• Russ Cox's avatar
    runtime: fix callwritebarrier · 001438bd
    Russ Cox authored
    Given a call frame F of size N where the return values start at offset R,
    callwritebarrier was instructing heapBitsBulkBarrier to scan the block
    of memory [F+R, F+R+N). It should only scan [F+R, F+N). The extra N-R
    bytes scanned might lead into the next allocated block in memory.
    Because the scan was consulting the heap bitmap for type information,
    scanning into the next block normally "just worked" in the sense of
    not crashing.
    
    Scanning the extra N-R bytes of memory is a problem mainly because
    it causes the GC to consider pointers that might otherwise not be
    considered, leading it to retain objects that should actually be freed.
    This is very difficult to detect.
    
    Luckily, juju turned up a case where the heap bitmap and the memory
    were out of sync for the block immediately after the call frame, so that
    heapBitsBulkBarrier saw an obvious non-pointer where it expected a
    pointer, causing a loud crash.
    
    Why is there a non-pointer in memory that the heap bitmap records as
    a pointer? That is more difficult to answer. At least one way that it
    could happen is that allocations containing no pointers at all do not
    update the heap bitmap. So if heapBitsBulkBarrier walked out of the
    current object and into a no-pointer object and consulted those bitmap
    bits, it would be misled. This doesn't happen in general because all
    the paths to heapBitsBulkBarrier first check for the no-pointer case.
    This may or may not be what happened, but it's the only scenario
    I've been able to construct.
    
    I tried for quite a while to write a simple test for this and could not.
    It does fix the juju crash, and it is clearly an improvement over the
    old code.
    
    Fixes #10844.
    
    Change-Id: I53982c93ef23ef93155c4086bbd95a4c4fdaac9a
    Reviewed-on: https://go-review.googlesource.com/10317Reviewed-by: 's avatarAustin Clements <austin@google.com>
    001438bd
mheap.go 29.5 KB