Contents

gc vs. gccgo
Go Delve Debugger
-O0 -g
Pretty Printing
1. Maps
2. Channels
gc Missing CFI
gc Wrong DW_TAG_variable
Missing Modules/Namespaces
Fedora Packaging
TODO list for gc in Fedora

gc vs. gccgo

There are two compilers: gc and gccgo

Packages like Docker require the most recent version of Go - 1.4.2, for gccgo available only since gcc-5.0 (Fedora 22).

Current Python pretty printers + helper commands /usr/lib/golang/src/runtime/runtime-gdb.py work only partially now:

written for old/incompatible PythonGDB API
written for old/incompatible gc
incompatible with gccgo

Go Delve Debugger

Delve is written in Go based on DWARF information. Therefore it suffers the same debug info problems like GDB. It cannot walk the stack frames.

break|b - Set break point at the entry point of a function,
continue|c - Run until breakpoint or program termination.
step|si - Single step through program.
next|n - Step over to next source line.
threads - Print out info for every traced thread.
clear - Deletes breakpoint.
goroutines - Print out info for every goroutine.
print|p - Evaluate a variable.
info - Provides info about source, locals, args, or funcs.

-O0 -g

gccgo features debugger-friendly mode by: -O0 -g
gc can reduce some optimizations by: go build -gcflags '-N -l'
- flag -N' currently cannot be used due to: runtime.atomicstorep: nosplit stack overflow

Pretty Printing

Maps

gccgo - missing DWARF:

(gdb) p *state
$5 = {__descriptor = 0x496480 <__go_map_MN3_int__N3_int>,
      __element_count = 4, __bucket_count = 5,
      __buckets = 0xc2080141e0}
(gdb) ptype state.__buckets
type = void **

gc - compatible with #gc runtime-gdb.py Bug:

(gdb) p state
$2 = map[int]int = {[0]= 85, [2]= 10}

But typedef-ed (type in Go) types do not get recognized by GDB.

Channels

Maybe provide an xmethod to access channel’s goroutine?

gccgo - missing DWARF:

(gdb) p reads
$1 = (__go_channel **) 0xc208006060
(gdb) p **reads
$2 = {<No data fields>}
DW_TAG_structure_type:
  DW_AT_byte_size: 0

gc - incompatible runtime-gdb.py + wrong DWARF for key (it is integer while it should be pointer to integer):

(gdb) p ’&reads’
$1 = (chan *main.readOp *) 0xc208048018
(gdb) ptype ’&reads’
type = struct hchan<*main.readOp> {
(gdb) p *((*’&reads’).sendq.first[0]).elem
$6 = {key = 833357914640, resp = 0xc20803e120}
(gdb) x/gx ((*’&reads’).sendq.first[0]).elem.key
0xc20800a210: 0x0000000000000002
(gdb) p **(struct ’hchan<*main.readOp>’ **)’&reads’
$13 = struct hchan<*main.readOp>

gc Missing CFI

gccgo works OK.
gc has no .cfi_* or .eh_frame/.debug_frame support
- assembler from 'Plan 9': /usr/lib/golang/pkg/tool/linux_amd64/6a
- linker from 'Plan 9': /usr/lib/golang/pkg/tool/linux_amd64/6l

(gdb) bt
#0 runtime.rt0_go () at /usr/lib/golang/src/runtime/asm_am
#1 0x3424e1ffe0 in __libc_start_main (main=0x4489c0 <main>
fini=<optimized out>, rtld_fini=<optimized out>, stack_
#2 0x403659 in _start ()
(gdb) bt
#0 runtime.epollwait () at /usr/lib/golang/src/runtime/sys
#1 0x41c353 in runtime.netpoll (block=true, gp=0x0) at /us
[...]
#7 0x43bdf4 in runtime.mstart () at /usr/lib/golang/src/ru
#8 0x445e4c in runtime.rt0_go () at /usr/lib/golang/src/ru
#9 0x3 in ?? ()

(gdb) bt
#0 crosscall_amd64 () at /builddir/build/BUILD/go/src/runt
#1 0x404513 in threadentry (v=<optimized out>) at /builddi
#2 0x342560752a in start_thread (arg=0x7ffff7e2f700) at pt
#3 0x3424f0079d in clone () at ../sysdeps/unix/sysv/linux/
(gdb) bt
#0 runtime.futex () at /usr/lib/golang/src/runtime/sys_lin
[...]
#6 0x43bdf4 in runtime.mstart () at /usr/lib/golang/src/ru
#7 0x404753 in crosscall_amd64 () at /builddir/build/BUILD
#8 0x7ffff7e2f9c0 in ?? ()
???
#13 0x43bd50 in runtime.starttheworld () at /usr/lib/golang
#14 0x7ffff7e2f700 in ?? ()
???

