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    SubjectRe: [PATCH] cfg80211: clear cfg80211_inform_bss() from kmemleak reports
    On Fri, 2009-09-04 at 07:04 +0200, Johannes Berg wrote:
    > On Thu, 2009-09-03 at 13:43 -0700, Luis R. Rodriguez wrote:
    > > On Thu, Sep 03, 2009 at 11:17:17AM -0700, Johannes Berg wrote:
    > > > On Thu, 2009-09-03 at 11:13 -0700, Luis R. Rodriguez wrote:
    > > >
    > > > > What I meant is it gobbles it up and spits another thing out. When it
    > > > > gobbles it up the routine then uses kref_put().
    > > > >
    > > > > > Why can it not track this?
    > > > >
    > > > > It probably can, just not sure if it follows kref_put(), I was under
    > > > > the impression here it doesn't and because of it we were getting false
    > > > > positives. Catalin, can you confirm?
    > > >
    > > > Ah I'd think that if it can't track it then that's because we use a
    > > > pointer to the middle of the struct to keep track of it much of the
    > > > time.
    > >
    > > So you agree with the patch but not the commit log entry?
    > I'm not sure -- I think kmemleak should be able to figure it out, and if
    > you were using IBSS then we actually have a leak that we need to plug,
    > but otherwise I'd prefer to get some more input from Catalin first.

    First of all, kmemleak_ignore() is not the right function to mark a
    false positive as it completely ignores an object even though it may
    have pointers to others. The kmemleak_not_leak() function should be
    used. However, there are only two places in the kernel where this was
    actually needed (one of them is a real leak but we ignore it as it makes
    the code more complicated).

    So, I think we should try to figure out why kmemleak reports it. There
    are a few common cases:
    1. transient false positive - this should disappear after a few
    2. a pointer leading to the reported object is stored in an area of
    memory not scanned by kmemleak - most commonly pages allocated
    explicitly (alloc_pages etc.) as kmemleak doesn't track these.
    The preferred solution is to inform kmemleak about such page
    (kmemleak_alloc/kmemleak_free) rather than marking the false
    3. a pointer leading to the reported object isn't actually pointing
    to anywhere inside the structure (i.e. using the physical
    address). Here we would use kmemleak_not_leak()

    > Catalin, is it conceivable that kmemleak reports false positives if we
    > use a struct like
    > struct pubbss {
    > ...
    > };
    > struct bss {
    > ...
    > struct pubbss pub;
    > };
    > and then keep track of &bss->pub; pointers instead of bss directly?

    It should not report false positive here. That's a pretty common case
    with struct list_head, struct device etc. and kmemleak handles them
    properly - if there is a memory location pointing to *anywhere* inside a
    structure, the object is considered referenced and not reported.


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