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cmd/cgo: _cgoCheckPointer call occurs too early for deferred function calls. #15921

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bcmills opened this issue Jun 1, 2016 · 2 comments

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@bcmills
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commented Jun 1, 2016

I found a related-but-different problem when investigating issue #13830.

Making the _cgoCheckPointer call explicitly in the function parameters of a deferred call causes the pointer check to occur at the wrong time. We end up checking whether the argument had a Go pointer at the point where the call was deferred, not whether the argument had a Go pointer at the point where the C call was actually made.

Consider this program:

package main

/*
#include <stdint.h>

struct sneaky {
  int *p;
};

void sneak(struct sneaky *s, uintptr_t *u) {
  *u = (uintptr_t)(s->p);
}
*/
import "C"

import "fmt"

func main() {
    var s C.struct_sneaky
    var u C.uintptr_t

    defer func() { fmt.Println(u) }()
    defer C.sneak(&s, &u)

    var escapee C.int
    s.p = &escapee
}

With go version go1.6.1 linux/amd64, the program (incorrectly) prints a leaked address and exits with code 0.

Changing the call to C.sneak to an explicit call at the end of main causes the program to (correctly) panic with a cgo pointer violation.

The problem is that the _cgoCheckPointer calls wrap the function's arguments, not the function call itself:

  defer _Cfunc_sneak(_cgoCheckPointer((*_Ctype_struct_sneaky)(&s), true).(*_Ctype_struct_sneaky), &u)

@bcmills bcmills changed the title cmd/cgo: _cgoCheckPointer call occurs at wrong time for deferred function calls. cmd/cgo: _cgoCheckPointer call occurs to early for deferred function calls. Jun 1, 2016

@ianlancetaylor ianlancetaylor added this to the Go1.7Maybe milestone Jun 1, 2016

@ianlancetaylor ianlancetaylor self-assigned this Jun 1, 2016

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commented Jun 1, 2016

Nice example.

@bcmills bcmills changed the title cmd/cgo: _cgoCheckPointer call occurs to early for deferred function calls. cmd/cgo: _cgoCheckPointer call occurs too early for deferred function calls. Jun 1, 2016

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commented Jun 1, 2016

CL https://golang.org/cl/23650 mentions this issue.

@gopherbot gopherbot closed this in 7b48020 Jun 3, 2016

@golang golang locked and limited conversation to collaborators Jun 3, 2017

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