Possible CFI fixes:
- Implement .cfi_* into Go assembler.
- Write .eh_frame as a relocated section.
- Implement gc-specific GDB frame sniffer either hard-coded arch+function+offset in .c or a command: set unwind i386:x86-64 runtime.goexit 112
- Switch to gccgo.

gc Wrong DW_TAG_variable

docker-1.4.1/daemon/daemon.go
func NewDaemonFromDirectory(config *Config, eng *engine.E
config.Root = realRoot

gccgo - correct:

(gdb) p config.Root
$1 = 0xc208001170 "/var/lib/docker"

gc - incorrect:

(gdb) p '&config'.Root
$2 = 0xc208159b10 "/var/lib/docker"
DW_TAG_subprogram:
  DW_TAG_variable:
    DW_AT_name: &config

Missing Modules/Namespaces

No DW_TAG_imported_* anywhere:

docker-1.4.1/daemon/graphdriver/driver.go
var (
DefaultDriver string
docker-1.4.1/daemon/daemon.go
import (
"github.com/docker/docker/daemon/graphdriver"
graphdriver.DefaultDriver = config.GraphDriver
^^^^^^^^^^^^^^^^^^^^^^^^^

gccgo:

(gdb) p 'github_com_docker_docker_daemon_graphdriver.DefaultDriver'
$4 = 0xc9d1a4 ""
DW_TAG_compile_unit:
  DW_TAG_variable:
    DW_AT_name: graphdriver.DefaultDriver
    DW_AT_linkage_name: github_com_docker_docker_daemon_graphdriver.Defaul
.symtab:
  github_com_docker_docker_daemon_graphdriver.DefaultDriver

gc:

(gdb) p 'github.com/docker.docker.daemon/graphdriver.DefaultDriver'
$6 = 0x0 ""
DW_TAG_compile_unit:
  DW_TAG_variable:
    DW_AT_name: github.com/docker.docker.daemon/graphdriver.DefaultDriver
.symtab:
  github.com/docker.docker.daemon/graphdriver.DefaultDriver

Fedora Packaging

Fedora Go packaging
Split DWARF to *-debuginfo.rpm - currently it is embedded: %global debug_package %{nil}
Provide build-ids, current build-id workaround:

# *** ERROR: No build ID note found in /.../BUILDROOT/etcd-2.0.0-1.rc1.fc22.x86_64/usr/bin/etcd
# build-id gets later regenerated by debugedit.
function gobuild { go build -a -ldflags "-B 0x$(head -c20 /dev/urandom|od -An -tx1|tr -d ' \n')" -v -x "$@"; }

gccgo Bug

gccgo overrides gc by default, see: alternatives --display go

Use: alternatives --set go /usr/lib/golang/bin/linux_amd64/go

Fedora Bug 1192716

gc runtime-gdb.py Bug

info goroutines, goroutine commands and Maps pretty printer fix:

Fedora Bug 1192721

gc variable references Bug

Fix #gc Wrong DW_TAG_variable; not completely clean fix:

Fedora Bug 1192742

gc Gerrit 7150

gc downloaded testsuite Bug

gc fails to rebuild outside of mock.

Fedora Bug 1191944

dockerinit Bug

Docker is not packageable as it contains embedded checksum of /usr/libexec/docker/dockerinit binary. This prevents debuginfo splitting and also prelink.

DWZ Bug

Fedora Bug 995136

DWZ will crash on gc-produced binaries. This does not prevent the packages to be built, just the DWZ optimization is not applied.

dwz: dwz.c:1984: checksum_die: Assertion `((!(multifile_mode & MULTIFILE_MODE_OP) && !(multifile_mode & MULTIFILE_MODE_RD) && !(multifile_mode & MULTIFILE_MODE_FI)) || cu != die_cu (ref)) && (!(multifile_mode & MULTIFILE_MODE_OP) || cu->cu_chunk == die_cu (ref)->cu_chunk)' failed.

At least a reduced level of DWZ optimization can be achieved by:

# https://bugzilla.redhat.com/show_bug.cgi?id=995136#c12
%global _dwz_low_mem_die_limit 0

debugedit Bug

Fedora Bug 1184221

Filenames of source files are invalid in *-debuginfo.rpm, they are not rewritten into /usr/src/debug/** subdirectory.

TODO list for gc in Fedora

Critical path:

#debugedit Bug - done, not yet accepted
#gc runtime-gdb.py Bug - 50% done, nothing accepted yet
#gc variable references Bug - done, a bit hacky, not yet accepted
#Channels - channels pretty printer in runtime-gdb.py
interfaces pretty printer fix in runtime-gdb.py

Optional TODOs:

GDB frame sniffer for #gc Missing CFI
Make build-id default for compilations by Fedora gc
#dockerinit Bug: Remove dockerinit checksum so that docker has its *-debuginfo.rpm.