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SubjectBug in sysctl table code?
Date

Hello to all and Al specifically,

Al, you were the last to touch kernel/sysctl.c and put in a WARN_ON(1) check on line 1930 which I seem to be hitting. Commit in question: http://git.kernel.org/?p=linux/kernel/git/stable/linux-2.6.27.y.git;a=commitdiff;h=ae7edecc9b8810770a8e5cb9a466ea4bdcfa8401

I thought I was not doing anything special, which also implies nothing wrong, but feel free to tell me correct me if not so. I have a test module which registers some sysctl tables to create this layout:

/proc/sys/test/
/proc/sys/test/a
/proc/sys/test/a/1
/proc/sys/test/a/1/item
/proc/sys/test/b
/proc/sys/test/b/1
/proc/sys/test/b/1/item
/proc/sys/test/b/2
/proc/sys/test/b/2/item
/proc/sys/test/b/3
/proc/sys/test/b/3/item
/proc/sys/test/b/4
/proc/sys/test/b/4/item
/proc/sys/test/b/5
/proc/sys/test/b/5/item

Unregistering is done in a different order and that seems to trigger this (ignore tainted status please since it is caused by my test module):

Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.064657] ------------[ cut here ]------------
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.064717] WARNING: at kernel/sysctl.c:1930 unregister_sysctl_table+0x9a/0xc5()
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.064760] Modules linked in: sysctlunreg(P-) psmouse af_packet serio_raw pcspkr i2c_piix4 container ac button evdev ext3 jbd mbcache ide_cd_mod cdrom floppy pcnet32 mii thermal processor
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.065416] Pid: 2799, comm: rmmod Tainted: P W 2.6.27-rc6 #3
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.065685] [<c0117078>] warn_on_slowpath+0x41/0x65
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.065839] [<c015853b>] ? check_bytes_and_report+0x21/0x8f
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.065881] [<c0158897>] ? check_object+0x111/0x1a4
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.065891] [<c011c5c0>] unregister_sysctl_table+0x9a/0xc5
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.065898] [<c0131f39>] ? trace_hardirqs_on+0xb/0xd
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.065959] [<e089600b>] unreg_one+0xb/0x15 [sysctlunreg]
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.066371] [<e08960e3>] test_exit+0x53/0x65 [sysctlunreg]
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.066381] [<c013a186>] sys_delete_module+0x163/0x1c1
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.066389] [<c012951d>] ? up_read+0x16/0x29
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.066397] [<c0262e09>] ? do_page_fault+0x300/0x5c1
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.066407] [<c0102b1d>] sysenter_do_call+0x12/0x35
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.066476] =======================
Oct 22 16:10:59 vm-kernel-32 kernel: [ 63.066612] ---[ end trace 4eaa2a86a8e2da22 ]---

Finally, code looks like this:

-----------------<CUT HERE>--------------------------------------------------------
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/sysctl.h>

static struct ctl_table_header *header[10];
static unsigned int d[10];
static ctl_table *tree[10];

static struct ctl_table_header *reg_one(ctl_table *tree, char *first, char *second, unsigned int *data)
{
const unsigned int items = 8;

tree = kzalloc(items*sizeof(ctl_table), GFP_KERNEL);

tree[0].procname = "test";
tree[0].mode = 0555;
tree[0].child = &tree[2];

tree[2].procname = first;
tree[2].mode = 0555;
tree[2].child = &tree[4];

tree[4].procname = second;
tree[4].mode = 0555;
tree[4].child = &tree[6];

tree[6].procname = "item";
tree[6].mode = 0555;
tree[6].data = data;
tree[6].maxlen = sizeof(unsigned int);
tree[6].proc_handler = proc_dointvec;

return register_sysctl_table(tree);
}

static void unreg_one(struct ctl_table_header *h, ctl_table *t)
{
unregister_sysctl_table(h);
kfree(t);
}

static int __init test_init(void)
{
header[0] = reg_one(tree[0], "a", "1", &d[0]);
header[1] = reg_one(tree[1], "b", "1", &d[1]);
header[2] = reg_one(tree[2], "b", "2", &d[2]);
header[3] = reg_one(tree[3], "b", "3", &d[3]);
header[4] = reg_one(tree[4], "b", "4", &d[4]);
header[5] = reg_one(tree[5], "b", "5", &d[5]);

return 0;
}

static void __exit test_exit(void)
{
unreg_one(header[1], tree[1]);
unreg_one(header[2], tree[2]);
unreg_one(header[5], tree[5]);
unreg_one(header[3], tree[3]);
unreg_one(header[4], tree[4]);
unreg_one(header[0], tree[0]);
}

module_init(test_init);
module_exit(test_exit);
-----------------<CUT HERE>--------------------------------------------------------

I would never expect unregistering having to be done in the same order, but a) I may be wrong, or b) there may be another bug in my test code. Hopefully you will know something about this.

Regards,

Tvrtko

P.S. Sorry for the dupe, I blame it on Notes!


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