You're violating quite a few unsafe.Pointer rules here. Particularly, casting C-allocated memory to a Go type that contains pointers, and particularly an interface, is very bad. The write barriers implicit in the *ptr = raw statement will be very confused.

Not sure why malloc itself is failing though. All the invalid code I see is post-malloc. A complete reproducible example would help.

I was told that you need to use malloc to pass Go types to C

You can do this for pointer-free types. You cannot do it for types containing pointers (which includes interfaces).

A better strategy is to use the new runtime.Pinner api. Then you do not need to call malloc at all.

I figured all that, but yeah, malloc itself failkng is weird.
Does it not just call gcc's malloc?

Does it not just call gcc's malloc?

It calls libc's malloc, yes.
That error (corrupted top size) makes it sound like the malloc heap itself got corrupted somehow. Does this error occur on the first call to your ResponseWriterIntoRaw function, or a subsequent one? I could believe it might happen on a second or subsequent call, as the code at the end of the call would put you squarely in undefined behavior territory.

Does this error occur on the first call to your ResponseWriterIntoRaw function, or a subsequent one?

First call.

It looks like C.malloc finally reached the code below
When the size of top chunk exceeds system_mem, this error will occur
I suggest you debug to this place to see if av(arena) is initialized correctly

      victim = av->top;
      size = chunksize (victim);

      if (__glibc_unlikely (size > av->system_mem))
        malloc_printerr ("malloc(): corrupted top size");

We can't do anything here without a complete, standalone, example that demonstrates the problem.

I actually can't.
I suppose then, the comment about the heap being corrupted is right.

A better strategy is to use the new runtime.Pinner api

Which seems to be poorly documented, and also I'm still getting that fault address error.
Am I not using this right?

func ResponseIntoRaw(raw *http.Response) C.Response {
	pinner := new(runtime.Pinner)
	pinner.Pin(raw)
	return C.Response{(unsafe.Pointer)(raw)}
}
func ResponseWriterIntoRaw(raw http.ResponseWriter) C.ResponseWriter {
	pinner := new(runtime.Pinner)
	pinner.Pin(raw)
	return C.ResponseWriter{(unsafe.Pointer)(&raw)}
}

As the runtime.Pinner docs say, "If the pinned object itself contains pointers to Go objects, these objects must be pinned separately if they are going to be accessed from C code." Just pinning the top level is not enough.

I suggest that you bring your actual problem to a forum. You will get better and faster answers there than you will on the issue tracker. See https://go.dev/wiki/Questions. Thanks.