Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH RFC] cred: Add WARN to detect wrong use of get/put_cred | Date | Fri, 12 Jun 2020 16:16:56 +0000 |
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From: Xiaoming Ni > Sent: 12 June 2020 11:28 > Cred release and usage check code flow: > 1. put_cred() > if (atomic_dec_and_test(&(cred)->usage)) > __put_cred(cred); > > 2. __put_cred() > BUG_ON(atomic_read(&cred->usage) != 0); > call_rcu(&cred->rcu, put_cred_rcu); > > 3. put_cred_rcu() > if (atomic_read(&cred->usage) != 0) > panic("CRED: put_cred_rcu() sees %p with usage %d\n", > cred, atomic_read(&cred->usage)); > kmem_cache_free(cred_jar, cred); > > If panic is triggered on put_cred_rcu(), there are two possibilities > 1. Call get_cred() after __put_cred(), usage > 0 > 2. Call put_cred() after __put_cred(), usage < 0 > Since put_cred_rcu is an asynchronous behavior, it is no longer the first > scene when panic, there is no information about the murderer in the panic > call stack... > > So, add WARN() in get_cred()/put_cred(), and pray to catch the murderer > at the first scene. > > Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com> > --- > include/linux/cred.h | 14 +++++++++++--- > 1 file changed, 11 insertions(+), 3 deletions(-) > > diff --git a/include/linux/cred.h b/include/linux/cred.h > index 18639c0..c00d5a1 100644 > --- a/include/linux/cred.h > +++ b/include/linux/cred.h > @@ -224,11 +224,16 @@ static inline bool cap_ambient_invariant_ok(const struct cred *cred) > * > * Get a reference on the specified set of new credentials. The caller must > * release the reference. > + * > + * Initialize usage to 1 during cred resource allocation, > + * so when calling get_cred, usage cannot be 0. > */ > static inline struct cred *get_new_cred(struct cred *cred) > { > - atomic_inc(&cred->usage); > - return cred; > + if (atomic_inc_not_zero(&cred->usage)) > + return cred; > + WARN(1, "get_new_cred after __put_cred"); > + return NULL; > } > > /** > @@ -280,11 +285,14 @@ static inline const struct cred *get_cred_rcu(const struct cred *cred) > static inline void put_cred(const struct cred *_cred) > { > struct cred *cred = (struct cred *) _cred; > + int usage; > > if (cred) { > validate_creds(cred); > - if (atomic_dec_and_test(&(cred)->usage)) > + usage = atomic_dec_return(&(cred)->usage); > + if (usage == 0) > __put_cred(cred); > + WARN(usage < 0, "put_cred after __put_cred"); > } > }
You really don't want to add WARN() to static inline functions. It will bloat horribly. It might be possible to the message into a called function.
One thing I've thought about for reference counts is for the code that allocates and frees the item to add a big number and code that only borrows a reference just adds 1. If the counter is large enough you can separately detect double frees and missing frees for the two different types of allocation.
David
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