Determine load address and entry point of stripped Linux Kernel image

Question

I have a crosscompiling toolchain for an embedded system (mipsel) on my x86 Linux. I know how to build a custom kernel (let's call the image "vmlinux") for it and how to strip that image via

objcopy -S -O binary vmlinux vmlinux.bin

For further processing I also need the load address and entry point of the image. Before stripping it is no problem to determine them via scripts/mksysmap or, more explicitly, via

nm -n vmlinux | grep -v '\( [aNUw] \)\|\(__crc_\)\|\( \$[adt]\)' > System.map

Then I can determine the load address and entry point via

awk '/A _text/ { print "0x"$1; }' < _System.map
awk '/T kernel_entry/ { print "0x"$1; }' < System.map

Now the challenge is that sometimes I do not build the kernel by myself, but get a pre-built kernel after it has already been stripped of its symbols via objcopy. Can anybody tell me how to do this? I am not very proficient in kernel building and toolchain usage. Both nm and objdump do not like the stripped image, saying

vmlinux.bin: File format not recognized

Answer 1

From the objcopy manual page

objcopy can be used to generate a raw binary file by using an output target of binary (e.g., use -O binary). When objcopy generates a raw binary file, it will essentially produce a memory dump of the contents of the input object file. All symbols and relocation information will be discarded. The memory dump will start at the virtual address of the lowest section copied into the output file.

Here is an example that could be used on the PowerPC architecture:

original vmlinux

bash-3.2$ file vmlinux
vmlinux: ELF 32-bit MSB executable, PowerPC or cisco 4500, version 1 (SYSV), statically linked, not stripped

stripped vmlinux is considered a "data" file

bash-3.2$ file vmlinux.bin
vmlinux.bin: data

convert binary to ELF format for the PowerPC

bash-3.2$ powerpc-440fp-linux-objcopy -I binary vmlinux.bin -B powerpc -O elf32-powerpc vmlinux.bin.x

output of vmlinux is now considered an ELF file

bash-3.2$ file vmlinux.bin.x
vmlinux.bin.x: ELF 32-bit MSB relocatable, PowerPC or cisco 4500, version 1 (SYSV), not stripped

You must pass the -I, -B and -O parameter. You can get this parameters from your objcopy documentation.

But since your binary is stripped already trying to decompile it might not be worthwhile since the section information is not available. All of the data in the file will be dumped into the .data secion.