Re: [PATCH 4/4] x86_64: enable text relative kallsyms for 64-bit targets

[Kees Cook]

On Wed, Jan 20, 2016 at 1:05 AM, Ard Biesheuvel
 wrote:
> This enables the newly introduced text-relative kallsyms support when
> building 64-bit targets. This cuts the size of the kallsyms address
> table in half, reducing the memory footprint of the kernel .rodata
> section by about 400 KB for a KALLSYMS_ALL build, and about 100 KB
> reduction in compressed size. (with CONFIG_RELOCATABLE=y)
>
> Signed-off-by: Ard Biesheuvel 

Tested-by: Kees Cook 

-Kees

> ---
> I tested this with my Ubuntu Wily box's config-4.2.0-23-generic, and
> got the following results:
>
> BEFORE:
> ===
> $ size vmlinux
>textdata bss dec hex filename
> 129729492213240 1482752 16668941 fe590d vmlinux
>
> $ readelf -S .tmp_kallsyms2.o |less
> There are 9 section headers, starting at offset 0x3e0788:
>
> Section Headers:
>   [Nr] Name  Type Address   Offset
>Size  EntSize  Flags  Link  Info  Align
>   ...
>   [ 4] .rodata   PROGBITS   0040
>001c7738     A   0 0 8
>   [ 5] .rela.rodata  RELA   001c7950
>00218e38  0018   I   7 4 8
>   [ 6] .shstrtab STRTAB     001c7778
>0039     0 0 1
>
> $ ls -l arch/x86/boot/bzImage
> -rw-rw-r-- 1 ard ard 6893168 Jan 20 09:36 arch/x86/boot/bzImage
>
> AFTER:
> ==
> $ size vmlinux
>textdata bss dec hex filename
> 126045012213240 1482752 16300493 f8b9cd vmlinux
>
> $ readelf -S .tmp_kallsyms2.o |less
> There are 8 section headers, starting at offset 0x16dd10:
>
> Section Headers:
>   [Nr] Name  Type Address   Offset
>Size  EntSize  Flags  Link  Info  Align
>   ...
>   [ 4] .rodata   PROGBITS   0040
>0016db20     A   0 0 8
>   [ 5] .shstrtab STRTAB     0016db60
>0034     0 0 1
>   ...
>
> $ ls -l arch/x86/boot/bzImage
> -rw-rw-r-- 1 ard ard 6790224 Jan 19 22:24 arch/x86/boot/bzImage
> ---
>  arch/x86/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 4a10ba9e95da..180a94bda8d4 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -142,6 +142,7 @@ config X86
> select HAVE_UNSTABLE_SCHED_CLOCK
> select HAVE_USER_RETURN_NOTIFIER
> select IRQ_FORCED_THREADING
> +   select KALLSYMS_TEXT_RELATIVE   if X86_64
> select MODULES_USE_ELF_RELA if X86_64
> select MODULES_USE_ELF_REL  if X86_32
> select OLD_SIGACTIONif X86_32
> --
> 2.5.0
>



-- 
Kees Cook
Chrome OS & Brillo Security
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[PATCH 4/4] x86_64: enable text relative kallsyms for 64-bit targets

[Ard Biesheuvel]

This enables the newly introduced text-relative kallsyms support when
building 64-bit targets. This cuts the size of the kallsyms address
table in half, reducing the memory footprint of the kernel .rodata
section by about 400 KB for a KALLSYMS_ALL build, and about 100 KB
reduction in compressed size. (with CONFIG_RELOCATABLE=y)

Signed-off-by: Ard Biesheuvel 
---
I tested this with my Ubuntu Wily box's config-4.2.0-23-generic, and
got the following results:

BEFORE:
===
$ size vmlinux
   textdata bss dec hex filename
129729492213240 1482752 16668941 fe590d vmlinux

$ readelf -S .tmp_kallsyms2.o |less
There are 9 section headers, starting at offset 0x3e0788:

Section Headers:
  [Nr] Name  Type Address   Offset
   Size  EntSize  Flags  Link  Info  Align
  ...
  [ 4] .rodata   PROGBITS   0040
   001c7738     A   0 0 8
  [ 5] .rela.rodata  RELA   001c7950
   00218e38  0018   I   7 4 8
  [ 6] .shstrtab STRTAB     001c7778
   0039     0 0 1

$ ls -l arch/x86/boot/bzImage
-rw-rw-r-- 1 ard ard 6893168 Jan 20 09:36 arch/x86/boot/bzImage

AFTER:
==
$ size vmlinux
   textdata bss dec hex filename
126045012213240 1482752 16300493 f8b9cd vmlinux

$ readelf -S .tmp_kallsyms2.o |less
There are 8 section headers, starting at offset 0x16dd10:

Section Headers:
  [Nr] Name  Type Address   Offset
   Size  EntSize  Flags  Link  Info  Align
  ...
  [ 4] .rodata   PROGBITS   0040
   0016db20     A   0 0 8
  [ 5] .shstrtab STRTAB     0016db60
   0034     0 0 1
  ...

$ ls -l arch/x86/boot/bzImage
-rw-rw-r-- 1 ard ard 6790224 Jan 19 22:24 arch/x86/boot/bzImage
---
 arch/x86/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 4a10ba9e95da..180a94bda8d4 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -142,6 +142,7 @@ config X86
select HAVE_UNSTABLE_SCHED_CLOCK
select HAVE_USER_RETURN_NOTIFIER
select IRQ_FORCED_THREADING
+   select KALLSYMS_TEXT_RELATIVE   if X86_64
select MODULES_USE_ELF_RELA if X86_64
select MODULES_USE_ELF_REL  if X86_32
select OLD_SIGACTIONif X86_32
-- 
2.5.0

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