What is the purpose of parse_elf() in misc.c

What is the purpose of parse_elf() in misc.c

[Graeme Russ]

Hi All,

In looking at booting a raw x86 kernel image, I discovered that the
compressed image in bzImage is actually an ELF image created with:

objcopy -R .comment -S vmlinux

After decompressing the image, the parse_elf() function in misc.c copies
the sections from the ELF image (there are 3 sections, only 2 are marked
PT_LOAD) into their final resting place in memory. This got me to thinking,
why not use:

objcopy -O binary -R .comment -S vmlinux

I wrote a little stand-alone parse_elf() program, and lo-and-behold, the
outputs are identical. So, in theory, if bzImage contained a raw binary, it
could be decompressed directly to the target address.

Now there is some adjustment in parse_elf() for relocatable kernels, but I
can't but think that this could be simply catered for in the decompression
target address

Surely this would speed up kernel loading by removing the additional copy
that parse_elf() introduces

Or am I missing something significant?

Regards,

Graeme

What is the purpose of parse_elf() in misc.c

[Mulyadi Santosa]

Hi Graeme....

On Fri, Nov 25, 2011 at 12:30, Graeme Russ <graeme.russ@gmail.com> wrote:
> I wrote a little stand-alone parse_elf() program, and lo-and-behold, the
> outputs are identical. So, in theory, if bzImage contained a raw binary, it
> could be decompressed directly to the target address.

I find your experiment quite interesting. After looking at parse_elf
(http://lxr.linux.no/#linux+v3.1.4/arch/x86/boot/compressed/misc.c#L276),
I guess it could be codes introduced long time before objcopy support
"binary" form. And since then is rarely touched again.

But maybe it's left there in case there is a platform that doesn't
have objcopy version that supports "binary" form conversion. So I
think it's more about portability issue.

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com