the
>> layout of the binary file, thereby preventing gc-sections from functioning
>> properly. Therefore, HAVE_LD_DEAD_CODE_DATA_ELIMINATION should only be
>> selected when they are not enabled.
>>
>> Enabling CONFIG_LTO_CLANG or CONFIG_X86_KERNEL_IBT will use vmlinux.o
>> instead of performing the slow LTO link again. This can also prevent
>> gc-sections from functioning properly. Therefore, using this optimization
>> when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is not enabled.
>>
>> The size comparison of zImage is as follows:
>> x86_def_defconfig  i386_defconfig    tinyconfig
>> 10892288           10826240          607232          no dce
>> 10748928           10719744          529408          dce
>> 1.3%               0.98%             12.8%           shrink
>>
>> When using smaller config file, there is a significant reduction in the
>> size of the zImage.
>> ---
>> arch/x86/Kconfig              |  1 +
>> arch/x86/kernel/vmlinux.lds.S | 24 ++++++++++++------------
>> scripts/link-vmlinux.sh       |  2 +-
>> 3 files changed, 14 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index a902680b6537..92dfbc8ee4e7 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -247,6 +247,7 @@ config X86
>> 	select HAVE_FUNCTION_ERROR_INJECTION
>> 	select HAVE_KRETPROBES
>> 	select HAVE_RETHOOK
>> +	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !CONFIG_UNWINDER_ORC && !CONFIG_MITIGATION_RETPOLINE
> 
> How to make sure only these two config affect the function?
> 

I have tested all the parameters of objtool, and only when these two 
config are removed can gc-sections take effect.

>> 	select HAVE_LIVEPATCH			if X86_64
>> 	select HAVE_MIXED_BREAKPOINTS_REGS
>> 	select HAVE_MOD_ARCH_SPECIFIC
>> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
>> index 3509afc6a672..aeee2b9b6a6a 100644
>> --- a/arch/x86/kernel/vmlinux.lds.S
>> +++ b/arch/x86/kernel/vmlinux.lds.S
>> @@ -253,7 +253,7 @@ SECTIONS
>>
>> 	.x86_cpu_dev.init : AT(ADDR(.x86_cpu_dev.init) - LOAD_OFFSET) {
>> 		__x86_cpu_dev_start = .;
>> -		*(.x86_cpu_dev.init)
>> +		KEEP(*(.x86_cpu_dev.init))
> 
> How you choose the sections to keep?
> 
> If my understanding is correct, keep the sections is for gc-section. Sounds we
> need to keep all possible section, otherwise it would mis-behave?
> 

Nicholas Piggin has mentioned it before.

> FYI, easiest way to check if you forgot to KEEP a linker table is
> to look at `readelf -S vmlinux` differences, and to see what is
> being trimmed, look at nm differences or use --print-gc-sections
> LD option to see what symbols you're trimming. Linker tables,
> boot entry, and exception entry tends to require anchoring.
https://lore.kernel.org/all/20170709031333.29443-1-npiggin@gmail.com/

