debug/elf: calling `Open` on an empty file does not return a `&FormatError`

[MusicalNinjaDad]

Go version

go1.24.9 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/ninjacoder/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/ninjacoder/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2922389718=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/workspaces/snaggle/go.mod'
GOMODCACHE='/workspaces/snaggle/.go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/workspaces/snaggle/.go'
GOPRIVATE=''
GOPROXY='direct'
GOROOT='/usr/lib/golang'
GOSUMDB='off'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/ninjacoder/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/lib/golang/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.9'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

shell: touch .empty

elffile, err = debug_elf.Open(".empty")

What did you see happen?

err is a naked io.EOF and not a &FormatError as would be expected. i.e.:

errors.Is(err, io.EOF) == true
var formaterr *debug_elf.FormatError
errors.As(err, &formaterr) == false

What did you expect to see?

As with other errors encountered by Open a &FormatError should be returned. Probably &FormatError{0, "no data"}

See MusicalNinjaDad/snaggle@3c69325#L172

[mknyszek]

In triage, we think that generally changing the type of an error returned is more iffy in terms of the backwards compatibility guarantee... but this is also a lesser used package and an odd corner case. Maybe it's fine.

[ianlancetaylor]

Currently the debug/elf package returns an error of type elf.FormatError for any error in the date of the ELF file. However, it does not return an error of that type for any error reading the ELF file. Errors reading the ELF file are returned unchanged. In particular, if the ELF file is too short, debug/elf will return io.EOF. This is true if the file is completely empty, and it is also true if the file is truncated at some point.

We certainly could change elf.Open and elf.NewFile to return elf.FormatError on any error reading from the file. But it's not obvious that we should do that.

I note that debug/macho and debug/pe act the same as debug/elf: they return FormatError for an error in the data, but return errors seen reading from the file unchanged.

I'm inclined to say that we shouldn't change anything here.

[MusicalNinjaDad]

Would you accept a simple documentation change to clarify this? The approach makes sense but isn't obvious - it's easy to assume that checking for a FormatError is sufficient to cover all cases.

I'd be happy to draft suitable wording for elf.Open, elf.NewFile and elf.FormatError

The alternative would be to wrap these cases in a fs.PathError which would be consistent with the rest of the stdlib but involve a chunk more work

[ianlancetaylor]

Documentation patches are always fine. Thanks.

At least the debug/macho package has documentation for its version of FormatError. The debug/elf and debug/macho docs should probably be identical or at least similar.

I see now that I was wrong about debug/pe. debug/pe defines FormatError but doesn't use it.