On Mon, May 16, 2011 at 9:14 AM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote: > 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@xxxxxxxxx> 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 > That was a great explanation. Thanks a lot. I think this will be very much useful for people who want to know how things work in the background. - Thanks, Vikram _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
