How vmlinux is recognized?

[Peter Teoh]

I loved this reply.......can I annotate it with references to the linux
kernel sources?

On Fri, May 13, 2011 at 9:42 AM, Dave Hylands <dhylands at gmail.com> wrote:

> Hi Vikram,
>
> ...snip...
> > So when compiling the kernel, what is the purpose of the other
> > files(mentioned below)
> > linux-2.6/vmlinux - ELF executable, not stripped
> > linux-2.6/arch/x86/boot/vmlinux.bin - Raw binary (Guess this is the
> > one which is inside the bzImage)
> > linux-2.6/arch/x86/boot/compressed/vmlinux.bin - ELF executable, stripped
> > linux-2.6/arch/x86/boot/compressed/vmlinux - ELF executable, not stripped
>
> Take luca's email and start at the bottom working towards the top.
>
> linux-2.6/vmlinux is the output of the linker. As such, it is an ELF file.
> A binary is then extracted from this to create
> arch/x86/boot/compressed/vmlinux.bin
>

yes:

See ./arch/x86/boot/Makefile


>  This binary is then compressed to produce
> arch/x86/boot/compressed/vmlinux.bin.gz
>

See ./arch/x86/boot/compressed/Makefile


> This gzipped binary is then converted into an object file (which just
> contains the gzipped data) but now we're back to having an ELF file
>

./arch/x86/boot/compressed/mkpiggy.c is compiled into a commandline binary -
mkpiggy which will generate the piggy.o.


> called arch/x86/boot/compressed/piggy.o
> The linker then compiles a decompressor (misc.o) and piggy.o together
>

Yes, the routine is called "decompress_kernel", residing inside
./arch/x86/boot/compressed/misc.c.   And this routine is called
from ./arch/x86/boot/compressed/head_32.S (or head_64.S) and at runtime, the
gzipped data is decompressed and immediately jumped into (perhaps after some
relocation if needed):

/*
 * Do the decompression, and jump to the new kernel..
 */
        leal    z_extract_offset_negative(%ebx), %ebp
                                /* push arguments for decompress_kernel: */
        pushl   %ebp            /* output address */
        pushl   $z_input_len    /* input_len */
        leal    input_data(%ebx), %eax
        pushl   %eax            /* input_data */
        leal    boot_heap(%ebx), %eax
        pushl   %eax            /* heap area */
        pushl   %esi            /* real mode pointer */
        call    decompress_kernel
        addl    $20, %esp


 to produce arch/x86/boot/compressed/vmlinux (an ELF file).
> objcopy is used again to convert this ELF into a binary:
> arch/x86/boot/compressed/vmlinux arch/x86/boot/vmlinux.bin
> Finally, the binary is compressed to produce bzImage.
>

Inside arch/x86/boot/Makefile:

Creating the vmlinux.bin from vmlinux via objcopy (note that this operation
will throw all relocation information):

$(obj)/vmlinux.bin: $(obj)/compressed/vmlinux FORCE
        $(call if_changed,objcopy)

And then packing together linearly to form the "bzImage" (output from make):

make -f scripts/Makefile.build obj=arch/x86/boot arch/x86/boot/bzImage

make -f scripts/Makefile.build obj=arch/x86/boot/compressed
arch/x86/boot/compressed/vmlinux

arch/x86/boot/tools/build arch/x86/boot/setup.bin arch/x86/boot/vmlinux.bin
CURRENT > arch/x86/boot/bzImage


> So what you get is a compressed binary which contains a decompressor
> and another compressed binary, this inner compressed binary being the
> kernel.
>
> GRUB loads bzImage into memory and decompresses it and then executes
> the resulting binary.
>

To be more precise, grub will load bzImage and jump into the startup_32
function located in arch/x86/boot/compressed/head_32.S at the following
fixed address (from source code):

/*
 *  head.S contains the 32-bit startup code.
 *
 * NOTE!!! Startup happens at absolute address 0x00001000, which is also
where
 * the page directory will exist. The startup code will be overwritten by
 * the page directory. [According to comments etc elsewhere on a compressed
 * kernel it will end up at 0x1000 + 1Mb I hope so as I assume this. - AC]
 *
 * Page 0 is deliberately kept safe, since System Management Mode code in
 * laptops may need to access the BIOS data stored there.  This is also
 * useful for future device drivers that either access the BIOS via VM86
 * mode.
 */

More info:

http://books.google.com/books?id=e8BbHxVhzFAC&pg=PA1224&lpg=PA1224&dq=grub+head_32.S&source=bl&ots=0MSdKwBoM6&sig=2RyEpprl25zueiqi332TQHLIj0E&hl=en&ei=y5vQTY7eBNDNrQeI3bTCCg&sa=X&oi=book_result&ct=result&resnum=3&ved=0CCkQ6AEwAg#v=onepage&q=grub%20head_32.S&f=false


> This binary starts with a decompressor which then decompresses the
> kernel, and executes the resulting binary.
> This binary may relocate itself (probably depends on the architecture)
> to a different spot in memory, and then runs.
> The kernel is now running.
>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
>
> _______________________________________________
> K
>

-- 
Regards,
Peter Teoh
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