[RFC][v5][PATCH 4/8]: Add target_pid parameter to alloc_pidmap()

[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>

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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