Hi, Jiri!
>>>>> On Thu, 25 Aug 2016 14:02:50 +0200, Jiri Olsa wrote:
[...]
> 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..
I see how it can do that:
Breakpoint 1, main () at 1.c:10
10 int a = 1;
(gdb) p &a
$1 = (int *) 0x7fffffffe9cc
(gdb) br *0x00000000004004d6
Breakpoint 2 at 0x4004d6: file 1.c, line 4.
(before the prologue, as you did)
Breakpoint 2, func (par=0) at 1.c:4
4 {
(gdb) p $rsp + 20
$2 = (void *) 0x7fffffffe9cc (same as &a).
So DW_OP_call_frame_cfa restores %rsp before the function call.
> 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
--
WBR,
Yauheni Kaliuta
