Re: [RFC][v8][PATCH 0/10] Implement clone3() system call

[Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>]

Sukadev Bhattiprolu [sukadev@linux.vnet.ibm.com] wrote:
| Eric W. Biederman [ebiederm@xmission.com] wrote:
| | > Anyway, is RESERVED_PIDS meant for initial kernel-threads/daemons - if so
| | > would it be ok enforce it only in init_pid_ns ?
| | 
| | It is mean for initial user space daemons, things that start on boot.
| | 
| | I don't know how much the protection matters at this date, but we have it.
| 
| Well, since it is not security or other critical restriction, can we allow
| set_pidmap() a free hand - even in init-pid-ns ? It could prevent a simple
| subtree C/R of one of the early daemons for debug for instance.

So here is how I have it at present. I would like to remove the RESERVED_PIDS
check in set_pidmap() if its ok to do so.

alloc_pid() does this:

	if (target_pids)
		set_pidmap(tmp, target_pids[i])
	else
		alloc_pidmap(tmp);

Sukadev
---

>From bc6093fc4fc2f01070647df6f1e85e45edc89d27 Mon Sep 17 00:00:00 2001
From: Sukadev Bhattiprolu <suka@suka.(none)>
Date: Thu, 22 Oct 2009 16:57:28 -0700
Subject: [PATCH] Define set_pidmap() function

Define a set_pidmap() interface which is like alloc_pidmap() only that
caller specifies the pid number to be assigned.

Changelog[v9]:
	- Complete rewrite this patch based on Eric Biederman's code.
Changelog[v7]:
        - [Eric Biederman] Generalize alloc_pidmap() to take a range of pids.
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@us.ibm.com>
---
 kernel/pid.c |   40 ++++++++++++++++++++++++++++++++--------
 1 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index c4d9914..9346755 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -147,18 +147,19 @@ static int alloc_pidmap_page(struct pidmap *map)
 	return 0;
 }
 
-static int alloc_pidmap(struct pid_namespace *pid_ns)
+static int do_alloc_pidmap(struct pid_namespace *pid_ns, int last, int min,
+		int max)
 {
-	int i, offset, max_scan, pid, last = pid_ns->last_pid;
+	int i, offset, max_scan, pid;
 	int rc = -EAGAIN;
 	struct pidmap *map;
 
 	pid = last + 1;
 	if (pid >= pid_max)
-		pid = RESERVED_PIDS;
+		pid = min;
 	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;
+	max_scan = (max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
 	for (i = 0; i <= max_scan; ++i) {
 		rc = alloc_pidmap_page(map);
 		if (rc)
@@ -168,7 +169,6 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
 					atomic_dec(&map->nr_free);
-					pid_ns->last_pid = pid;
 					return pid;
 				}
 				offset = find_next_offset(map, offset);
@@ -179,16 +179,16 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 			 * bitmap block and the final block was the same
 			 * as the starting point, pid is before last_pid.
 			 */
-			} while (offset < BITS_PER_PAGE && pid < pid_max &&
+			} while (offset < BITS_PER_PAGE && pid < max &&
 					(i != max_scan || pid < last ||
 					    !((last+1) & BITS_PER_PAGE_MASK)));
 		}
-		if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
+		if (map < &pid_ns->pidmap[(max-1)/BITS_PER_PAGE]) {
 			++map;
 			offset = 0;
 		} else {
 			map = &pid_ns->pidmap[0];
-			offset = RESERVED_PIDS;
+			offset = min;
 			if (unlikely(last == offset)) {
 				rc = -EAGAIN;
 				break;
@@ -199,6 +199,30 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 	return rc;
 }
 
+static int alloc_pidmap(struct pid_namespace *pid_ns)
+{
+	int nr;
+
+	nr = do_alloc_pidmap(pid_ns, pid_ns->last, RESERVED_PIDS, pid_max);
+	if (nr >= 0)
+		pid_ns->last_pid = nr;
+	return nr;
+}
+
+static int set_pidmap(struct pid_namespace *pid_ns, int target)
+{
+	if (!target)
+		return alloc_pidmap(pid_ns);
+
+	if (target >= pid_max)
+		return -EINVAL;
+
+	if ((target < 0) || (target < RESERVED_PIDS && pid_ns == &init_pid_ns))
+		return -EINVAL;
+
+	return do_alloc_pidmap(pid_ns, target - 1, target, target + 1);
+}
+
 int next_pidmap(struct pid_namespace *pid_ns, int last)
 {
 	int offset;
-- 
1.6.0.4