linux-kernel - Re: [PATCH 2/2] perf tools: Fix find_perf_probe_point_from

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Date:	Fri, 6 Nov 2015 17:27:06 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>,
	"'ltc-kernel@...yrl.intra.hitachi.co.jp'" 
	<ltc-kernel@...yrl.intra.hitachi.co.jp>,
	"'acme@...nel.org'" <acme@...nel.org>
CC:	"namhyung@...nel.org" <namhyung@...nel.org>,
	"lizefan@...wei.com" <lizefan@...wei.com>,
	"pi3orama@....com" <pi3orama@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jolsa@...nel.org" <jolsa@...nel.org>
Subject: Re: [PATCH 2/2] perf tools: Fix find_perf_probe_point_from_map()
 which incorrectly returns success



On 2015/11/6 16:30, Wangnan (F) wrote:
>
>
> On 2015/11/6 15:12, 平松雅巳 / HIRAMATU，MASAMI wrote:
>> From: acme@...nel.org [mailto:acme@...nel.org]
>>>> Em Thu, Nov 05, 2015 at 02:08:48PM +0000, 平松雅巳 / 
>>>> HIRAMATU，MASAMI escreveu:
>>>>> From: Wang Nan [mailto:wangnan0@...wei.com]

[SNIP]
>> Ah, finally I got what happened. I guess the problem may happen when 
>> we put
>> a probe on the kernel somewhere outside of any functions and run 
>> "perf probe -l".
>> I think it should not be allowed to put the probe outside any symbol.
>>
>> The background is here, at first "perf-probe -a somewhere" defines a 
>> probe in
>> the kernel but its address is relative from "_text". (thus, vfs_read 
>> becomes "_text+2348080"
>>   for example). Since it is not readable by human, perf probe -l 
>> tries to get an appropriate
>> symbol from the "_text+OFFSET".
>> For the purpose, the first kernel_get_symbol_address_by_name() is for 
>> translating _text to
>> an address, and the second  __find_kernel_function() is for finding a 
>> symbol from the
>> address+OFFSET.
>> Then, if the address+OFFSET is out of the symbol map, the second one 
>> can fail.
>> This means the first symbol and the second symbol is not same.
>>
>> So, the direction of Wang solution is good :). Just a cleanup is 
>> required.
>>
>> Thank you!
>
> I also tried to finger out the problem for all day and made some 
> progress. It is another
> problem. It happeneds when probing an address reside in a module on 
> aarch64 system.
>
> On my aarch64 system I use kcore. Different from x86, on aarch64, 
> modules address is lower
> than normal kernel. For example:
>
> On x86_64:
>
> # readelf -a /proc/kcore
>
>   Type           Offset             VirtAddr           PhysAddr
>                  FileSiz            MemSiz              Flags Align
>   ...
>   LOAD           0x00007fff81003000 0xffffffff81000000 
> 0x0000000000000000   <-- kernel
>                  0x0000000001026000 0x0000000001026000  RWE 1000
>   LOAD           0x00007fffa0003000 0xffffffffa0000000 
> 0x0000000000000000   <-- module
>                  0x000000005f000000 0x000000005f000000  RWE 1000
>
> On aarch64:
>
>   Type           Offset             VirtAddr           PhysAddr
>                  FileSiz            MemSiz              Flags Align
>   ...
>   LOAD           0x0000000000002000 0xffffffc000000000 
> 0x0000000000000000    <-- kernel
>                  0x000000007fc00000 0x000000007fc00000  RWE 1000
>   LOAD           0xfffffffffc002000 0xffffffbffc000000 
> 0x0000000000000000    <-- module
>                  0x0000000004000000 0x0000000004000000  RWE 1000
>
> See? On aarch64, Offset field of module address area is negative.
>

One thing should be noticed that, even if normal kernel code and modules 
use different
'struct map', they share a same dso. Please see dso__load_kcore, notice 
how it initialize
parameters (md) before calling file__read_maps().

> Which causes a problem in dso__split_kallsyms_for_kcore(): when it 
> adjusting symbols
> using "pos->start -= curr_map->start - curr_map->pgoff", the relative 
> order between
> module functions and normal kernel function is changed.
>
> For example:
>
> funca at 0xffffffc00021b428 is a normal kernel function.
> funcb at 0xffffffbffc000000 is a function in kernel.
>
> During parsing /proc/kallsyms, address of funca > address of funcb.
>
> However, after the adjusting:
>
> funca becomes:
>
> 0xffffffc00021b428 - (0xffffffc000000000 - 0x2000) = 0x21d428
>
> funcb becomes:
>
> 0xffffffbffc000000 - (0xffffffbffc000000 - 0xfffffffffc002000) = 
> 0xfffffffffc002000
>
> address of funca < address of funcb.
>
> Unfortunately, the rbtree is not adjusted in this case.
>

Even if they are in different maps, they share a same dso here, so a 
same rbtree.

Thank you.

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