os: Race condition in (* os.File).readdir()

[mitmitmitm]

Go version

go version go1.23.1 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='~/.cache/go-build'
GOENV='~/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='~/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='~/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.1'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='~/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='~/git/test-proj/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1913440061=/tmp/go-build -gno-record-gcc-switches'

What did you do?

In os/dir_unix.go and in os/dir_plan9.go, (* os.File).readdir() has

func (f *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
	// If this file has no dirInfo, create one.
	d := f.dirinfo.Load()
	if d == nil {
		d = new(dirInfo)
		f.dirinfo.Store(d)
	}
	d.mu.Lock()
	defer d.mu.Unlock()

	// [...]

	for n != 0 {
			// [...]
			d.nbuf, errno = f.pfd.ReadDirent(*d.buf)

This code can execute concurrently in two goroutines such that the condition d == nil is true in both of them. In this case, each goroutine creates and works with a separate instance of d = new(dirInfo), locks a separate mutex d.mu.Lock() and possibly races with the otherone over calls to f.pfd.ReadDirent(*d.buf).

What did you see happen?

/

What did you expect to see?

f.dirinfo.Store(d) should probably use CompareAndSwap instead.

[gabyhelp]

Related Issues

• os: File.ReadDir is not concurrency safe on some platforms #66498 (closed)
• runtime/race: os.File.ReadAt defeats race detector #51618 (closed)
• os: race between os.(*File).Close() and os.(*File).Read() #10001 (closed)
• os: small-count Readdirnames followed by seek-to-zero can lead to duplicate names #37161 (closed)
• os: document that fs.ReadDirFile.ReadDir doesn't restart each time #69301
• runtime/race: potential false positive when synchronizing with unix.Flock() #50139 (closed)
• os/exec: .Wait(): non-deterministic behavior (race?) #68308 (closed)
• io: race condition with use of Pipe #67633

Related Code Changes

• os: make File.Readdir et al concurrency-safe

Related Discussions

• file_unix.go race?

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[gopherbot]

Change https://go.dev/cl/645720 mentions this issue: Fix: race condition in readdir by ensuring atomic initialization of d…

[jianfengtony]

why not backport this issue to old version?

[ianlancetaylor]

It's not really a race condition, and it's a fairly minor bug. I don't think this meets the guidelines at https://go.dev/wiki/MinorReleases.