Messages in this thread | | | Date | Thu, 25 Aug 2016 14:02:50 +0200 | From | Jiri Olsa <> | Subject | [BUG] perf tool: uprobe displays wrong argument value |
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hi, Michael reported wrong values for function arguments being shown when adding uprobe on function with argument, I dig in and got even more confused.. ;-)
This is the testing binary:
--- #include <stdio.h>
int func(int par) { return par; }
int main(void) { int a = 1; return func(a); } ---
Adding the uprobe:
# ./perf probe -x ./ex --add 'func par' Added new event: probe_ex:func (on func in /home/jolsa/linux-perf/tools/perf/ex with par)
You can now use it in all perf tools, such as:
perf record -e probe_ex:func -aR sleep 1
Enabling globaly: # echo 1 > events/probe_ex/func/enable
After running the binary, this is the 'trace' output: # cat trace ... ex-15942 [014] d... 371.608516: func: (0x4004f6) par=0
Same with perf record: # ./perf record -e probe_ex:func -aR ./ex [ perf record: Woken up 1 times to write data ] [ perf record: Captured and wrote 0.157 MB perf.data (1 samples) ] # ./perf script ex 15954 [017] 489.298178: probe_ex:func: (4004f6) par=0
So, there's clearly bug in there..
Now here's where I got confused.. please continue reading only on your own risk ;-)
The uprobe_events shows following record: # cat uprobe_events p:probe_ex/func /home/jolsa/linux-perf/tools/perf/ex:0x00000000000004f6 par=-12(%sp):s32
I can't see how ($rsp - 12) address could hold the 'par' value, when we stop at the 'func' addreess, so where did it come from?
I figured it's the debug info, namely the par argument's CU:
$ readelf --debug-dump ./ex | less ...
<2><94>: Abbrev Number: 5 (DW_TAG_formal_parameter) <95> DW_AT_name : par <99> DW_AT_decl_file : 1 <9a> DW_AT_decl_line : 3 <9b> DW_AT_type : <0x57> <9f> DW_AT_location : 2 byte block: 91 6c (DW_OP_fbreg: -20)
which says the value is frame buffer reg -20.. I can't see this will get the proper value for any of $rbp or $rsp even after new func's stack frame is set..
Also if I set gdb to stop directly on the function address, it shows wrong value:
# gdb ./ex (gdb) b *0x4004f6 Breakpoint 1 at 0x4004f6: file ex.c, line 4. (gdb) r Starting program: /home/jolsa/linux-perf/tools/perf/ex Missing separate debuginfos, use: dnf debuginfo-install glibc-2.21-13.fc22.x86_64
Breakpoint 1, func (par=0) at ex.c:4 4 { (gdb)
Apart from when I set the breakpoint after the new stack frame is set:
(gdb) b func Breakpoint 1 at 0x4004fd: file ex.c, line 5. (gdb) r Starting program: /home/jolsa/linux-perf/tools/perf/ex Missing separate debuginfos, use: dnf debuginfo-install glibc-2.21-13.fc22.x86_64
Breakpoint 1, func (par=1) at ex.c:5 5 return par;
I'm clearly missing something..
thanks for help, jirka
--- kernel version: 4.8.0-rc2 perf version: latest Arnaldo's perf/core
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