Re: [PATCH] bpf: Call rcu_read_unlock() before copy_to_user()

[Alexei Starovoitov]

On Thu, Jan 22, 2015 at 7:57 AM, Michael Holzheu
<holzheu@xxxxxxxxxxxxxxxxxx> wrote:
> We must not hold locks when calling copy_to_user():
>
> BUG: sleeping function called from invalid context at mm/memory.c:3732
> in_atomic(): 0, irqs_disabled(): 0, pid: 671, name: test_maps
> 1 lock held by test_maps/671:
>  #0:  (rcu_read_lock){......}, at: [<0000000000264190>] map_lookup_elem+0xe8/0x260
> Preemption disabled at:[<00000000001be3b6>] vprintk_default+0x56/0x68
>
> CPU: 0 PID: 671 Comm: test_maps Not tainted 3.19.0-rc5-00117-g5eb11d6-dirty #424
>        000000001e447bb0 000000001e447c40 0000000000000002 0000000000000000
>        000000001e447ce0 000000001e447c58 000000001e447c58 0000000000115c8a
>        0000000000000000 0000000000c08246 0000000000c27e8a 000000000000000b
>        000000001e447ca0 000000001e447c40 0000000000000000 0000000000000000
>        0000000000000000 0000000000115c8a 000000001e447c40 000000001e447ca0
> Call Trace:
> ([<0000000000115b7e>] show_trace+0x12e/0x150)
>  [<0000000000115c40>] show_stack+0xa0/0x100
>  [<00000000009b163c>] dump_stack+0x74/0xc8
>  [<000000000017424a>] ___might_sleep+0x23a/0x248
>  [<00000000002b58e8>] might_fault+0x70/0xe8
>  [<0000000000264230>] map_lookup_elem+0x188/0x260
>  [<0000000000264716>] SyS_bpf+0x20e/0x840
>  [<00000000009bbe3a>] system_call+0xd6/0x24c
>  [<000003fffd15f566>] 0x3fffd15f566
> 1 lock held by test_maps/671:
>  #0:  (rcu_read_lock){......}, at: [<0000000000264190>] map_lookup_elem+0xe8/0x260
>
> So call rcu_read_unlock() before copy_to_user(). We can
> release the lock earlier because it is not needed for copy_to_user().

we cannot move the rcu unlock this way, since it protects the value.
So we need to copy the value while still under rcu.
I'm puzzled how I missed this warning.
I guess you have CONFIG_PREEMPT_RCU=y ?
and if (in_atomic()) return; as part of might_fault() hid it.
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