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

From: Daniel Shaulov
Date: Thu Apr 02 2020 - 16:25:32 EST


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.