Segmentation fault in matcher/queryoptimiser

[Robert Stepanek]

Since a couple of weeks we are experiencing occasional segmentation
faults within Xapian 1.5. We can't reproduce the crashes, but we have
strong hints that they are due to memory corruption. We have narrowed
down our root cause analysis to phrase searches on multi-databases that
fail on reading the `hint` field in the`QueryOptimiser`class [1].

We'd appreciate any hints on how to fix this. I've written up our
findings and solution attempts below. Should we post this on trac?

Our findings so far
================
In a core dump we see that calling the `open_nearby_postlist` function
on the `hint` variable [2] falls of a cliff, resulting in a segfault:

    (gdb) bt 2
    #0  0x000000000001eaa1 in ?? ()
    #1  0x00007fa19d09231f in LocalSubMatch::open_post_list
    (this=0x13527d0, term=..., wqf=1, factor=1,
        need_positions=<optimized out>, in_synonym=<optimized out>,
        qopt=0x7ffe66370940, lazy_weight=false)
        at matcher/localsubmatch.cc:289

the line at localsubmatch.cc:289 is

    pl = hint->open_nearby_postlist(term);

Unfortunately, the compiler had optimised a lot of debugging information
away. Still, it's clear where the system crashed. 

Following a similar crash and re-running the query we could not
reproduce the crash, but valgrind catched a read-after-free on the
`hint` field (full valgrind log attached):

    ==2265126== Invalid read of size 8
    ==2265126==    at 0x9A6B313:
    LocalSubMatch::open_post_list(std::string const&, unsigned int,
    double, bool, bool, QueryOptimiser*, bool) (localsubmatch.cc:289)

which got free'd in this codepath:

    ==2265126==  Address 0xb3fdfd0 is 0 bytes inside a block of size 216
    free'd
    ==2265126==    at 0x4C2A360: operator delete(void*) (in
    /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==2265126==    by 0x99C0B75: operator() (queryinternal.cc:65)
    ==2265126==    by 0x99C0B75:
    for_each<__gnu_cxx::__normal_iterator<Xapian::PostingIterator::Internal**,
    std::vector<Xapian::PostingIterator::Internal*> >,
    delete_ptr<Xapian::PostingIterator::Internal> > (stl_algo.h:3755)
    ==2265126==    by 0x99C0B75: Xapian::Internal::Context::reset()
    (queryinternal.cc:155)
    ==2265126==    by 0x99C481F:
    Xapian::Internal::QueryWindowed::postlist_windowed(Xapian::Query::op,
    Xapian::Internal::AndContext&, QueryOptimiser*, double) const
    (queryinternal.cc:1668)

which is triggered by this condition in queryinternal.cc#L1665 ([2]);

        if (!qopt->db.has_positions()) {
	// No positions in this subdatabase so this matches nothing,
	// which means the whole andcontext matches nothing.
	ctx.reset();
	return;
    }

How to fix this
============
Here is what I believe is happening:

We are using subdatabases (all glass) for caching recent database
additions, and some of these do not contain positional information. This
makes the internal query code reset the AND-context, which in effect
frees its postlist. One of the postlist entries is still pointed at by
the `hint` field of QueryOptimiser from a previous submatch, and the
next call to `get_hint_postlist` in queryoptimiser.h#L106 references the
bogus address.

A fix to avoid this is simple: just reset the QueryOptimiser hint field
when free'ing the context postlist. I've written a one-liner for this
here [3]. But I'm not yet convinced that's all there is: could the hint
field be used already somewhere else? Should we probably keep it and
make QueryOptimiser take ownership?

[1]
https://github.com/xapian/xapian/blob/master/xapian-core/matcher/queryoptimiser.h#L51
[2]
https://github.com/xapian/xapian/blob/master/xapian-core/api/queryinternal.cc#L1665
[3]
https://github.com/rsto/xapian/commit/3e7d65b25eef00347f5c764af5ff4d770433ac9b
[4]
https://github.com/xapian/xapian/blob/master/xapian-core/matcher/queryoptimiser.h#L106

Cheers,
Robert

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