Messages in this thread | | | Date | Tue, 13 Jun 2006 09:26:46 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 2.6.17-rc6 7/9] Remove some of the kmemleak false positives |
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* Pekka J Enberg <penberg@cs.Helsinki.FI> wrote:
> Hi Ingo, > > On Mon, 12 Jun 2006, Ingo Molnar wrote: > > i dont know - i feel uneasy about the 'any pointer' method - it has a > > high potential for false negatives, especially for structures that > > contain strings (or other random data), etc. > > Is that a problem in practice? Structures that contain data are > usually allocated from the slab. There needs to be a link to that > struct from the gc roots to get a false negative. Or am I missing > something here?
you should think of this in terms of a 'graph of data', where each node is a block of memory. The edges between nodes are represented by pointers. The graph roots from .data/bss, but it may go indefinitely into dynamically allocated blocks as well - just think of a hash-list where the hash list table is in .data, but all the chain entries are in allocated blocks and the chaining can be arbitrarily deep.
Furtermore, each block of data has a couple of fields within it that contain 'outgoing pointers', and each block of data has a couple of addresses associated with it that are valid targets for 'incoming pointers'.
The task of kmemleak is to find orphan blocks of memory - the ones that are not connected to the graph via any edge. For that it starts scanning in .data/bss and recursively searches through the blocks of memory (marking all scanned blocks, to avoid circular walking of the graph) until it has walked the whole graph. Blocks that were registered but were not touched during this recursive walking are the leaks.
Currently kmemleak does not track the per-block position of 'outgoing pointers': it assumes that all fields within a block may be an outgoing pointer. This is a source of false negatives. (fields that do not contain a real pointer might accidentally contain a value that is interpreted as a false edge - falsely connecting a leaked block to the graph.)
Kmemleak does recognize 'incoming pointers' via the offsetof tracking method, but it's limited in that it is not a type-accurate method either: it tracks per-size offsets, so two types accidentally having the same size merges their 'possible incoming pointer offset' lists, which introduces false negatives. (a pointer may be considered an incoming edge while in reality the pointer is not validly pointing into this structure)
The full matching that was suggested before would further weaken the 'incoming pointers' logic and would introduce yet another source of false negatives: we'd match every block pointer against every possible target address that points to within another block.
My suggestion would be to attempt to achieve perfect matches: annotate structures to figure out the offset of pointers, and thus to figure out the precise source addresses and a precise list of valid target addresses. This is a quite elaborate task to pull off though, and i'm not sure it's possible without intolerable maintainance overhead, but we should consider it nevertheless. It will also be _much_ faster, because per block we'd only have to scan a handful of outgoing pointers.
Perhaps a hybrid method could be used: by default we assume the most lenient structure: if the block type is 'unknown' (which is the default for not-yet-annotated structures) then we'd assume that all fields are pointers, and that they could all be targets too.
Once a structure is annotated, the scope of scanning is drastically reduced: only the annotated fields are scanned for pointers (and at that point we'd also _enforce_ that those pointers do indeed point to valid blocks of memory - i.e. this would also serve as a pointer-correctness checker), and annotated blocks will also restrict the scope of 'incoming pointers'.
Naturally, there would be two types of annotations: one that finetunes the scanning of outgoing pointers to happen only for fields that are true pointers, and one that finetunes incoming pointer matching to only those addresses within the block that program logic allows. All in a strictly per-type manner.
This also means that by default we'd have no false positives at all, but that there is a capable annotation method to reduce the amount of false negatives, in a gradual and managable way - down to zero if everything is annotated.
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