[PATCH 02/46] perf tools: Adjust symbol for shared objects

Wang Nan
2016-02-26T09:37:12Z

He Kuang reported a problem that perf fails to get correct symbol on
Android platform in [1]. The problem can be reproduced on normal x86_64
platform. I will describe the reproducing steps in detail at the end of
commit message.

The reason of this problem is the missing of symbol adjustment for normal
shared objects. In most of the cases it is okay skipping adjustment. However,
the result is wrong when '.text' section have different 'address' and 'offset'.
I checked all shared objects in my working platform, only wine dll objects and
debug objects (in .debug) have this problem. However, it is common on Android.
For example:

$ readelf -S ./libsurfaceflinger.so | grep \.text
[10] .text PROGBITS 0000000000029030 00012030

This patch enables symbol adjustment for dynamic objects so the symbol
address got from elfutils would be adjusted correctly.

Now nearly all type of ELF file should adjust symbols. Makes
ss->adjust_symbols default to true.

Steps to reproduce the problem:

$ cat ./Makefile
PWD := $(shell pwd)
LDFLAGS += "-Wl,-rpath=$(PWD)"
CFLAGS += -g
main: main.c libbuggy.so
libbuggy.so: buggy.c
gcc -g -shared -fPIC -Wl,-Ttext-segment=0x200000 $< -o $@
clean:
rm -rf main libbuggy.so *.o

$ cat ./buggy.c
int fib(int x)
{
return (x == 0) ? 1 : (x == 1) ? 1 : fib(x - 1) + fib(x - 2);
}

$ cat ./main.c
#include <stdio.h>

extern int fib(int x);
int main()
{
int i;

for (i = 0; i < 40; i++)
printf("%d\n", fib(i));
return 0;
}

$ make
$ perf record ./main
...
$ perf report --stdio
# Overhead Command Shared Object Symbol
# ........ ....... ................. ...............................
#
14.97% main libbuggy.so [.] 0x000000000000066c
8.68% main libbuggy.so [.] 0x00000000000006aa
8.52% main libbuggy.so [.] fib@plt
7.95% main libbuggy.so [.] 0x0000000000000664
5.94% main libbuggy.so [.] 0x00000000000006a9
5.35% main libbuggy.so [.] 0x0000000000000678
...

The correct result should be (after this patch):

# Overhead Command Shared Object Symbol
# ........ ....... ................. ...............................
#
91.47% main libbuggy.so [.] fib
8.52% main libbuggy.so [.] fib@plt
0.00% main [kernel.kallsyms] [k] kmem_cache_free

[1] http://lkml.kernel.org/g/1452567507-54013-1-git-send-email-hekuang@huawei.com

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Cody P Schafer <dev@codyps.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kirill Smelkov <kirr@nexedi.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: pi3orama@163.com
---
tools/perf/util/symbol-elf.c | 13 +++----------
1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index bc229a7..3f9d679 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -709,17 +709,10 @@ int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
if (ss->opdshdr.sh_type != SHT_PROGBITS)
ss->opdsec = NULL;

- if (dso->kernel == DSO_TYPE_USER) {
- GElf_Shdr shdr;
- ss->adjust_symbols = (ehdr.e_type == ET_EXEC ||
- ehdr.e_type == ET_REL ||
- dso__is_vdso(dso) ||
- elf_section_by_name(elf, &ehdr, &shdr,
- ".gnu.prelink_undo",
- NULL) != NULL);
- } else {
+ if (dso->kernel == DSO_TYPE_USER)
+ ss->adjust_symbols = true;
+ else
ss->adjust_symbols = elf__needs_adjust_symbols(ehdr);
- }

ss->name = strdup(name);
if (!ss->name) {
--
1.8.3.4