CODEWITHSUNDEEP
cx86-64decompiling
randy
randy

Reputation: 53

Manual decompilation of asm snippet

I've been trying to decompile the following asm snippet(that's all I have):

55                      push   %rbp
48 89 e5                mov    %rsp,%rbp
48 81 ec d0 00 00 00    sub    $0xd0,%rsp
64 48 8b 04 25 28 00    mov    %fs:0x28,%rax
00 00 
48 89 45 f8             mov    %rax,-0x8(%rbp)
31 c0                   xor    %eax,%eax
48 c7 85 30 ff ff ff    movq   $0x0,-0xd0(%rbp)
00 00 00 00 
48 8d b5 38 ff ff ff    lea    -0xc8(%rbp),%rsi
b8 00 00 00 00          mov    $0x0,%eax
ba 18 00 00 00          mov    $0x18,%edx
48 89 f7                mov    %rsi,%rdi
48 89 d1                mov    %rdx,%rcx
f3 48 ab                rep stos %rax,%es:(%rdi)
48 8b 15 19 06 20 00    mov    0x200619(%rip),%rdx
48 8d 85 30 ff ff ff    lea    -0xd0(%rbp),%rax
be ce 0f 40 00          mov    $0x400fce,%esi
48 89 c7                mov    %rax,%rdi
b8 00 00 00 00          mov    $0x0,%eax
e8 4e fc ff ff          callq  4008a0 <sprintf@plt>

Here is my attempt:

char buf[192] = {0};
sprintf(buf, "hello %s", name);

I've compiled this with gcc 4.8.5, and it gave me:

55                      push   %rbp
48 89 e5                mov    %rsp,%rbp
48 81 ec d0 00 00 00    sub    $0xd0,%rsp
64 48 8b 04 25 28 00    mov    %fs:0x28,%rax
00 00 
48 89 45 f8             mov    %rax,-0x8(%rbp)
31 c0                   xor    %eax,%eax
48 8d b5 30 ff ff ff    lea    -0xd0(%rbp),%rsi
b8 00 00 00 00          mov    $0x0,%eax
ba 18 00 00 00          mov    $0x18,%edx
48 89 f7                mov    %rsi,%rdi
48 89 d1                mov    %rdx,%rcx
f3 48 ab                rep stos %rax,%es:(%rdi)
48 8b 15 14 14 20 00    mov    0x201414(%rip),%rdx
48 8d 85 30 ff ff ff    lea    -0xd0(%rbp),%rax
be 2e 10 40 00          mov    $0x40102e,%esi
48 89 c7                mov    %rax,%rdi
b8 00 00 00 00          mov    $0x0,%eax
e8 cb fb ff ff          callq  4008a0 <sprintf@plt>

I'm struggling to figure out why this exists:

movq   $0x0,-0xd0(%rbp)

and also the subsequent usage of -0xd0(%rbp) as a pointer for the argument to sprintf. I'm puzzled because the rep stos begin at -0xc8(%rbp) and not -0xd0(%rbp).

This is probably compiler specific, but still I'm curious what could possibly be the original code that produced that asm.

Upvotes: 4

Views: 196

Answers (1)

davmac
davmac

Reputation: 20641

I imagine something like:

char buf[192] = {0, 0, 0, 0, 0, 0, 0, 0};
sprintf(buf + 8, "hello %s", name);

... would give you that output.

The movq instruction you refer to stores 0 (an 8-byte quantity) at the beginning of an array. The -0xc8(%rbp) comes from copying a string to an offset within the array.

Upvotes: 1

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