LinuxQuestions.org - Issues with open() (asm)

- Programming (https://www.linuxquestions.org/questions/programming-9/)

- - Issues with open() (asm) (https://www.linuxquestions.org/questions/programming-9/issues-with-open-asm-178298/)

Issues with open() (asm)

I wrote a small program that is supposed to (keyword: supposed) create a file and put the word BORING! inside. The problem I have is that it wont take O_RDONLY (or anything else for that matter)

I copied a program using this exact method from Programming from the Ground up and it worked. So... here is the code.

Code:

.section .data



.equ    OPEN,  5

.equ    WRITE,  4

.equ    CLOSE,  6



crack:

        .ascii  "bored!"

        len = . - crack



.section .text

.global _start



_start:



create_file:

        movl    $OPEN, %eax

        movl    $crack,%ebx

        movl    $O_RDONLY, %ecx

        movl    $0666, %edx

        int    $0x80



copy_descriptor:

        push    %eax



write_to_file:

        movl    $4, %eax

        movl    4(%ebp), %ebx

        movl    $crack, %ecx

        movl    $len, %ecx

        int    $0x80



close_file:

        movl    $6,%eax

        movl    4(%ebp), %ebx

        pop    %eax

PS: if you download the pdf of Programming from the ground up, check page 89.

1) i dont see where u have defined the constant O_RDONLY, 2) if you are writing to afile, why would u use O_RDONLY, and 3) the args to open are supposed to be:
int open(const char *pathname, int flags, mode_t mode);
so ebx should have the name of the file, 'crack' seems to be what you want to write to the file?? and if u want to create the file for writing, here is constant i used from a project i did:
OUTFILE_FLAGS = 0x241 ;//O_WRONLY | O_CREAT | O_TRUNC

just look in the header files from man open to see the other constant values.

Ok, so I fixed some of the errors. Code still doesn't work of course.

Code:

.section .data



.equ    OPEN,  5

.equ    WRITE,  4

.equ    CLOSE,  6

.equ    O_WRONLY, 1



filename:

        .ascii  "file.tickle"



crack:

        .ascii  "bored!"

        len = . - crack



.section .text

.global _start



_start:



create_file:

        movl    $OPEN, %eax

        movl    $filename,%ebx

        movl    $O_WRONLY, %ecx

        movl    $0666, %edx

        int    $0x80



copy_descriptor:

        push    %eax



write_to_file:

        movl    $WRITE, %eax

        movl    4(%ebp), %ebx

        movl    $crack, %ecx

        movl    $len, %edx

        int    $0x80



close_file:

        movl    $6,%eax

        movl    4(%ebp), %ebx

        pop    %eax

when I strace it, I get the following, which hints that there is something wrong with my messages :

Code:

execve("./harhar", ["./harhar"], [/* 46 vars */]) = 0

open("file.ticklebored!", O_WRONLY)      = -1 ENOENT (No such file or directory)

--- SIGSEGV (Segmentation fault) @ 0 (0) ---

+++ killed by SIGSEGV +++

I've done some work on the code. In the past say, 20 minutes. (probably less). Here is the strace once more:

Code:

execve("./harhar", ["./harhar"], [/* 47 vars */]) = 0

open("file.PNX", O_WRONLY|O_CREAT, 0666) = 3

--- SIGSEGV (Segmentation fault) @ 0 (0) ---

+++ killed by SIGSEGV +++

here is the code:

Code:

.section .data



.equ    OPEN,  5

.equ    WRITE,  4

.equ    CLOSE,  6

.equ    OPEN_FLAGS, 0x41 ## Set O_CREAT and O_WRONLY. Changed 0101 to hex





filename:

        .ascii  "file.PNX\0"



crack:

        .ascii  "bored!\0"

        len = . - crack



.section .text

.global _start



_start:



create_file:

        movl    $OPEN, %eax

        movl    $filename,%ebx

        movl    $OPEN_FLAGS, %ecx

        movl    $0666, %edx

        int    $0x80



copy_descriptor:

        push    %eax



write_to_file:

        movl    $WRITE, %eax

        #pop    %ebx

        movl    4(%ebp), %ebx

        movl    $crack, %ecx

        movl    $len, %edx

        int    $0x80



close_file:

        movl    $CLOSE,%eax

        pop    %ebx

        #movl  4(%esp), %ebx

        int    $0x80

Code:

(gdb) r

Starting program: /tmp/a.out 



Program received signal SIGSEGV, Segmentation fault.

0x08048090 in write_to_file ()

(gdb) disas write_to_file

Dump of assembler code for function write_to_file:

0x804808b <write_to_file>:      mov    $0x4,%eax

0x8048090 <write_to_file+5>:    mov    0x4(%ebp),%ebx

0x8048093 <write_to_file+8>:    mov    $0x80490b1,%ecx

0x8048098 <write_to_file+13>:  mov    $0x7,%edx

0x804809d <write_to_file+18>:  int    $0x80

End of assembler dump.

(gdb) i r ebp

ebp            0x0      0x0

(gdb) q

The program is running.  Exit anyway? (y or n) y

you can't reference ebp with out properly settin up the stack frame, ie:

Code:

.section .data



.equ    STACK_FRAME_SZ, 100

.equ    OPEN,  5

.equ    EXIT, 1

.equ    WRITE,  4

.equ    CLOSE,  6

.equ    OPEN_FLAGS, 0x41 ## Set O_CREAT and O_WRONLY. Changed 0101 to hex





filename:

        .ascii  "file.PNX\0"



crack:

        .ascii  "bored!\0"

        len = . - crack



.section .text

.global _start



_start:



create_file:

        enter  $STACK_FRAME_SZ, $0 

        movl    $OPEN, %eax

        movl    $filename,%ebx

        movl    $OPEN_FLAGS, %ecx

        movl    $0666, %edx

        int    $0x80



copy_descriptor:

        push    %eax



write_to_file:

        movl    $WRITE, %eax

        #pop    %ebx

        movl    4(%ebp), %ebx

        movl    $crack, %ecx

        movl    $len, %edx

        int    $0x80



close_file:

        movl    $CLOSE,%eax

        pop    %ebx

        #movl  4(%esp), %ebx

        int    $0x80



      leave 

        movl    $EXIT, %eax

        int    $0x80

enter is the shortcut for:
pushl %ebp
movl %esp, %ebp
subl STACK_SZ, %esp
and leave is:
movl %ebp, %esp
popl %ebp

also notice i added the sys_exit function. when u enter a program via _start, there is not stack frame setup by libc init code, so u can't just "return" from main. if u ran your code rfurther, u would find it segfaulting after close_file.

Do you have to use a ret after?

you CANNOT ret when u call _start directly, u have to call exit like above. else u need to define main as a global function and use gcc to compile so that libc init code can be inserted, rather than 'ld && as' like i imagine ur using now.

Ok, I understand that now.

One last thing, why "enter $STACK_FRAME_SZ, $0"?

I understand that enter is equivalent to building the stack, but why the operands? what do those serve?

the first arg is the size of the frame, which is the amount to subtract from esp to create it, usually it looks like:
movl %esp, %ebp
pushl %ebp
subl $STACK_FRAME_SZ, %esp
that is just a shortcut for those 3 ins. and the 2nd arg to enter is some crap about nested frames that i dont recall, its not important for simple use.