Using the GNU Assembler I'm trying to call malloc to create a C struct, insert values into that struct and return a pointer to that struct. Below is the struct:
struct node {
void *next;
void *last;
char *name;
int id;
uint8_t number;
uint8_t numberTwo;
uint8_t numberThree;
uint8_t numberFour;
}
Below is the GNU Assembly that I'm using:
push %ebp
movl %esp, %ebp
subl $0x1c, %esp
#movl $20, (%esp) #I've tried this but it doesn't seem to help
push $20 #Struct is 20 bytes
call malloc
test %eax, %eax
jz allocateFailed
jmp allocateOK
allocateFailed:
leave
ret
allocateOK:
movl 8(%ebp), %edx
movl %edx, 8(%eax)
movl 12(%ebp), %edx
movl %edx, 12(%eax)
leave
ret
I've updated the code to stay away from using the %ebx register and taking a look at an Objdump of the function that is how the assembler handled this.
I'm getting the parameters fine but when I try to put them in the newly created struct I get a seg fault. Below is the core dump.
Program terminated with signal 6, Aborted.
0 0xb7727424 in __kernel_vsyscall ()
Below is what I get what I run a backtrace in gdb:
0 allocateOK () at new_student.s:25
1 0x00000014 in ?? () #Not sure what this is coming from
2 0xb7fc5a20 in ?? () from /lib/i386-linux-gnu/libc.so.6
3 0x080485ed in test () #function that calls and passes in the parameters
4 0x080487b4 in main ()
I've also written this function in C and compiled with the --gstabs and -o and used objdump to get a look at it. I seems to be pretty close to mine but I just can't seem to figure it out.
sub $0x1c,%esp
movl $0x14,(%esp)
call 8048350 <malloc@plt>
mov 0x20(%esp),%edx
mov %edx,0xc(%eax)
mov 0x24(%esp),%edx
mov %edx,0x8(%eax)
Any help is greatly appreciated. I've never used assembly (only C) to create and work with structs. I've done some research and have been trying a few things but I just seem to get it.
