Quoting Sukadev Bhattiprolu (sukadev@xxxxxxxxxxxxxxxxxx):
>
>
> Subject: [RFC][v5][PATCH 4/8]: Add target_pid parameter to alloc_pidmap()
>
> With support for setting a specific pid number for a process,
> alloc_pidmap() will need a 'target_pid' parameter.
>
> Changelog[v3]:
> - (Eric Biederman): Avoid set_pidmap() function. Added couple of
> checks for target_pid in alloc_pidmap() itself.
> Changelog[v2]:
> - (Serge Hallyn) Check for 'pid < 0' in set_pidmap().(Code
> actually checks for 'pid <= 0' for completeness).
>
> Signed-off-by: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>
Acked-by: Serge Hallyn <serue@xxxxxxxxxx>
>
> Index: linux-mmotm/kernel/pid.c
> ===================================================================
> --- linux-mmotm.orig/kernel/pid.c 2009-09-05 21:17:36.000000000 -0700
> +++ linux-mmotm/kernel/pid.c 2009-09-05 21:19:11.000000000 -0700
> @@ -146,16 +146,22 @@
> return 0;
> }
>
> -static int alloc_pidmap(struct pid_namespace *pid_ns)
> +static int alloc_pidmap(struct pid_namespace *pid_ns, int target_pid)
> {
> int i, offset, max_scan, pid, last = pid_ns->last_pid;
> int rc = -EAGAIN;
> int pid_max = pid_ns->pid_max;
> struct pidmap *map;
>
> - pid = last + 1;
> - if (pid >= pid_max)
> - pid = RESERVED_PIDS;
> + if (target_pid) {
> + pid = target_pid;
> + if (pid < 0 || pid >= pid_max)
> + return -EINVAL;
> + } else {
> + pid = last + 1;
> + if (pid >= pid_max)
> + pid = RESERVED_PIDS;
> + }
> offset = pid & BITS_PER_PAGE_MASK;
> map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
> max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
> @@ -168,9 +174,14 @@
> do {
> if (!test_and_set_bit(offset, map->page)) {
> atomic_dec(&map->nr_free);
> - pid_ns->last_pid = pid;
> + if (!target_pid)
> + pid_ns->last_pid = pid;
> return pid;
> + } else if (target_pid) {
> + rc = -EBUSY;
> + break;
> }
> +
> offset = find_next_offset(map, offset);
> pid = mk_pid(pid_ns, map, offset);
> /*
> @@ -196,6 +207,7 @@
> }
> pid = mk_pid(pid_ns, map, offset);
> }
> +
> return rc;
> }
>
> @@ -272,7 +284,7 @@
>
> tmp = ns;
> for (i = ns->level; i >= 0; i--) {
> - nr = alloc_pidmap(tmp);
> + nr = alloc_pidmap(tmp, 0);
> if (nr < 0)
> goto out_free;
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