• Tejun Heo's avatar
    rculist: list_first_or_null_rcu() should use list_entry_rcu() · a7179b89
    Tejun Heo authored
    
    
    commit c34ac00caefbe49d40058ae7200bd58725cebb45 upstream.
    
    list_first_or_null() should test whether the list is empty and return
    pointer to the first entry if not in a RCU safe manner.  It's broken
    in several ways.
    
    * It compares __kernel @__ptr with __rcu @__next triggering the
      following sparse warning.
    
      net/core/dev.c:4331:17: error: incompatible types in comparison expression (different address spaces)
    
    * It doesn't perform rcu_dereference*() and computes the entry address
      using container_of() directly from the __rcu pointer which is
      inconsitent with other rculist interface.  As a result, all three
      in-kernel users - net/core/dev.c, macvlan, cgroup - are buggy.  They
      dereference the pointer w/o going through read barrier.
    
    * While ->next dereference passes through list_next_rcu(), the
      compiler is still free to fetch ->next more than once and thus
      nullify the "__ptr != __next" condition check.
    
    Fix it by making list_first_or_null_rcu() dereference ->next directly
    using ACCESS_ONCE() and then use list_entry_rcu() on it like other
    rculist accessors.
    
    v2: Paul pointed out that the compiler may fetch the pointer more than
        once nullifying the condition check.  ACCESS_ONCE() added on
        ->next dereference.
    
    v3: Restored () around macro param which was accidentally removed.
        Spotted by Paul.
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    Reported-by: default avatarFengguang Wu <fengguang.wu@intel.com>
    Cc: Dipankar Sarma <dipankar@in.ibm.com>
    Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Cc: "David S. Miller" <davem@davemloft.net>
    Cc: Li Zefan <lizefan@huawei.com>
    Cc: Patrick McHardy <kaber@trash.net>
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    a7179b89
rculist.h 17.4 KB