Stackpointer subtraction in listings 14-4, 14-5, 14-6 #28
Description
The assembly code for listing 14-4 in listing 14-5 seems a bit wierd. RSP is not subtracted enough (only 0xf90) to hold char buffer[1024]. buffer is partly outside the stack and both a and b are completly outside the stack.
...
4004ba: 48 81 ec 90 0f 00 00 sub rsp,0xf90
4004c1: 89 bd fc ef ff ff mov DWORD PTR [rbp-0x1004],edi
4004c7: 89 b5 f8 ef ff ff mov DWORD PTR
...
Same issue with 14-6.
When I compiled and disassembled myself I got some extra bloat at 40055d-40056a and 400588-400597 (maybe you know why?), but RSP is subtracted more than enough (x1020) to hold buffeer, a and b:
$ gcc --version
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc main.c -o test
$objdump -d -M intel test
...
0000000000400546 <maximum>:
400546: 55 push rbp
400547: 48 89 e5 mov rbp,rsp
40054a: 48 81 ec 20 10 00 00 sub rsp,0x1020
400551: 89 bd ec ef ff ff mov DWORD PTR [rbp-0x1014],edi
400557: 89 b5 e8 ef ff ff mov DWORD PTR [rbp-0x1018],esi
40055d: 64 48 8b 04 25 28 00 mov rax,QWORD PTR fs:0x28
400564: 00 00
400566: 48 89 45 f8 mov QWORD PTR [rbp-0x8],rax
40056a: 31 c0 xor eax,eax
40056c: 8b 85 ec ef ff ff mov eax,DWORD PTR [rbp-0x1014]
400572: 3b 85 e8 ef ff ff cmp eax,DWORD PTR [rbp-0x1018]
400578: 7d 08 jge 400582 <maximum+0x3c>
40057a: 8b 85 e8 ef ff ff mov eax,DWORD PTR [rbp-0x1018]
400580: eb 06 jmp 400588 <maximum+0x42>
400582: 8b 85 ec ef ff ff mov eax,DWORD PTR [rbp-0x1014]
400588: 48 8b 55 f8 mov rdx,QWORD PTR [rbp-0x8]
40058c: 64 48 33 14 25 28 00 xor rdx,QWORD PTR fs:0x28
400593: 00 00
400595: 74 05 je 40059c <maximum+0x56>
400597: e8 84 fe ff ff call 400420 <__stack_chk_fail@plt>
40059c: c9 leave
40059d: c3 ret
...