lkml.org 
[lkml]   [2011]   [Mar]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    Subject[tip:core/futexes] futex: Deobfuscate handle_futex_death()
    Commit-ID:  6e0aa9f8a8190e0879a29bd67aa606b51734a122
    Gitweb: http://git.kernel.org/tip/6e0aa9f8a8190e0879a29bd67aa606b51734a122
    Author: Thomas Gleixner <tglx@linutronix.de>
    AuthorDate: Mon, 14 Mar 2011 10:34:35 +0100
    Committer: Thomas Gleixner <tglx@linutronix.de>
    CommitDate: Mon, 14 Mar 2011 21:08:47 +0100

    futex: Deobfuscate handle_futex_death()

    handle_futex_death() uses futex_atomic_cmpxchg_inatomic() without
    disabling page faults. That's ok, but totally non obvious.

    We don't hold locks so we actually can and want to fault here, because
    the get_user() before futex_atomic_cmpxchg_inatomic() does not
    guarantee a R/W mapping.

    We could just add a big fat comment to explain this, but actually
    changing the code so that the functionality is entirely clear is
    better.

    Use the helper function which disables page faults around the
    futex_atomic_cmpxchg_inatomic() and handle a fault with a call to
    fault_in_user_writeable() as all other places in the futex code do as
    well.

    Pointed-out-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Acked-by: Darren Hart <darren@dvhart.com>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Matt Turner <mattst88@gmail.com>
    Cc: Russell King <linux@arm.linux.org.uk>
    Cc: David Howells <dhowells@redhat.com>
    Cc: Tony Luck <tony.luck@intel.com>
    Cc: Michal Simek <monstr@monstr.eu>
    Cc: Ralf Baechle <ralf@linux-mips.org>
    Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
    Cc: Paul Mundt <lethal@linux-sh.org>
    Cc: "David S. Miller" <davem@davemloft.net>
    Cc: Chris Metcalf <cmetcalf@tilera.com>
    LKML-Reference: <alpine.LFD.2.00.1103141126590.2787@localhost6.localdomain6>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    ---
    kernel/futex.c | 17 ++++++++++++++---
    1 files changed, 14 insertions(+), 3 deletions(-)

    diff --git a/kernel/futex.c b/kernel/futex.c
    index c6bef6e..e9251d9 100644
    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -2458,9 +2458,20 @@ retry:
    * userspace.
    */
    mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
    - if (futex_atomic_cmpxchg_inatomic(&nval, uaddr, uval, mval))
    - return -1;
    -
    + /*
    + * We are not holding a lock here, but we want to have
    + * the pagefault_disable/enable() protection because
    + * we want to handle the fault gracefully. If the
    + * access fails we try to fault in the futex with R/W
    + * verification via get_user_pages. get_user() above
    + * does not guarantee R/W access. If that fails we
    + * give up and leave the futex locked.
    + */
    + if (cmpxchg_futex_value_locked(&nval, uaddr, uval, mval)) {
    + if (fault_in_user_writeable(uaddr))
    + return -1;
    + goto retry;
    + }
    if (nval != uval)
    goto retry;


    \
     
     \ /
      Last update: 2011-03-14 21:19    [W:0.039 / U:62.760 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site