[PATCH 03/10] Containers(V10): Add tasks file interface

[Cedric Le Goater]

Hello Paul !

menage at google.com wrote:
> This patch adds the per-directory "tasks" file for containerfs mounts;
> this allows the user to determine which tasks are members of a
> container by reading a container's "tasks", and to move a task into a
> container by writing its pid to its "tasks".

here's a small fix for 2.6.22-rc4-mm2.

C.

when there's no tasks in a container, opening 
	
<containerfs>/<container>/tasks

spits the following warning because we are trying to 
kmalloc(0). 

WARNING: at mm/slab.c:777 __find_general_cachep()
 [<c0102b01>] show_trace_log_lvl+0x1a/0x2f
 [<c0103627>] show_trace+0x12/0x14
 [<c010363e>] dump_stack+0x15/0x17
 [<c0148040>] __kmalloc+0x56/0xf3
 [<c012d030>] container_tasks_open+0x56/0x131
 [<c012cdf8>] container_file_open+0x32/0x3a
 [<c014964d>] __dentry_open+0x99/0x13c
 [<c0149771>] nameidata_to_filp+0x27/0x37
 [<c01497b4>] do_filp_open+0x33/0x3b
 [<c0149801>] do_sys_open+0x45/0xc9
 [<c01498bd>] sys_open+0x1c/0x1e
 [<c0102540>] syscall_call+0x7/0xb

Signed-off-by: Cedric Le Goater <clg at fr.ibm.com>
Cc: Paul Menage <menage at google.com>
---
 kernel/container.c |   33 +++++++++++++++++++--------------
 1 file changed, 19 insertions(+), 14 deletions(-)

Index: 2.6.22-rc4-mm2/kernel/container.c
===================================================================
--- 2.6.22-rc4-mm2.orig/kernel/container.c
+++ 2.6.22-rc4-mm2/kernel/container.c
@@ -1651,21 +1651,26 @@ static int container_tasks_open(struct i
 	 * show up until sometime later on.
 	 */
 	npids = container_task_count(cont);
-	pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
-	if (!pidarray)
-		goto err1;
-
-	npids = pid_array_load(pidarray, npids, cont);
-	sort(pidarray, npids, sizeof(pid_t), cmppid, NULL);
-
-	/* Call pid_array_to_buf() twice, first just to get bufsz */
-	ctr->bufsz = pid_array_to_buf(&c, sizeof(c), pidarray, npids) + 1;
-	ctr->buf = kmalloc(ctr->bufsz, GFP_KERNEL);
-	if (!ctr->buf)
-		goto err2;
-	ctr->bufsz = pid_array_to_buf(ctr->buf, ctr->bufsz, pidarray, npids);
+	if (npids) {
+		pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
+		if (!pidarray)
+			goto err1;
+
+		npids = pid_array_load(pidarray, npids, cont);
+		sort(pidarray, npids, sizeof(pid_t), cmppid, NULL);
+
+		/* Call pid_array_to_buf() twice, first just to get bufsz */
+		ctr->bufsz = pid_array_to_buf(&c, sizeof(c), pidarray, npids) + 1;
+		ctr->buf = kmalloc(ctr->bufsz, GFP_KERNEL);
+		if (!ctr->buf)
+			goto err2;
+		ctr->bufsz = pid_array_to_buf(ctr->buf, ctr->bufsz, pidarray, npids);
 
-	kfree(pidarray);
+		kfree(pidarray);
+	} else {
+		ctr->buf = 0;
+		ctr->bufsz = 0;
+	}
 	file->private_data = ctr;
 	return 0;