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cmd/cgo: false msan reports when C function returns struct on stack #26209

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ianlancetaylor opened this issue Jul 4, 2018 · 1 comment

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@ianlancetaylor
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commented Jul 4, 2018

When using -msan with the C compiler option -fsanitize=memory, when a C function returns a struct on the Go stack, the memory sanitizer code can mark a portion of the Go stack as uninitialized. If that portion of the Go stack is later passed to msanread, perhaps in a different function, the memory sanitizer will report an error.

Test case, which shows the problem when built with go build -msan.

package main

/*
#cgo LDFLAGS: -fsanitize=memory
#cgo CPPFLAGS: -fsanitize=memory

#include <stdint.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
	uintptr_t a[20];
} S;

S f() {
	S *p;

	p = (S *)(malloc(sizeof(S)));
	p->a[0] = 0;
	return *p;
}
*/
import "C"

// allocateStack extends the stack so that stack copying doesn't
// confuse the msan data structures.
//go:noinline
func allocateStack(i int) int {
	if i == 0 {
		return i
	}
	return allocateStack(i - 1)
}

// F1 marks a chunk of stack as uninitialized.
// C.f returns an uninitialized struct on the stack, so msan will mark
// the stack as uninitialized.
//go:noinline
func F1() uintptr {
	s := C.f()
	return uintptr(s.a[0])
}

// F2 allocates a struct on the stack and converts it to an empty interface,
// which will call msanread and see that the data appears uninitialized.
//go:noinline
func F2() interface{} {
	return C.S{}
}

func poisonStack(i int) int {
	if i == 0 {
		return int(F1())
	}
	F1()
	r := poisonStack(i - 1)
	F2()
	return r
}

func main() {
	allocateStack(16384)
	poisonStack(128)
}
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commented Jul 4, 2018

Change https://golang.org/cl/122196 mentions this issue: cmd/cgo: mark C result as written for msan

@gopherbot gopherbot closed this in e2f8766 Jul 5, 2018

@golang golang locked and limited conversation to collaborators Jul 5, 2019

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