Re: [PATCH 2/2] perf tools: Fix find_perf_probe_point_from_map() which incorrectly returns success
From: Wangnan (F)
Date: Fri Nov 06 2015 - 04:32:01 EST
On 2015/11/6 16:30, Wangnan (F) wrote:
On 2015/11/6 15:12, åæéå / HIRAMATUïMASAMI wrote:
From: acme@xxxxxxxxxx [mailto:acme@xxxxxxxxxx]
Em Thu, Nov 05, 2015 at 02:08:48PM +0000, åæéå /
HIRAMATUïMASAMI escreveu:
From: Wang Nan [mailto:wangnan0@xxxxxxxxxx]
[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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