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    Subject[ 145/153] usermodehelper: implement UMH_KILLABLE
    3.2-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Oleg Nesterov <oleg@redhat.com>

    commit d0bd587a80960d7ba7e0c8396e154028c9045c54 upstream.

    Implement UMH_KILLABLE, should be used along with UMH_WAIT_EXEC/PROC.
    The caller must ensure that subprocess_info->path/etc can not go away
    until call_usermodehelper_freeinfo().

    call_usermodehelper_exec(UMH_KILLABLE) does
    wait_for_completion_killable. If it fails, it uses
    xchg(&sub_info->complete, NULL) to serialize with umh_complete() which
    does the same xhcg() to access sub_info->complete.

    If call_usermodehelper_exec wins, it can safely return. umh_complete()
    should get NULL and call call_usermodehelper_freeinfo().

    Otherwise we know that umh_complete() was already called, in this case
    call_usermodehelper_exec() falls back to wait_for_completion() which
    should succeed "very soon".

    Note: UMH_NO_WAIT == -1 but it obviously should not be used with
    UMH_KILLABLE. We delay the neccessary cleanup to simplify the back
    porting.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Cc: Rusty Russell <rusty@rustcorp.com.au>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: David Rientjes <rientjes@google.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    include/linux/kmod.h | 2 ++
    kernel/kmod.c | 27 +++++++++++++++++++++++++--
    2 files changed, 27 insertions(+), 2 deletions(-)

    --- a/include/linux/kmod.h
    +++ b/include/linux/kmod.h
    @@ -54,6 +54,8 @@ enum umh_wait {
    UMH_WAIT_PROC = 1, /* wait for the process to complete */
    };

    +#define UMH_KILLABLE 4 /* wait for EXEC/PROC killable */
    +
    struct subprocess_info {
    struct work_struct work;
    struct completion *complete;
    --- a/kernel/kmod.c
    +++ b/kernel/kmod.c
    @@ -199,7 +199,15 @@ EXPORT_SYMBOL(call_usermodehelper_freein

    static void umh_complete(struct subprocess_info *sub_info)
    {
    - complete(sub_info->complete);
    + struct completion *comp = xchg(&sub_info->complete, NULL);
    + /*
    + * See call_usermodehelper_exec(). If xchg() returns NULL
    + * we own sub_info, the UMH_KILLABLE caller has gone away.
    + */
    + if (comp)
    + complete(comp);
    + else
    + call_usermodehelper_freeinfo(sub_info);
    }

    /* Keventd can't block, but this (a child) can. */
    @@ -250,6 +258,9 @@ static void __call_usermodehelper(struct
    enum umh_wait wait = sub_info->wait;
    pid_t pid;

    + if (wait != UMH_NO_WAIT)
    + wait &= ~UMH_KILLABLE;
    +
    /* CLONE_VFORK: wait until the usermode helper has execve'd
    * successfully We need the data structures to stay around
    * until that is done. */
    @@ -440,9 +451,21 @@ int call_usermodehelper_exec(struct subp
    queue_work(khelper_wq, &sub_info->work);
    if (wait == UMH_NO_WAIT) /* task has freed sub_info */
    goto unlock;
    +
    + if (wait & UMH_KILLABLE) {
    + retval = wait_for_completion_killable(&done);
    + if (!retval)
    + goto wait_done;
    +
    + /* umh_complete() will see NULL and free sub_info */
    + if (xchg(&sub_info->complete, NULL))
    + goto unlock;
    + /* fallthrough, umh_complete() was already called */
    + }
    +
    wait_for_completion(&done);
    +wait_done:
    retval = sub_info->retval;
    -
    out:
    call_usermodehelper_freeinfo(sub_info);
    unlock:



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