验证golang中unsafe包不安全

在go中,uintptr不能持有对象,unsafe包不安全,但是我之前一直没有时间验证,今天写了段代码验证了一下。

代码

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package main

import (
"fmt"
"unsafe"
)

func main() {
a := f()
b := f2()
fmt.Println(a)
fmt.Println(b)
}

//go:noinline
func f() unsafe.Pointer {
d := 1
p := unsafe.Pointer(&d)
return p
}

//go:noinline
func f2() uintptr {
d := 1
p := uintptr(unsafe.Pointer(&d))
return p
}

逃逸分析

根据逃逸分析可以看出来f和f2这两个函数中的d变量分别分配在哪里:

编译参数方法

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$ go build -gcflags '-m -m' unsafe.go
# command-line-arguments
./unsafe.go:16:6: cannot inline f: marked go:noinline
./unsafe.go:23:6: cannot inline f2: marked go:noinline
./unsafe.go:8:6: cannot inline main: function too complex: cost 260 exceeds budget 80
./unsafe.go:11:13: inlining call to fmt.Println func(...interface {}) (int, error) { return fmt.Fprintln(io.Writer(os.Stdout), fmt.a...) }
./unsafe.go:12:13: inlining call to fmt.Println func(...interface {}) (int, error) { return fmt.Fprintln(io.Writer(os.Stdout), fmt.a...) }
./unsafe.go:18:22: &d escapes to heap
./unsafe.go:18:22: from p (assigned) at ./unsafe.go:18:4
./unsafe.go:18:22: from ~r0 (return) at ./unsafe.go:19:2
./unsafe.go:17:2: moved to heap: d
./unsafe.go:25:30: f2 &d does not escape
./unsafe.go:11:13: a escapes to heap
./unsafe.go:11:13: from ~arg0 (assign-pair) at ./unsafe.go:11:13
./unsafe.go:11:13: io.Writer(os.Stdout) escapes to heap
./unsafe.go:11:13: from io.Writer(os.Stdout) (passed to call[argument escapes]) at ./unsafe.go:11:13
./unsafe.go:12:13: io.Writer(os.Stdout) escapes to heap
./unsafe.go:12:13: from io.Writer(os.Stdout) (passed to call[argument escapes]) at ./unsafe.go:12:13
./unsafe.go:12:13: b escapes to heap
./unsafe.go:12:13: from ~arg0 (assign-pair) at ./unsafe.go:12:13
./unsafe.go:12:13: from []interface {} literal (slice-literal-element) at ./unsafe.go:12:13
./unsafe.go:12:13: from fmt.a (assigned) at ./unsafe.go:12:13
./unsafe.go:12:13: from *fmt.a (indirection) at ./unsafe.go:12:13
./unsafe.go:12:13: from fmt.a (passed to call[argument content escapes]) at ./unsafe.go:12:13
./unsafe.go:11:13: main []interface {} literal does not escape
./unsafe.go:12:13: main []interface {} literal does not escape
<autogenerated>:1: os.(*File).close .this does not escape

可以看出来在函数f中,d逃逸到堆上;但是在函数f2中,d没有发生逃逸,uintptr没有持有对象。

汇编

再来看看汇编的结果:

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$ go tool compile -S unsafe.go | grep unsafe.go:24
0x000e 00014 (unsafe.go:24) PCDATA $2, $0
0x000e 00014 (unsafe.go:24) PCDATA $0, $0
0x000e 00014 (unsafe.go:24) MOVQ $1, "".d(SP)
$ go tool compile -S unsafe.go | grep unsafe.go:17
0x001d 00029 (unsafe.go:17) PCDATA $2, $1
0x001d 00029 (unsafe.go:17) PCDATA $0, $0
0x001d 00029 (unsafe.go:17) LEAQ type.int(SB), AX
0x0024 00036 (unsafe.go:17) PCDATA $2, $0
0x0024 00036 (unsafe.go:17) MOVQ AX, (SP)
0x0028 00040 (unsafe.go:17) CALL runtime.newobject(SB)
0x002d 00045 (unsafe.go:17) PCDATA $2, $1
0x002d 00045 (unsafe.go:17) MOVQ 8(SP), AX
0x0032 00050 (unsafe.go:17) MOVQ $1, (AX)

可以看出来,结果也是一样的,f中的d调用了newobject,但是f2中没有。

结论

所以为什么说unsafe包不安全呢,原因之一就是因为go不保证地址一定是有效的,当然还有其它的原因,有时间再验证分享。

请博主喝杯咖啡~