x/debug/cmd/viewcore: confusing error message when not specifying --exe

[stapelberg]

What version of Go are you using (go version)?

$ go version
go version go1.11.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/michael/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/michael/go"
GOPROXY=""
GORACE=""
GOROOT="/home/michael/sdk/go1.11.1"
GOTMPDIR=""
GOTOOLDIR="/home/michael/sdk/go1.11.1/pkg/tool/linux_amd64"
GCCGO="/usr/bin/gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/tmp/vc/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build665857060=/tmp/go-build -gno-record-gcc-switches"

What did you do?

% viewcore /tmp/core.dcs-package-imp.23131.ex61.1543603411 breakdown
can't read DWARF info from /lib/x86_64-linux-gnu/libc-2.24.so: decoding dwarf section info at offset 0x0: too short

% viewcore /tmp/core.dcs-package-imp.23131.ex61.1543603411 --exe ./bin/dcs-package-importer breakdown 
 all                  13451362304 100.00% 
[…]

I’m not sure what’s happening without --exe, but the error message should suggest I use --exe in that situation, I suppose?

[agnivade]

/cc @randall77 @aclements