Re: [PATCH v5 08/17] x86/entry: Add enter_from_user_mode and use it in syscalls

[Andy Lutomirski <luto@amacapital.net>]

On Mon, Dec 21, 2015 at 12:50 PM, Sasha Levin <sasha.levin@oracle.com> wrote:
> On 07/03/2015 03:44 PM, Andy Lutomirski wrote:
>> Changing the x86 context tracking hooks is dangerous because there
>> are no good checks that we track our context correctly.  Add a
>> helper to check that we're actually in CONTEXT_USER when we enter
>> from user mode and wire it up for syscall entries.
>>
>> Subsequent patches will wire this up for all non-NMI entries as
>> well.  NMIs are their own special beast and cannot currently switch
>> overall context tracking state.  Instead, they have their own
>> special RCU hooks.
>>
>> This is a tiny speedup if !CONFIG_CONTEXT_TRACKING (removes a
>> branch) and a tiny slowdown if CONFIG_CONTEXT_TRACING (adds a layer
>> of indirection).  Eventually, we should fix up the core context
>> tracking code to supply a function that does what we want (and can
>> be much simpler than user_exit), which will enable us to get rid of
>> the extra call.
>
> Hey Andy,
>
> I see the following warning in today's -next:

Weird.  I wonder if you might have hit this while switching context
tracking on a runtime.  (Can you even do that?)

--Andy


>
> [ 2162.706868] ------------[ cut here ]------------
> [ 2162.708021] WARNING: CPU: 4 PID: 28801 at arch/x86/entry/common.c:44 enter_from_user_mode+0x1c/0x50()
> [ 2162.709466] Modules linked in:
> [ 2162.709998] CPU: 4 PID: 28801 Comm: trinity-c375 Tainted: G    B           4.4.0-rc5-next-20151221-sasha-00020-g840272e-dirty #2753
> [ 2162.711847]  0000000000000000 00000000f17e6fcd ffff880292d5fe08 ffffffffa4045334
> [ 2162.713108]  0000000041b58ab3 ffffffffaf66686b ffffffffa4045289 ffff880292d5fdc0
> [ 2162.714544]  0000000000000000 00000000f17e6fcd ffffffffa23cf466 0000000000000004
> [ 2162.715793] Call Trace:
> [ 2162.716229] dump_stack (lib/dump_stack.c:52)
> [ 2162.719021] warn_slowpath_common (kernel/panic.c:484)
> [ 2162.721014] warn_slowpath_null (kernel/panic.c:518)
> [ 2162.721950] enter_from_user_mode (arch/x86/entry/common.c:44 (discriminator 7) include/linux/context_tracking_state.h:30 (discriminator 7) include/linux/context_tracking.h:30 (discriminator 7) arch/x86/entry/common.c:45 (discriminator 7))
> [ 2162.722911] syscall_trace_enter_phase1 (arch/x86/entry/common.c:94)
> [ 2162.726914] tracesys (arch/x86/entry/entry_64.S:241)
> [ 2162.727704] ---[ end trace 1e5b49c361cbfe8b ]---
> [ 2162.728468] BUG: scheduling while atomic: trinity-c375/28801/0x00000401
> [ 2162.729517] Modules linked in:
> [ 2162.730020] Preemption disabled param_attr_store (kernel/params.c:625)
> [ 2162.731304]
> [ 2162.731579] CPU: 4 PID: 28801 Comm: trinity-c375 Tainted: G    B   W       4.4.0-rc5-next-20151221-sasha-00020-g840272e-dirty #2753
> [ 2162.733432]  0000000000000000 00000000f17e6fcd ffff880292d5fe20 ffffffffa4045334
> [ 2162.734778]  0000000041b58ab3 ffffffffaf66686b ffffffffa4045289 ffff880292d5fde0
> [ 2162.736036]  fffffffface198f9 00000000f17e6fcd ffff880292d5fe50 0000000000000282
> [ 2162.737309] Call Trace:
> [ 2162.737718] dump_stack (lib/dump_stack.c:52)
> [ 2162.740566] __schedule_bug (kernel/sched/core.c:3102)
> [ 2162.741498] __schedule (./arch/x86/include/asm/preempt.h:27 kernel/sched/core.c:3116 kernel/sched/core.c:3225)
> [ 2162.742391] schedule (kernel/sched/core.c:3312 (discriminator 1))
> [ 2162.743221] exit_to_usermode_loop (arch/x86/entry/common.c:246)
> [ 2162.744331] syscall_return_slowpath (arch/x86/entry/common.c:282 include/linux/context_tracking_state.h:30 include/linux/context_tracking.h:24 arch/x86/entry/common.c:284 arch/x86/entry/common.c:344)
> [ 2162.745364] int_ret_from_sys_call (arch/x86/entry/entry_64.S:282)
>
>
> Thanks,
> Sasha



-- 
Andy Lutomirski
AMA Capital Management, LLC