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
<ard.biesheuvel@xxxxxxxxxx> 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 <ard.biesheuvel@xxxxxxxxxx>

Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
> I tested this with my Ubuntu Wily box's config-4.2.0-23-generic, and
> got the following results:
>
> BEFORE:
> =======
> $ size vmlinux
>    text    data     bss     dec     hex filename
> 12972949        2213240 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         0000000000000000  00000040
>        00000000001c7738  0000000000000000   A       0     0     8
>   [ 5] .rela.rodata      RELA             0000000000000000  001c7950
>        0000000000218e38  0000000000000018   I       7     4     8
>   [ 6] .shstrtab         STRTAB           0000000000000000  001c7778
>        0000000000000039  0000000000000000           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
>    text    data     bss     dec     hex filename
> 12604501        2213240 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         0000000000000000  00000040
>        000000000016db20  0000000000000000   A       0     0     8
>   [ 5] .shstrtab         STRTAB           0000000000000000  0016db60
>        0000000000000034  0000000000000000           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_SIGACTION                    if X86_32
> --
> 2.5.0
>



-- 
Kees Cook
Chrome OS & Brillo Security