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    SubjectRe: BUG? "Call fasync() functions without the BKL" is racy
    On Wed, 03 Dec 2008 22:21:02 +0100
    Andi Kleen <ak@linux.intel.com> wrote:

    > > Serioulsy, I think the best (partial, yes) fix for now is to restore
    > > lock_kernel() in setfl() and change ioctl_fioxxx() accordingly.
    > > At least this protect us from tty too.
    >
    > For 2.6.28 I agree.

    OK, what do you all think of the following? It returns fcntl.c to its
    previous state, and adds locking in fs/ioctl.c. It's worth noting that
    ioctl_fioasync() has always been racy in exactly the same way.

    If this passes muster I'll send it upward shortly.

    jon

    --

    Restore BKL protection to manipulations of f_flags

    Changeset a238b790d5f99c7832f9b73ac8847025815b85f7 (Call fasync()
    functions without the BKL) introduced a race which could leave
    file->f_flags in a state inconsistent with what the underlying
    driver/filesystem believes. Revert that change, and also fix the same
    races in ioctl_fioasync() and ioctl_fionbio().

    This is a minimal, short-term fix; the real fix will not involve the
    BKL.

    Reported-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: Jonathan Corbet <corbet@lwn.net>

    diff --git a/fs/fcntl.c b/fs/fcntl.c
    index ac4f7db..549daf8 100644
    --- a/fs/fcntl.c
    +++ b/fs/fcntl.c
    @@ -19,6 +19,7 @@
    #include <linux/signal.h>
    #include <linux/rcupdate.h>
    #include <linux/pid_namespace.h>
    +#include <linux/smp_lock.h>

    #include <asm/poll.h>
    #include <asm/siginfo.h>
    @@ -175,6 +176,11 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
    if (error)
    return error;

    + /*
    + * We still need a lock here for now to keep multiple FASYNC calls
    + * from racing with each other.
    + */
    + lock_kernel();
    if ((arg ^ filp->f_flags) & FASYNC) {
    if (filp->f_op && filp->f_op->fasync) {
    error = filp->f_op->fasync(fd, filp, (arg & FASYNC) != 0);
    @@ -185,6 +191,7 @@ static int setfl(int fd, struct file * filp, unsigned long arg)

    filp->f_flags = (arg & SETFL_MASK) | (filp->f_flags & ~SETFL_MASK);
    out:
    + unlock_kernel();
    return error;
    }

    diff --git a/fs/ioctl.c b/fs/ioctl.c
    index d152856..fc8fcaa 100644
    --- a/fs/ioctl.c
    +++ b/fs/ioctl.c
    @@ -400,11 +400,9 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,

    /* Did FASYNC state change ? */
    if ((flag ^ filp->f_flags) & FASYNC) {
    - if (filp->f_op && filp->f_op->fasync) {
    - lock_kernel();
    + if (filp->f_op && filp->f_op->fasync)
    error = filp->f_op->fasync(fd, filp, on);
    - unlock_kernel();
    - } else
    + else
    error = -ENOTTY;
    }
    if (error)
    @@ -440,11 +438,17 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
    break;

    case FIONBIO:
    + /* BKL needed to avoid races tweaking f_flags */
    + lock_kernel();
    error = ioctl_fionbio(filp, argp);
    + unlock_kernel();
    break;

    case FIOASYNC:
    + /* BKL needed to avoid races tweaking f_flags */
    + lock_kernel();
    error = ioctl_fioasync(fd, filp, argp);
    + unlock_kernel();
    break;

    case FIOQSIZE:

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