[SSSD] [PATCHES] mmap_cache: Skip records which doesn't have same hash

Jakub Hrozek jhrozek at redhat.com
Sat Aug 24 16:33:13 UTC 2013


On Wed, Aug 21, 2013 at 05:56:48PM +0200, Lukas Slebodnik wrote:
> On (20/08/13 21:50), Simo Sorce wrote:
> >On Tue, 2013-08-20 at 10:32 +0200, Lukas Slebodnik wrote:
> >> On (19/08/13 16:49), Simo Sorce wrote:
> >> >On Mon, 2013-08-19 at 21:05 +0200, Lukas Slebodnik wrote:
> >> >> MAIN_PROBLEM: We know that main problem is when hash table entry referes
> >> >> to record, which hashes are different.
> >> >>    (hash_key != rec->hash1 && hash_key != rec->hash2)
> >> >> 
> >> >> This can only happend if data in hash table are changed. Data in hash table can
> >> >> be changed in two situation (adding record, removing record)
> >> >> 
> >> >> A) Adding record to hash table:
> >> >>     1) hash_table[hash_key] == MC_INVALID VAL
> >> >>        there is no record for hash_key and we can safely refer to new record.
> >> >>        hash_table[hash_key] refers to valid first record.
> >> >>        MAIN_PROBLEM cannot occur in this situation.
> >> >>     2) hash_table[hash_key] refers to valid object.
> >> >>        adding record to chain.
> >> >>        hash_table[hash_key] refers to valid record.
> >> >>        MAIN_PROBLEM cannot occur in this situation.
> >> >> 
> >> >> AS we can see, there is no problem with adding record to hash table.
> >> >> 
> >> >> 
> >> >> B) Removing record from hash table
> >> >>     We need to remove record from chain (function sss_mc_rm_rec_from_chain)
> >> >>     1) Record is the first in chain and *HAS NOT* successor
> >> >>               (rec->next == MC_INVALID_VAL)
> >> >>        hash_table[hash_key] will be set to MC_INVALID_VAL
> >> >>        this mean that there is no record for hash_key.
> >> >>        MAIN_PROBLEM cannot occur in this situation.
> >> >>     2) Record is the first in chain and *HAS* successor
> >> >>        hash_table[hash_key] will be set to rec->next
> >> >>        *** MAIN_PROBLEM can occur ***
> >> >>        This will not be problem with attached patch
> >> >>     3) Record is in the middle of chain.
> >> >>        record will be removed from chain. (nothing else)
> >> >>        hash_table[hash_key] refers to valid record, because
> >> >>            1st record is untouched.
> >> >>        MAIN_PROBLEM cannot occur in this situation.
> >> >>     4) Record is at the end of chain.
> >> >>        The situation is the same as in previous point.
> >> >>        MAIN_PROBLEM cannot occur in this situation.
> >> >> 
> >> >> 
> >> >> We can see that only one operation is problematic (B3).
> >> >> 
> >> >> Attached patch fixes this problematic situation. Chains will not be changed.
> >> >> on hash_table[hash_key] will refer to first valid record after removed record.
> >> >> 
> >> >> Simo, thank you very much for review. I hope this version is correct.
> >> >
> >> >After quite some thinking I couldn't find a way to break this, indeed I
> >> >guess the only case where a record can remain orphaned and accessible on
> >> >a chain is if it is the second record on the chain but brought there
> >> >only because it is a next record on another chain, and the first record
> >> >on that chain is removed.
> >> >
> >> >So this new patch seem to address the problem correctly, however I would
> >> >like to see comments before bot the critical assignments in the code
> >> >that points out that the difference in treatment is intentional (so that
> >> >if someone needs to modify the code at a later time he will not 'fix'
> >> >the discrepancy bymistake).
> >> done.
> >> 
> >> >
> >> >Also I think we need to rewrite the comment of the commit, I still do
> >> >not think it is clear enough to convey what the actual issue is.
> >> >
> >> >What about this:
> >> >
> >> >"The code uses 2 hashes for each record, but only one hash table to
> >> >index them both, furthermore each record has only one single 'next'
> >> >pointer.
> >> >
> >> >This means that in certain conditions a record main end up being on a
> >> >hash chain even though its hashes do not match the hash chain. This can
> >> >happen when another record 'drags' it in from another hash chain where
> >> >they both belong.
> >> >
> >> >If the record without matching hashes happens to be the second of the
> >> >chain and the first record is removed, then the non matching record is
> >> >left on the wrong chain. On removal of the non-matching record the hash
> >> >chain will not be updated and the hash chain will end up pointing to an
> >> >invalid slot.
> >> >This slot may be later reused for another record and may not be the
> >> >first slot of this new record. In this case the hash chain will point to
> >> >arbitrary data and may cause issues if the slot is interpreted as the
> >> >head of a record.
> >> >
> >> >By skipping any block that has no matching hashes upon removing the
> >> >first record in a chain we insure that dangling references cannot be
> >> >left in the hash table"
> >> 
> >> I am highly influenced by debugging this, so my point of view is not objective.
> >> I reused your proposal without any change.
> >> 
> >> I also changed resolves section to https://fedorahosted.org/sssd/ticket/2049,
> >> because I used reproducers from BZ997406 and reporter used sssd from master
> >> (including Michal's patch).
> >> 
> >> Functionality of 1st patch is not changed.
> >> 
> >> Thank you very much for review.
> >
> >Looks good to me!
> >
> >Simo.
> >
> 
> Reproducer for this bug: https://bugzilla.redhat.com/show_bug.cgi?id=997406#c9
> 
> While I was investigating the root of the problem I wrote two helper functions,
> because it was impossible to find out solution from coredump.
> Those functions check each entry in hash table, whether hash_key refers to
> valid slot. Functions are called after adding new record and after removing
> record. I think patches can be used as a base for unit tests.
> 
> Patches are really ugly. Macro EXCEPTION causes SEGFAULT :-)
> But it was an aim, because I debug optimized code at the beginning.
> I had an idea about optimized code, but luckily it was not a true.
> 
> LS

Nice work, we should really turn these into unit tests:
https://fedorahosted.org/sssd/ticket/2055



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