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    SubjectRe: [PATCH] net, cgroup: Fix boot failure due to iteration of uninitialized list
    On Mon, Jul 23, 2012 at 09:15:05AM +0800, Gao feng wrote:
    > 于 2012年07月20日 00:27, Srivatsa S. Bhat 写道:
    > > After commit ef209f15 (net: cgroup: fix access the unallocated memory in
    > > netprio cgroup), boot fails with the following NULL pointer dereference:
    > >
    > > Initializing cgroup subsys devices
    > > Initializing cgroup subsys freezer
    > > Initializing cgroup subsys net_cls
    > > Initializing cgroup subsys blkio
    > > Initializing cgroup subsys perf_event
    > > Initializing cgroup subsys net_prio
    > > BUG: unable to handle kernel NULL pointer dereference at 0000000000000698
    > > IP: [<ffffffff8145e8d6>] cgrp_create+0xf6/0x190
    > > PGD 0
    > > Oops: 0000 [#1] SMP
    > > CPU 0
    > > Modules linked in:
    > >
    > > Pid: 0, comm: swapper/0 Not tainted 3.5.0-rc7-mandeep #1 IBM IBM System x -[7870C4Q]-/68Y8033
    > > RIP: 0010:[<ffffffff8145e8d6>] [<ffffffff8145e8d6>] cgrp_create+0xf6/0x190
    > > RSP: 0000:ffffffff81a01ea8 EFLAGS: 00010213
    > > RAX: 0000000000000000 RBX: ffffffffffffff10 RCX: 0000000000000000
    > > RDX: 0000000000000000 RSI: 0000000000000246 RDI: ffffffff81aa70a0
    > > RBP: ffffffff81a01ed8 R08: 0000000000000000 R09: 0000000000000000
    > > R10: ffff8808ff8641c0 R11: 6e697a696c616974 R12: 0000000000000001
    > > R13: ffff8808ff8641c0 R14: 0000000000000000 R15: 0000000000093970
    > > FS: 0000000000000000(0000) GS:ffff8808ffc00000(0000) knlGS:0000000000000000
    > > CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
    > > CR2: 0000000000000698 CR3: 0000000001a0b000 CR4: 00000000000006b0
    > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    > > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
    > > Process swapper/0 (pid: 0, threadinfo ffffffff81a00000, task ffffffff81a13420)
    > > Stack:
    > > ffffffff81a01eb8 ffffffff818060ff ffffffff81d75ec8 ffffffff81aa8960
    > > ffffffff81aa8960 ffffffff81b4c2c0 ffffffff81a01ef8 ffffffff81b1cb78
    > > 0000000000000018 0000000000000048 ffffffff81a01f18 ffffffff81b1ce13
    > > Call Trace:
    > > [<ffffffff81b1cb78>] cgroup_init_subsys+0x83/0x169
    > > [<ffffffff81b1ce13>] cgroup_init+0x36/0x119
    > > [<ffffffff81affef7>] start_kernel+0x3ba/0x3ef
    > > [<ffffffff81aff95b>] ? kernel_init+0x27b/0x27b
    > > [<ffffffff81aff356>] x86_64_start_reservations+0x131/0x136
    > > [<ffffffff81aff45e>] x86_64_start_kernel+0x103/0x112
    > > Code: 01 48 3d f8 e1 ec 81 48 8d 98 10 ff ff ff 75 1b eb 73 0f 1f 00 48 8b 83 f0 00 00 00 48 3d f8 e1 ec 81 48 8d 98 10 ff ff ff 74 5a <48> 8b 83 88 07 00 00 48 85 c0 74 de 44 3b 60 10 76 d8 44 89 e6
    > > RIP [<ffffffff8145e8d6>] cgrp_create+0xf6/0x190
    > > RSP <ffffffff81a01ea8>
    > > CR2: 0000000000000698
    > > ---[ end trace a7919e7f17c0a725 ]---
    > > Kernel panic - not syncing: Attempted to kill the idle task!
    > >
    > > The code corresponds to:
    > >
    > > update_netdev_tables():
    > > for_each_netdev(&init_net, dev) {
    > > map = rtnl_dereference(dev->priomap); <---- HERE
    > >
    > >
    > > The list head is initialized in netdev_init(), which is called much
    > > later than cgrp_create(). So the problem is that we are calling
    > > update_netdev_tables() way too early (in cgrp_create()), which will
    > > end up traversing the not-yet-circular linked list. So at some point,
    > > the dev pointer will become NULL and hence dev->priomap becomes an
    > > invalid access.
    > >
    > > To fix this, just remove the update_netdev_tables() function entirely,
    > > since it appears that write_update_netdev_table() will handle things
    > > just fine.
    >
    > The reason I add update_netdev_tables in cgrp_create is to avoid additional
    > bound checkings when we accessing the dev->priomap.priomap.
    >
    > Eric,can we revert this commit 91c68ce2b26319248a32d7baa1226f819d283758 now?
    > I think it's safe enough to access priomap without bound check.
    >
    > Thanks
    >

    I think its probably safe, yes, but lets leave it there for just a bit. Its not
    hurting anything, and I'd like to look into getting Srivatsa' patch in first.
    Neil

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