buf(9) woes: when does bcopy do nothing at all?

[Alan Somers]

How is it possible that bcopy() doesn't affect its output array at
all?  While investigating a data corruption issue in fuse, I narrowed
the problem down to a bcopy operation that apparently has no affect.
The code in question is:

bcopy(cp, iov->iov_base, cnt);
r = memcmp(cp, iov->iov_base, cnt);
if (r)
    printf("uiomove: miscompare\n");

Rationally, I would expect that line never to be printed.  But it
does.  The destination is always all zeros, even though the source is
not.  I can only guess that there's something wrong about the way that
I I'm using buf(9), because the output is part of a buffer allocated
by bread(9).  I've been able to rule out:

1) Race conditions.  The bug is 100% reproducible, and doubling the
bcopy or changing the timing in other ways has no effect.
2) Unmapped buffer.  I verified that the buf is not unmapped_buf.
3) Overlapping src and dst
4) Duplicated pages.  I verified that each of the buf's pages has a
unique physical address
5) Bad RAM.  My machine passes memtest86, and anyway the failure is
too specific and reproducible to be caused by bad hardware.

What could I be missing?  Do I need to do something to prepare the buf
before I can use it?  The code that allocates the buffer is here:
https://svnweb.freebsd.org/base/projects/fuse2/sys/fs/fuse/fuse_io.c?view=markup#l240

-Alan