Copying and calling Functions in x86 AT&T-Assembler in gcc?

You have the following problems: when setting up the stack for your call to copy_bytes, you want pushl $to_gen_inner not pushl to_gen_inner (the latter pushes the contents of memory to_gen_inner points to) when copying values to the local stackframe inside copy_bytes, you need to write the register you just read the parameter into, instead of always writing EAX lcall *(%eax) expects to read an address from the memory pointed to by EAX, and jump there. Moreover, it expects to read 48 bytes, with the first 16 being the segment. I've replaced your lcall with call *%eax; also replaced the lrets with rets accordingly.

The call to printregs is assembled as a relative call, which blows up since the instruction you're executing is no longer at the same relative offset to the target as it was when it was assembled. I've replaced it with movl $printregs, %ecx call *%ecx (which trashes %ecx) finally, to_gen_inner sets up the stackframe on entry, but fails to destroy it on exit With all those fixed, the code looks like this: . Global main .

Section . Data to_gen_inner: #x f, implicit n pushl %ebp movl %esp, %ebp movl $0xFF00FF00, %eax movl $printregs, %ecx call *%ecx movl %ebp, %esp popl %ebp ret . Set to_gen_inner_len, .

- to_gen_inner . Section . Text main: pushl %ebp movl %esp, %ebp #allocate memory pushl $to_gen_inner_len call malloc popl %ecx pushl $to_gen_inner_len pushl $to_gen_inner pushl %eax call copy_bytes popl %eax popl %ecx popl %ecx call *%eax movl %ebp, %esp popl %ebp ret printfregs: .

Ascii "eax: %8X\nebx: %8X\necx: %8X\nedx: %8X\n\0" printregs: pushl %edx pushl %ecx pushl %ebx pushl %eax pushl $printfregs call printf popl %ecx popl %eax popl %ebx popl %ecx popl %edx ret copy_bytes: #dest source length pushl %ebp movl %esp, %ebp subl $24, %esp movl 8(%ebp), %ecx # dest movl %ecx, -4(%ebp) movl 12(%ebp), %ebx # source movl %ebx, -8(%ebp) movl 16(%ebp), %eax # length movl %eax, -12(%ebp) addl %eax, %ecx # last dest-byte movl %ecx, -16(%ebp) addl %eax, %edx # last source-byte movl %ecx, -20(%ebp) movl -4(%ebp), %eax movl -8(%ebp), %ebx movl -16(%ebp), %ecx copy_bytes_2: movb (%ebx), %dl movb %dl, (%eax) incl %eax incl %ebx cmp %eax, %ecx jne copy_bytes_2 movl %ebp, %esp popl %ebp ret ...which builds and runs here for me. Hope that helps.

Hm. Strange. Your code also works with just call *%eax (what I did first).

Seems like this wasnt the main reason why my code failed. Anyway. Thank you very much.

– schoppenhauer Mar 12 '09 at 23:40 Ah - looks like it's call *%eax vs call *(%eax) - the former branches to the address at %eax, the latter calls the address contained in the memory pointed to by %eax. Will edit answer. – moonshadow Mar 13 '09 at 0:07.

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