>> 		__x86_cpu_dev_end = .;
>> 	}
>>
>> @@ -261,7 +261,7 @@ SECTIONS
>> 	.x86_intel_mid_dev.init : AT(ADDR(.x86_intel_mid_dev.init) - \
>> 								LOAD_OFFSET) {
>> 		__x86_intel_mid_dev_start = .;
>> -		*(.x86_intel_mid_dev.init)
>> +		KEEP(*(.x86_intel_mid_dev.init))
>> 		__x86_intel_mid_dev_end = .;
>> 	}
>> #endif
>> @@ -275,21 +275,21 @@ SECTIONS
>> 	. = ALIGN(8);
>> 	.retpoline_sites : AT(ADDR(.retpoline_sites) - LOAD_OFFSET) {
>> 		__retpoline_sites = .;
>> -		*(.retpoline_sites)
>> +		KEEP(*(.retpoline_sites))
>> 		__retpoline_sites_end = .;
>> 	}
>>
>> 	. = ALIGN(8);
>> 	.return_sites : AT(ADDR(.return_sites) - LOAD_OFFSET) {
>> 		__return_sites = .;
>> -		*(.return_sites)
>> +		KEEP(*(.return_sites))
>> 		__return_sites_end = .;
>> 	}
>>
>> 	. = ALIGN(8);
>> 	.call_sites : AT(ADDR(.call_sites) - LOAD_OFFSET) {
>> 		__call_sites = .;
>> -		*(.call_sites)
>> +		KEEP(*(.call_sites))
>> 		__call_sites_end = .;
>> 	}
>> #endif
>> @@ -298,7 +298,7 @@ SECTIONS
>> 	. = ALIGN(8);
>> 	.ibt_endbr_seal : AT(ADDR(.ibt_endbr_seal) - LOAD_OFFSET) {
>> 		__ibt_endbr_seal = .;
>> -		*(.ibt_endbr_seal)
>> +		KEEP(*(.ibt_endbr_seal))
>> 		__ibt_endbr_seal_end = .;
>> 	}
>> #endif
>> @@ -307,7 +307,7 @@ SECTIONS
>> 	. = ALIGN(8);
>> 	.cfi_sites : AT(ADDR(.cfi_sites) - LOAD_OFFSET) {
>> 		__cfi_sites = .;
>> -		*(.cfi_sites)
>> +		KEEP(*(.cfi_sites))
>> 		__cfi_sites_end = .;
>> 	}
>> #endif
>> @@ -320,7 +320,7 @@ SECTIONS
>> 	. = ALIGN(8);
>> 	.altinstructions : AT(ADDR(.altinstructions) - LOAD_OFFSET) {
>> 		__alt_instructions = .;
>> -		*(.altinstructions)
>> +		KEEP(*(.altinstructions))
>> 		__alt_instructions_end = .;
>> 	}
>>
>> @@ -330,13 +330,13 @@ SECTIONS
>> 	 * get the address and the length of them to patch the kernel safely.
>> 	 */
>> 	.altinstr_replacement : AT(ADDR(.altinstr_replacement) - LOAD_OFFSET) {
>> -		*(.altinstr_replacement)
>> +		KEEP(*(.altinstr_replacement))
>> 	}
>>
>> 	. = ALIGN(8);
>> 	.apicdrivers : AT(ADDR(.apicdrivers) - LOAD_OFFSET) {
>> 		__apicdrivers = .;
>> -		*(.apicdrivers);
>> +		KEEP(*(.apicdrivers));
>> 		__apicdrivers_end = .;
>> 	}
>>
>> @@ -406,7 +406,7 @@ SECTIONS
>> 	.brk : AT(ADDR(.brk) - LOAD_OFFSET) {
>> 		__brk_base = .;
>> 		. += 64 * 1024;		/* 64k alignment slop space */
>> -		*(.bss..brk)		/* areas brk users have reserved */
>> +		KEEP(*(.bss..brk))	/* areas brk users have reserved */
>> 		__brk_limit = .;
>> 	}
>>
>> @@ -432,7 +432,7 @@ SECTIONS
>> 	. = ALIGN(HPAGE_SIZE);
>> 	.init.scratch : AT(ADDR(.init.scratch) - LOAD_OFFSET) {
>> 		__init_scratch_begin = .;
>> -		*(.init.scratch)
>> +		KEEP(*(.init.scratch))
>> 		. = ALIGN(HPAGE_SIZE);
>> 		__init_scratch_end = .;
>> 	}
>> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
>> index 7862a8101747..7287b5a9f17d 100755
>> --- a/scripts/link-vmlinux.sh
>> +++ b/scripts/link-vmlinux.sh
>> @@ -60,7 +60,7 @@ vmlinux_link()
>> 	# skip output file argument
>> 	shift
>>
>> -	if is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT; then
>> +	if [ is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT ] && [ ! is_enabled CONFIG_LD_DEAD_CODE_DATA_ELIMINATION ]; then
>> 		# Use vmlinux.o instead of performing the slow LTO link again.
>> 		objs=vmlinux.o
>> 		libs=
>> -- 
>> 2.34.1
>