Re: [PATCH] perf probe: Add support for DW_OP_call_frame_cfa vars

[Daniel Shaulov]

On Thu, Apr 02, 2020 at 03:49:43PM -0300, Arnaldo Carvalho de Melo wrote:
> Daniel, next time please consider providing precise steps to show what
> wasn't working and then how the patch solves the problem, i.e. get some
> go program, show the command lines used to build, probe, etc.
>
> - Arnaldo

Sure thing:

---- contents of example.go ----
package main

import "fmt"

func example(param1 int, param2 int) {
	fmt.Println(param1, param2)
}

func main() {
	example(111, 222)
}
---- end contents of example.go ----

Build with:
$ go build example.go

Note that the output file is named after the directory.

Then try to probe it with:
$ perf probe -x ./example 'main.example param1 param2'

The result is:
> Target program is compiled without optimization. Skipping prologue.
> Probe on address 0x4820f0 to force probing at the function entry.
>
> Sorry, we don't support this variable location yet.
>   Error: Failed to add events.

With my changes the event is added successfuly.
The reason for the change can be observed in the output of:
$ readelf --debug-dump=info ./example

Where we have the following for the main.example function (with go >= 1.10):
 <1><63dd2>: Abbrev Number: 2 (DW_TAG_subprogram)
    <63dd3>   DW_AT_name        : main.example
    <63de0>   DW_AT_low_pc      : 0x4820f0
    <63de8>   DW_AT_high_pc     : 0x4821c2
    <63df0>   DW_AT_frame_base  : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
    <63df2>   DW_AT_decl_file   : 0x1
    <63df6>   DW_AT_external    : 1
 <2><63df7>: Abbrev Number: 14 (DW_TAG_formal_parameter)
    <63df8>   DW_AT_name        : param1
    <63dff>   DW_AT_variable_parameter: 0
    <63e00>   DW_AT_decl_line   : 5
    <63e01>   DW_AT_type        : <0x220f>
    <63e05>   DW_AT_location    : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
 <2><63e07>: Abbrev Number: 14 (DW_TAG_formal_parameter)
    <63e08>   DW_AT_name        : param2
    <63e0f>   DW_AT_variable_parameter: 0
    <63e10>   DW_AT_decl_line   : 5
    <63e11>   DW_AT_type        : <0x220f>
    <63e15>   DW_AT_location    : 2 byte block: 91 8 	(DW_OP_fbreg: 8)
 <2><63e18>: Abbrev Number: 0

The go compiler (after 1.10) uses DW_OP_call_frame_cfa for the first parameter and DW_OP_fbreg for the rest.
On go versions lower than 1.10, it used DW_OP_call_frame_cfa for all parameters.
Here is example output of the readelf on main.example, compiled with go1.9.7:
 <1><2634d>: Abbrev Number: 2 (DW_TAG_subprogram)
    <2634e>   DW_AT_name        : main.example
    <2635b>   DW_AT_low_pc      : 0x487db0
    <26363>   DW_AT_high_pc     : 0x487e99
    <2636b>   DW_AT_frame_base  : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
    <2636d>   DW_AT_external    : 1
 <2><2636e>: Abbrev Number: 5 (DW_TAG_formal_parameter)
    <2636f>   DW_AT_name        : param1
    <26376>   DW_AT_location    : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
    <26378>   DW_AT_type        : <0x27f1c>
 <2><2637c>: Abbrev Number: 5 (DW_TAG_formal_parameter)
    <2637d>   DW_AT_name        : param2
    <26384>   DW_AT_location    : 4 byte block: 9c 11 8 22 	(DW_OP_call_frame_cfa; DW_OP_consts: 8; DW_OP_plus)
    <26389>   DW_AT_type        : <0x27f1c>
 <2><2638d>: Abbrev Number: 0

Hope this makes things easier to understand.

Daniel.