A study of the principle of coredump Problem Linux x86 5.8 section C-style data structure memory layout of the Consortium

In C, a union (union) is a bit like a struct, which organizes different types of data, but unlike structs, each member of a struct has its own memory space, and the total length of a struct object is the sum of the lengths of each member. In a union, each member shares a memory space, and the length of a Union object is equal to the longest length of each member.

As described above, a consortium should have a multifaceted nature, that is, at the assembly level, it sometimes shows the structure's characteristics, or array features, or other basic data type characteristics.

Let's take a look at the example:

1 #include <stdio.h>  2 Union xuzhina_dump_c05_s4  3 {  4     int i;  5     Char hello[4];  6};  7   8 int main ()  9 {Ten     Union XUZHINA_DUMP_C05_S4 test;     test.i = 0x656463; n for     (int i = 0; I & Lt 4; i++) (     ("         %c", Test.hello[i]),     ("\ n"),     0;

Assembly Code:

(GDB) disassemble maindump of assembler code for function main:0x08048570 <+0>: Push%EBP 0x08048571 < +1&GT: mov%esp,%ebp 0x08048573 <+3>: and $0xfffffff0,%esp 0x08048576 <+6>: Sub $0x2 0,%esp 0x08048579 <+9>: Movl $0x656463,0x18 (%ESP) 0x08048581 <+17>: Movl $0x0,0x1c (%ESP) 0x08 048589 <+25>: jmp 0x80485a8 <main+56> 0x0804858b <+27>: Lea 0x18 (%ESP),%edx 0x0804858f & LT;+31&GT: mov 0x1c (%ESP),%eax 0x08048593 <+35>: add%edx,%eax 0x08048595 <+37>: movzbl (% EAX),%eax 0x08048598 <+40>: MOVSBL%al,%eax 0x0804859b <+43>: mov%eax, (%ESP) 0x0804859e <+4  6&gt: Call 0x8048430 <[email protected]> 0x080485a3 <+51>: Addl $0x1,0x1c (%ESP) 0x080485a8 <+56&gt: Cmpl $0x3,0x1c (%ESP) 0X080485AD <+61>: Setle%al 0x080485b0 <+64>: Test%al,%a L 0X080485B2 &LT;+66&Gt;: jne 0x804858b <main+27> 0x080485b4 <+68>: Movl $0xa, (%ESP) 0X080485BB <+75>: Call    0x8048430 <[email protected]> 0x080485c0 <+80>: mov $0x0,%eax 0x080485c5 <+85>: jmp  0X80485CF <main+95> 0x080485c7 <+87>: mov%eax, (%ESP) 0X080485CA <+90>: Call 0x8048460 <[email protected]> 0x080485cf <+95>: Leave 0x080485d0 <+96>: ret End of Assembler Dump.

From the assembly code above, UNIONXUZHINA_DUMP_C05_S4 is actually accessed with an int and a char array. See the following two sets of instructions

   0x08048579 <+9>:     movl   $0x656463,0x18 (%esp)   0x0804858b <+27>:    Lea    0x18 (%ESP),%edx   0x0804858f <+31>:    mov    0x1c (%esp),%eax   0x08048593 <+35>:    add    %edx,%eax   0x08048595 <+37>:    movzbl (%eax),%eax

From the above discussion, the union member type is best avoided as a pointer type. Because the pointer is easily overwritten, a "accessviolation" error occurs. Assuming the pointer is a function pointer, the coredump of the previous section appears.

A study of the principle of coredump Problem Linux x86 5.8 section C-style data structure memory layout of the Consortium