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SubjectRe: [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@kernel.org [mailto:acme@kernel.org]
>>>> Em Thu, Nov 05, 2015 at 02:08:48PM +0000, 平松雅巳 /
>>>> HIRAMATU,MASAMI escreveu:
>>>>> From: Wang Nan [mailto:wangnan0@huawei.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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