Re: [Libguestfs] libnbd golang failure on RISC-V
On Thu, Jun 09, 2022 at 05:00:46PM +0100, Daniel P. Berrangé wrote:
Well it isn't use-after-free, because we've cleared the
pointer we freed.
Yes, indeed that's what I meant to say!
> It seems a bit of an odd function however. Wouldn't it be
better to
> changes the Bytes function so that it tests if the pointer is NULL and
> panics?
In theory I guess both should be equivalent in terms of
semantics for the caller.
Also I feel like 'Free' ought to set 'b.Size = 0' after
it set 'b.P = nul'. That should solve the problem for
the Bytes & Slice method tests at least, but probably not
the Get method test.
Making this change has very unexpected effects (although I think I
partially understand what's going on). Note this is replicable on
x86-64.
=== RUN TestAioBufferBytesAfterFree
libnbd_020_aio_buffer_test.go:108: Did not recover from panic calling Bytes() after
Free()
--- FAIL: TestAioBufferBytesAfterFree (0.00s)
This is failing because C.GoBytes does _not_ segfault, since it is
being asked to memcpy a zero length buffer.
=== RUN TestAioBufferSliceAfterFree
--- PASS: TestAioBufferSliceAfterFree (0.00s)
I think I understand why this test now passes, because of the way that
slices are created using b.Size which is now 0.
=== RUN TestAioBufferGetAfterFree
--- PASS: TestAioBufferGetAfterFree (0.00s)
Why does this test now pass?! It's still dereferencing a NULL
pointer.
I wonder why Get doesn't do bounds checking? That seems like a
general bug.
> NB: this _does not_ address the other problem where
GODEBUG=cgocheck=2
> complains about "fatal error: Go pointer stored into non-Go memory".
Maybe that message across comes from the Go signal handler that's
trying to cope with the SEGV from the NULL reference, causing it
to trip over itself & thus not turn the problem into a pnaic.
We're rebuilding the latest Fedora packages, so my plan here is to try
with the newest golang when it becomes available and hope it's been
fixed upstream (if it's a golang problem).
Rich.
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