Initialize char[] fails, esi contains wrong value - c

Question

I want to initialize a char array, but during I do this my programm crashes. Here's my code:
void kernelEnteredMsg() {
char str[] = "Kernel successfully entered!";
}
Here's the disassembly:
push ebp
mov ebp,esp
push edi
push esi
push ebx
sub esp,byte +0x30
lea edx,[ebp-0x2d]
mov ebx,0x402000 ; load an address outside my data segment
mov eax,0x1d
mov edi,edx
mov esi,ebx ; move this address to edi
mov ecx,eax
rep movsb ; here the programm crashes
add esp,byte +0x30
pop ebx
pop esi
pop edi
pop ebp
ret
I don't understand why it loads esi with 0x402000. But this seems to cause the error. Can somebody explain what happens here and how to fix it?
PS: "Kernel successful entered!" is at 0x1000 in binary file.
C code:
void kernelEnteredMsg();
void entryPoint() {
kernelEnteredMsg();
}
void kernelEnteredMsg() {
char str[] = "Kernel successfully entered!";
int size = 28;
}
Calling assembly code:
extern _entryPoint
global _main
section .text
_main: ; start of kernel
nop
; setup ds, es, ss and gs
mov ax, 16
mov ds, ax
mov es, ax
mov ss, ax
mov sp, 0x4000
mov ax, 24
mov gs, ax
mov [gs:0], dword 0x07690748 ; test graphics
call _entryPoint ; enter kernel C code
jmp $

Answer 1

This code does copy the string from the .text section to the local stack, because the char array is not 'const'. This may provide a simple solution if you do not need the string to be modified - just make it const char.
I don't understand why it loads esi with 0x402000.
ESI is the source of the string copy instruction 'rep movsb', EDI is the destination.
The address is constructed by IMAGE_BASE+SECTION (IIRC) in the PE file(assuming it is PE.)
Remember in the file there is a FILE_ALIGN and a SECTION_VIRTUAL_ADDRESS, so a section may be
at position 0x1000 in the file(FILE_ALIGN) and at 0x2000 in memory(VIRTUAL_ADDRESS) resulting in IMAGE_BASE+VIRTUAL_ADDRESS=0x402000.
You can use a PE explorer like CFF Explorer(http://www.ntcore.com/exsuite.php)
to display this(if it's a .bin file it may be unapplicable but it has to have some kind of format)
Another possibility may be a wrong state of the DF-Flag leading to wrong behaviour of the string copy instruction (should not happen, because the compiler should take care of this).
Try inserting
__asm__ ("cld");
before the char str[] or in the __main procedure to set string increment to 'UP'.