How to place kernel_entry function beginning of the text section - c

Question

My Kernel text section starts at address 0x80100000 & kernel_entry function is at the address 0x80585f70. I want kernel_entry should place beginning of the text section at address 0x80100000.
Starting address of text section
$ head -n 10 ../../../System.map
80100000 A _text
80100400 T __kernel_entry
80100400 T _stext
Entry point address in initrd image
$ readelf -h vmlinuz-initrd
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: MIPS R3000
Version: 0x1
Entry point address: 0x80585f70
Start of program headers: 52 (bytes into file)
Start of section headers: 63759852 (bytes into file)
Flags: 0x50001001, noreorder, o32, mips32
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 2
Size of section headers: 40 (bytes)
Number of section headers: 25
Section header string table index: 22
$
I tried to solve the problem using linux linker script(vmlinux.lds) by adding head.o module at the beginning of text section.
vmlinux.lds linux linker script
SECTIONS
{
. = 0x80100000;
/* read-only */
_text = .; /* Text and read-only data */
.text : {
. = ALIGN(8); head.o(.ref.text)
. = ALIGN(8); *(.text.hot) *(.text) *(.ref.text) *(.devinit.text) *(.devexit.text) *(.cpuinit.text) *(.cpuexit.text) *(.text.unlikely)
. = ALIGN(8); __sched_text_start = .; *(.sched.text) __sched_text_end = .;
. = ALIGN(8); __lock_text_start = .; *(.spinlock.text) __lock_text_end = .;
. = ALIGN(8); __kprobes_text_start = .; *(.kprobes.text) __kprobes_text_end = .;
kernel_entry function format .ref.text
$ objdump -t head.o
head.o: file format elf32-little
SYMBOL TABLE:
00000000 l d .text 00000000 .text
00000000 l d .data 00000000 .data
00000000 l d .bss 00000000 .bss
00000000 l d .ref.text 00000000 .ref.text
00000000 l d .cpuinit.text 00000000 .cpuinit.text
00000000 l d .reginfo 00000000 .reginfo
00000000 l d .pdr 00000000 .pdr
00000400 g O .text 00000000 _stext
00000400 g F .text 00000000 __kernel_entry
00000000 g F .ref.text 000000c8 kernel_entry
But I am unable to change this due to vmlinux.lds automatically generated through vmlinux.lds.S file.
I tried to put the same line ". = ALIGN(8); head.o(.ref.text)"
from vmlinux.lds but head.o is not found while building the kernel.
Please help me how can I resolve the problem.

Answer 1

First of all, if you insert a symbol into a section, then all subsequent symbols in the section are shifted. If some code relies on the assumption that there is a certain object located at .text + hardcoded offset then the code will be broken.
If you decided to do that anyway, then you need to be able to edit the vmlinux.lds.S file or at least patch the generated vmlinux.lds before the linking stage. The vmlinux.lds.S is essentially a linker script + a bunch of C macros, so the syntax is pretty similar.
The basic idea is to put the kernel_entry() to a separate section called .kernel_entry and add a record to the vmlinux.lds.S that puts .kernel_entry to the .text before all other sections. The separate section is needed to ensure there is only one symbol in the section.
How to do that:
Put the kernel_entry() function to its own section. You may do that in the source code with a GCC attribute like this:
void kernel_entry() __attribute__((section(".kernel_entry")));
{
/* function body */
}
Put the .kernel_entry section to beginning of the .text in vmlinux.lds.S.
Each architecture has its own vmlinux.lds.S, so the exact contents of the file may be different, but in general, you may find a .text section defined in vmlinux.lds.S like this:
.text : {
TEXT_TEXT
SCHED_TEXT
LOCK_TEXT
KPROBES_TEXT
IRQENTRY_TEXT
*(.text.*)
} :text = 0
The TEXT_TEXT, SCHED_TEXT and so on are actually C macros that expand to some linker script commands. You may use the regular syntax and add *(.kernel_entry) to the beginning of the section, like this:
.text : {
*(.kernel_entry)
TEXT_TEXT
SCHED_TEXT
LOCK_TEXT
KPROBES_TEXT
IRQENTRY_TEXT
*(.text.*)
} :text = 0
That is it. The linker will put .kernel_entry (containing kernel_entry()) to the beginning of the .text which is seems to be what you want.
Good luck!