Re: [PATCH] perf: unwind: fix segbase for libunwind.

From: Wang Nan
Date: Thu Apr 02 2015 - 01:19:42 EST

On 2015/4/2 13:07, Wang Nan wrote:
> On 2015/4/1 22:48, Ingo Molnar wrote:
>>
>> * Wang Nan <wangnan0@xxxxxxxxxx> wrote:
>>
>>> Perf passes incorrect segbase and table_data to libunwind when
>>> map->pgoff != 0, causes unwind failure. This patch fixes this
>>> problem.
>>>
>>> segbase is an absolute offset from the head of object file, directly
>>> read from ELF file. Original code computes corresponding virtual
>>> address using map->start + segbase, doesn't consider map->pgoff.
>>> Which causes libunwind read from incorrect offset.
>>
>> What's the effect of this bug in practice?
>>
>> Is there any before/after output you can show that demonstrates the
>> fix?
>>
>
> I found the problem when testing my '--map-adjustment' argument. I tried to
> construct a test case for normal case, but it seems triggers futher bugs.
>
> Following is the reproducing steps. The first two
>
> Step 1: create a C file like this (libtest-load.c):
>
> --------test-load.c-----------
> #define DATA_SZ 65535
> int data[DATA_SZ] = {1,1,2,3,5,8,13,};
>
> int fib(int x)
> {
> if (x >= DATA_SZ)
> return -1;
> if ((x == 0) || (x == 1))
> return 1;
> data[x] = fib(x - 1) + fib(x - 2);
> return data[x];
> }
> ----------------------
>
> Step 2: create a shared object with folowing ld-script (test-load.lds) using:
>
> $ gcc -shared -fPIC ./test-load.c -Wl,-T./test-load.lds -O0 -o libtest-load.so
>
> --------------test-load.lds---------------
> SECTIONS
> {
> .note.gnu.build-id : { *(.node.*) } :text
> .gnu.hash : { *(.gnu.hash) } :text
> .dynsym : { *(.dynsym) } :text
> .dynstr : { *(.dynstr) } :text
> .gnu.version : { *(.gnu.version) } :text
> .gnu.version_r : { *(.gnu.version_r) } :text
> .rela.dyn : { *(.rela.dyn) } :text
> .rela.plt : { *(.rela.plt) } :text
> .init : { *(.init) } :text
> .plt : { *(.plt) } :text
> .text : { *(.text) } :text
> .fini : { *(.fini) } :text
> .eh_frame_hdr : { *(.eh_frame_hdr) } :text
> .eh_frame : { *(.eh_frame) } :text
>
> .ctors : { *(.ctors) } :data
> .dtors : { *(.dtors) } :data
> .jcr : { *(.jcr) } :data
> .got : { *(.got) } :data
> .got.plt : { *(.got.plt) } :data
> .data : { *(.data) } :data
> .bss : { *(.bss) } :data
> .dynamic : { *(.dynamic) } :dynamic
>
> }
>
> PHDRS
> {
> text PT_LOAD FLAGS(7);
> data PT_LOAD FLAGS(7);
> dynamic PT_DYNAMIC FLAGS(7);
> note PT_NOTE FLAGS(4);
> }
> -----------------------------
>
> In my environment, I get a shared object with:
>
> $ readelf -l ./libtest-load.so
>
> Elf file type is DYN (Shared object file)
> Entry point 0x390
> There are 4 program headers, starting at offset 64
>
> Program Headers:
> Type Offset VirtAddr PhysAddr
> FileSiz MemSiz Flags Align
> LOAD 0x0000000000200000 0x0000000000000000 0x0000000000000000
> 0x00000000000005c4 0x00000000000005c4 RWE 200000
> LOAD 0x00000000002005c8 0x00000000000005c8 0x00000000000005c8
> 0x00000000000400a0 0x00000000000400b0 RWE 200000
> DYNAMIC 0x0000000000240678 0x0000000000040678 0x0000000000040678
> 0x0000000000000180 0x0000000000000180 RWE 8
> NOTE 0x0000000000000000 0x0000000000000000 0x0000000000000000
> 0x0000000000000000 0x0000000000000000 R 8
>
> Section to Segment mapping:
> Segment Sections...
> 00 .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.plt .init .plt .text .fini .eh_frame_hdr .eh_frame .rela.dyn .note.gnu.build-id
> 01 .ctors .dtors .jcr .got .got.plt .data .data.rel .bss
> 02 .dynamic
> 03
>
> The goal of the first 2 steps is to create a shared object which will be mmap-ed with
> pgoff != 0 (offset of a PT_LOAD segment != 0). I'm not sure which part of the ld scripts
> leads to this, so I paste all of them.
>
> Step 3: Use that shared object: compile following C file using:
>
> $ gcc ./test.c -O0 -ltest-load -L.
>
> --------- test.c ---------
> #include <stdio.h>
> extern int fib(int x);
> int main()
> {
> printf("%d\n", fib(30));
> return 0;
> }
> -----------------
>
> Step 4: perf record:
>
> $ perf record -g --call-graph=dwarf ./a.out
>
> Step 5: perf report:
>
> $ perf report --stdio --no-children
>
> # To display the perf.data header info, please use --header/--header-only options.
> #
> # Samples: 40 of event 'cycles'
> # Event count (approx.): 28432005
> #
> # Overhead Command Shared Object Symbol
> # ........ ....... ................ ......................
> #
> 18.40% a.out libtest-load.so [.] 0x00000000002004ee
> |
> ---0x7f17127964ee
>
> 16.85% a.out libtest-load.so [.] 0x00000000002004bf
> |
> ---0x7f17127964bf
>
> 15.20% a.out libtest-load.so [.] 0x00000000002004c2
> |
> ---0x7f17127964c2
>
> 9.33% a.out libtest-load.so [.] 0x000000000020049c
> |
> ---0x7f171279649c
>
>
> Perf failed to extract symbol name and also failed to unwind.
>
>
> With my patch, perf unwind works:
>
> #
> # Overhead Command Shared Object Symbol
> # ........ ....... ................ ......................
> #
> 18.40% a.out libtest-load.so [.] 0x00000000002004ee
> |
> ---0x7f17127964ee
> |
> |--50.52%-- 0x7f17127964cc
> | |
> | |--64.45%-- 0x7f17127964cc
> | | 0x7f17127964db
> | | |
> | | |--98.69%-- 0x7f17127964db
> | | | |
> | | | |--93.86%-- 0x7f17127964db
> | | | | 0x7f17127964cc
> | | | | 0x7f17127964cc
> | | | | 0x7f17127964cc
> | | | | 0x7f17127964db
> | | | | 0x7f17127964cc
> | | | | 0x7f17127964cc
> | | | | 0x7f17127964cc
> | | | | 0x7f17127964db
> | | | | 0x7f17127964cc
> | | | | 0x7f17127964db
> | | | | 0x7f17127964cc
> | | | | 0x7f17127964cc
> | | | | 0x7f17127964db
> | | | | 0x7f17127964db
> | | | | 0x7f17127964cc
> | | | | main
> | | | | __libc_start_main
> | | | | _start
> | | | |
> | | | --6.14%-- 0x7f17127964cc
>
> However, still unable to find symbol names.
>
>> Thanks,
>>
>> Ingo
>>
>


