Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines

[Masami Hiramatsu]

(2014/07/14 16:16), Namhyung Kim wrote:
> Hi Masami,
> 
> On Mon, 14 Jul 2014 10:35:21 +0900, Masami Hiramatsu wrote:
>> (2014/07/11 23:29), Josh Poimboeuf wrote:
>> [...]
>>>
>>> >From 951d2aec17885a62905df6b910dc705d99c63993 Mon Sep 17 00:00:00 2001
>>> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>>> Date: Fri, 11 Jul 2014 08:58:33 -0500
>>> Subject: [PATCH] x86/dumpstack: fix stack traces for generated code
>>>
>>> If a function in the stack trace is dynamically generated, for example an
>>> ftrace dynamically generated trampoline, print_context_stack() gets confused
>>> and ends up showing all the following addresses as unreliable:
>>>
>>>   [  934.546013]  [<ffffffff81700312>] dump_stack+0x45/0x56
>>>   [  934.546020]  [<ffffffff8125f5b0>] ? meminfo_proc_open+0x30/0x30
>>>   [  934.546027]  [<ffffffffa080a494>] kpatch_ftrace_handler+0x14/0xf0 [kpatch]
>>>   [  934.546058]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
>>>   [  934.546062]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
>>>   [  934.546067]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
>>>   [  934.546071]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
>>>   [  934.546075]  [<ffffffff8121423a>] ? seq_read+0x16a/0x3b0
>>>   [  934.546081]  [<ffffffff8125768d>] ? proc_reg_read+0x3d/0x80
>>>   [  934.546088]  [<ffffffff811f0668>] ? vfs_read+0x98/0x170
>>>   [  934.546093]  [<ffffffff811f1345>] ? SyS_read+0x55/0xd0
>>>   [  934.546099]  [<ffffffff81707969>] ? system_call_fastpath+0x16/0x1b
>>>
>>> Once it encounters an address which is not in the kernel's text area, it gets
>>> confused and stops updating the frame pointer.
>>
>> Right, this uses a module_alloc to get a memory for trampline, but
>> it just allocates a page in executable vmalloc area. We need a hack
>> in __kernel_text_address if we really want to use that.
>>
>>> The __kernel_text_address() check isn't needed when determining whether an
>>> address is reliable.  It's only needed when deciding whether to print an
>>> unreliable address.
>>
>> Yeah, I guess that is for the case that the frame pointer is broken.
>>
>>>
>>> Here's the same stack trace with this patch:
>>>
>>>   [ 1314.612287]  [<ffffffff81700312>] dump_stack+0x45/0x56
>>>   [ 1314.612290]  [<ffffffff8125f5b0>] ? meminfo_proc_open+0x30/0x30
>>>   [ 1314.612293]  [<ffffffffa080a494>] kpatch_ftrace_handler+0x14/0xf0 [kpatch]
>>>   [ 1314.612306]  [<ffffffffa00160c4>] 0xffffffffa00160c3
>>
>> Here, this still has a wrong entry. Maybe the trampline doesn't setup
>> frame pointer (bp) correctly.
> 
> Hmm.. are you saying about the hex address above?  I guess it's a valid
> entry in the (dynamic) trampoline, but has no symbol..

Ah, indeed. (BTW, why is it one less than the address ...? printk's spec?)

>>>   [ 1314.612309]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
>>>   [ 1314.612311]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
>>>   [ 1314.612312]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
>>>   [ 1314.612314]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
>>>   [ 1314.612315]  [<ffffffff8121423a>] ? seq_read+0x16a/0x3b0
> 
> But these seem to be wrong - there're duplicate entries and they should
> show some of these functions (at least) correctly IMHO.  I guess it's
> because the trampoline didn't save rbp to the stack right below the
> return address as dumpstack requires.

Right, the last seq_read should be reliable. Thank you for pointing out.

Thanks!


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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