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    Subject[PATCH] fix SEM_UNDO with namespaces, take 2
    Hi,

    below is the second attempt to fix SEM_UNDO + unshare():
    lookup_undo (in ipc/sem.c) is not namespace-aware, thus all entries in
    sysvsem.undo_list must be from the same namespace.
    The patch enforces that by detaching the current thread from
    sysvsem.undo_list in switch_task_namespaces() if the ipc namespace is
    changed.

    The patch boots and passes simple sysvsem+unshare tests.

    Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>
    diff --git a/ipc/sem.c b/ipc/sem.c
    index 0b45a4d..35841bd 100644
    --- a/ipc/sem.c
    +++ b/ipc/sem.c
    @@ -1298,6 +1298,7 @@ void exit_sem(struct task_struct *tsk)
    undo_list = tsk->sysvsem.undo_list;
    if (!undo_list)
    return;
    + tsk->sysvsem.undo_list = NULL;

    if (!atomic_dec_and_test(&undo_list->refcnt))
    return;
    diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
    index f5d332c..ddeb9d1 100644
    --- a/kernel/nsproxy.c
    +++ b/kernel/nsproxy.c
    @@ -211,6 +211,18 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)

    might_sleep();

    + if ((p->nsproxy == NULL && new != NULL) ||
    + (p->nsproxy != NULL && new == NULL) ||
    + (p->nsproxy != NULL && new != NULL && p->nsproxy->ipc_ns != new->ipc_ns)) {
    + /* switching the IPC namespace is considered equivalent to sys_exit() wrt.
    + * to outstanding SEM_UNDO undos: After switching to the new IPC namespace,
    + * the semaphore arrays from the old namespace are not accessible anymore.
    + *
    + * Additionally, an implicit sys_unshare(CLONE_SYSVSEM) is performed.
    + */
    + exit_sem(p);
    + }
    +
    ns = p->nsproxy;

    rcu_assign_pointer(p->nsproxy, new);
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