Quoting Oren Laadan (orenl@xxxxxxxxxxx):
> The main challenge with restoring the pgid of tasks is that the
> original "owner" (the process with that pid) might have exited
> already. I call these "ghost" pgids. 'mktree' does create these
> processes, but they then exit without participating in the restart.
>
> To solve this, this patch introduces a RESTART_GHOST flag, used for
> "ghost" owners that are created only to pass their pgid to other
> tasks. ('mktree' now makes them call restart(2) instead of exiting).
>
> When a "ghost" task calls restart(2), it will be placed on a wait
> queue until the restart completes and then exit. This guarantees that
> the pgid that it owns remains available for all (regular) restarting
> tasks for when they need it.
>
> Regular tasks perform the restart as before, except that they also
> now restore their old pgrp, which is guaranteed to exist.
>
> Changelog [v1]:
> - Verify that pgid owner is a thread-group-leader.
> - Handle the case of pgid/sid == 0 using root's parent pid-ns
>
> Signed-off-by: Oren Laadan <orenl@xxxxxxxxxxxxxxx>
> ---
> checkpoint/process.c | 106 ++++++++++++++++++++++++-
> checkpoint/restart.c | 158 ++++++++++++++++++++++++++------------
> checkpoint/sys.c | 3 +-
> include/linux/checkpoint.h | 11 ++-
> include/linux/checkpoint_hdr.h | 3 +
> include/linux/checkpoint_types.h | 6 +-
> 6 files changed, 230 insertions(+), 57 deletions(-)
>
> diff --git a/checkpoint/process.c b/checkpoint/process.c
> index 40b2580..5d6bdb9 100644
> --- a/checkpoint/process.c
> +++ b/checkpoint/process.c
> @@ -23,6 +23,57 @@
> #include <linux/syscalls.h>
>
>
> +pid_t ckpt_pid_nr(struct ckpt_ctx *ctx, struct pid *pid)
> +{
> + return pid ? pid_nr_ns(pid, ctx->root_nsproxy->pid_ns) : CKPT_PID_NULL;
> +}
> +
> +/* must be called with tasklist_lock or rcu_read_lock() held */
> +struct pid *_ckpt_find_pgrp(struct ckpt_ctx *ctx, pid_t pgid)
> +{
> + struct task_struct *p;
> + struct pid *pgrp;
> +
> + if (pgid == 0) {
> + /*
> + * At checkpoint the pgid owner lived in an ancestor
> + * pid-ns. The best we can do (sanely and safely) is
> + * to examine the parent of this restart's root: if in
> + * a distinct pid-ns, use its pgrp; otherwise fail.
> + */
> + p = ctx->root_task->real_parent;
> + if (p->nsproxy->pid_ns == current->nsproxy->pid_ns)
> + return NULL;
> + pgrp = task_pgrp(p);
> + } else {
> + /*
> + * Find the owner process of this pgid (it must exist
> + * if pgrp exists). It must be a thread group leader.
> + */
> + pgrp = find_vpid(pgid);
> + p = pid_task(pgrp, PIDTYPE_PID);
> + if (!p || !thread_group_leader(p))
> + return NULL;
> + /*
> + * The pgrp must "belong" to our restart tree (compare
> + * p->checkpoint_ctx to ours). This prevents malicious
> + * input from (guessing and) using unrelated pgrps. If
> + * the owner is dead, then it doesn't have a context,
> + * so instead compare against its (real) parent's.
> + */
> + if (p->exit_state == EXIT_ZOMBIE)
> + p = p->real_parent;
> + if (p->checkpoint_ctx != ctx)
> + return NULL;
> + }
> +
> + if (task_session(current) != task_session(p))
> + return NULL;
> +
> + return pgrp;
> +}
> +
> +
> #ifdef CONFIG_FUTEX
> static void save_task_robust_futex_list(struct ckpt_hdr_task *h,
> struct task_struct *t)
> @@ -94,8 +145,8 @@ static int checkpoint_task_struct(struct ckpt_ctx *ctx, struct task_struct *t)
> h->exit_signal = t->exit_signal;
> h->pdeath_signal = t->pdeath_signal;
>
> - h->set_child_tid = t->set_child_tid;
> - h->clear_child_tid = t->clear_child_tid;
> + h->set_child_tid = (unsigned long) t->set_child_tid;
note that set_child_tid is an int (signed), not a long. Same on
x86, but not on other arches. Shouldn't lose info so could be worse.
On the whole,
Acked-by: Serge Hallyn <serue@xxxxxxxxxx>
-serge
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