13 replies to this topic

[untitled_1]

Hi, am having a bit of trouble with dynamic linking, i am using linux mint the 64bit version, I get errors when i try dynamically link.
here is a simple program that uses calls to c functions:

.section .data
    values: .int 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60
    output: .asciz "The value is %d\n"
    
.section .text
.globl _start
    _start:
        nop
        movl $0, %edi
        xor %rax, %rax
        loop:
            movl values(, %edi, 4 ), %eax
            pushq %rax
            pushq $output
            call printf
            
            inc %edi
        cmpl $11, %edi
        jne loop
        
        movl $1, %eax
        movl $0, %ebx
        int $0x80

have tried linking with two different libraries

the first one i typed: ld -dynamic-link /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2 -lc -o frog frog.o
with this i got the following error : "Segmentation fault.
I know the main cause is the the call to printf

the second thing i tried was : ld -dynamic-link /lib/ld-linux.so.2 -lc -o frog frog.o
with this the error i got was: "bash: ./frog: Accessing a corrupted shared library"

is there like i specific library i must use or download

Edited by dargueta, 03 December 2011 - 02:23 PM.

[dargueta]

You're not cleaning up the stack after calling printf. When you try to return, your data is still on the stack and the processor will interpret that as a return address.

As for the second error, fix the first and we'll go from there.

[untitled_1]

Thanks, the clean up of the stack seemed to have helped, but now when i run the program nothing gets printed in the terminal.
I also have another question: When i try to type the following pushl eax, i get an error "invalid suffix for push" i dont understand why because this instrustion is used in source code

[dargueta]

pushl %eax

Try replacing the call to printf with a function of your own that prints out something hard-coded. If that doesn't show up, then printf is never getting called.

[untitled_1]

oh, ye that was just a typo, however I have found out that you cant push 32 bits onto the stack any more when programming for 64bits, so basically it seems like pushl has been dropped, I find that strange but there must be some reason or another.

[RhetoricalRuvim]

I wonder if you can do it manually.

I don't know much of the 64-bit language, but I think it might be similar to this:

mov rcx, rax 

mov rax, qword [rsp] 

shl rax, 32 

mov eax, ecx 

sub rsp, 4 

mov qword [rsp], rax 

Once again, I don't know 64-bit, so I'm not completely sure if everything in the above code is efficient, or even correct.

[dargueta]

Don't try to push 8-, 16-, or 32-bit data onto the stack. It needs to remain aligned on a 64-bit boundary for hardware reasons.

[untitled_1]

So the command line arguments are passed using 64 bits as well right?

[dargueta]

Those are all passed in as strings, actually. The program converts the strings into numbers or whatever else it needs them to be.

[untitled_1]

I see, so the pointers to the command line arguments will be 64 bit addresses though? The amd64 manual I am using doesn't say much on this.

[dargueta]

That's because it's operating-system dependent; there's no real standard convention as far as that goes, but the C standard guarantees that command-line arguments are passed in as an array of string pointers.

[RhetoricalRuvim]

And pointers, in the 64-bit world, are 64 bits in size, correct?