[RFC][v8][PATCH 4/10]: Add target_pid parameter to alloc_pidmap()

[Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>]

Subject: [RFC][v8][PATCH 4/10]: 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[v6]:
	- Separate target_pid > 0 case to minimize the number of checks needed.
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>
---
 kernel/pid.c |   26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

Index: linux-2.6/kernel/pid.c
===================================================================
--- linux-2.6.orig/kernel/pid.c	2009-09-09 19:07:20.000000000 -0700
+++ linux-2.6/kernel/pid.c	2009-09-10 10:23:27.000000000 -0700
@@ -146,16 +146,22 @@ static int alloc_pidmap_page(struct pidm
 	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;
@@ -164,6 +170,15 @@ static int alloc_pidmap(struct pid_names
 		if (rc)
 			break;
 
+		if (target_pid) {
+			rc = -EBUSY;
+			if (!test_and_set_bit(offset, map->page)) {
+				atomic_dec(&map->nr_free);
+				rc = pid;
+			}
+			return rc;
+		}
+
 		if (likely(atomic_read(&map->nr_free))) {
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
@@ -196,6 +211,7 @@ static int alloc_pidmap(struct pid_names
 		}
 		pid = mk_pid(pid_ns, map, offset);
 	}
+
 	return rc;
 }
 
@@ -272,7 +288,7 @@ struct pid *alloc_pid(struct pid_namespa
 
 	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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