The download driver code loops through the downloaded_data buffer
but doesn't ensure that it always passes a valid pointer to the
Go side. In particular, if the malloc'd memory ends against an
unmapped page, and the test passes a pointer one past the end
of the memory region, and since the Go side always dereferences
the pointer when creating a slice, it will attempt to read
unmapped memory, causing a segfault.
This bug doesn't always present. Indeed, it depends on the details
of your system's memory allocator. I validated that this could be
a cause of observed crashes on OS X by using mmap and mprotect
to do the allocations ensuring that the page directly after the
memory we use was unmapped/protected. The crash happened exactly
as seen, and was fixed by changing this condition in the while
loop.
Change-Id: I685dac07ff9b904097375dbf850f387450858753
e22c0bff94