Additional information:

With following patch it seems to work:

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 476268c..1177f02 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -1049,8 +1049,10 @@ new_symbol:
if (demangled != NULL)
elf_name = demangled;
}
- f = symbol__new(sym.st_value, sym.st_size,
+ f = symbol__new(sym.st_value + map->pgoff, sym.st_size,
GELF_ST_BIND(sym.st_info), elf_name);
free(demangled);
if (!f)
goto out_elf_end;


result:

Overhead Command Shared Object Symbol
# ........ ....... ................ ......................
#
86.37% a.out libtest-load.so [.] fib
|
---fib
fib
fib
fib
fib
fib
fib
fib
fib
fib
fib
fib
fib
fib
|
|--94.35%-- fib
| fib
| |
| |--93.42%-- fib
| | |
| | |--93.12%-- fib
| | | fib
| | | |
| | | |--84.90%-- fib
| | | | |
| | | | |--74.34%-- fib
| | | | | |
| | | | | |--90.56%-- fib
| | | | | | |
| | | | | | |--57.37%-- fib
| | | | | | | |
| | | | | | | |--83.51%-- main
| | | | | | | | __libc_start_main
| | | | | | | | _start
| | | | | | | |
| | | | | | | --16.49%-- fib
| | | | | | | main
| | | | | | | __libc_start_main
| | | | | | | _start
...

However, I think this is only a temporary solutsion. What we need is to obay both
Offset and VirtAddr in PHDR.

Thank you.


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