libct: simplify exec fifo wait using poll(2) by captainmo1 · Pull Request #5271 · opencontainers/runc

captainmo1 · 2026-05-09T00:20:37Z

Replace the goroutine + channel + 100ms time.After + blocking-open in handleFifo with a poll(2) loop on a non-blocking-open. Use pidfd_open(2) where available to wait for init exit without timeout, falling back to /proc state checks with 100ms timeout on older kernels. Allowing us to wait on both write to execfifo and state of init process simultaneously.

Fixes #5251

kolyshkin · 2026-05-13T02:51:47Z

+// or the init process has exited. It does not consume the data — it only
+// returns once a subsequent read on f will not block indefinitely.
+func waitForFifoReady(f *os.File, pid int) error {
+	pidFd, err := unix.PidfdOpen(pid, 0)


You can use https://pkg.go.dev/os#Process.WithHandle here:

proc, _ := os.FindProcess(pid) // or add process() to parentProcess interface var readyErr error err := proc.WithHandle(func(pidfd uintptr) { readyErr = waitForReadyPidfd(f, int(pidfd)) }) if err != nil { // pidfd not supported return waitForFifoReadyPolling(f, pid) }

This works better because WithHandle will only call the function if pidfd is fully supported by the kernel (there are many kernels with different nuances and you can rely on the Go runtime check).

I agree its much better

os.Process.WithHandle became available in Go 1.26 but runc's go.mod is currently at go 1.25.0. We'll need to bump the version or I can continue using raw unix.PidfOpen for now.

Maybe add a // TODO: switch to os.Process.WithHandle once go < 1.26 is no longer supported. or somesuch.

kolyshkin

Thanks for working on this! Left a few comments, but overall this looks good.

KhurshidMurotov · 2026-05-13T11:18:14Z

this is nice one

lifubang

Great refactoring work! LGTM overall, with a few minor nits.

lifubang · 2026-06-01T15:07:37Z

+	// ENOSYS: kernel < 5.3, no pidfd_open at all.
+	// ESRCH:  process is already gone — fall through to the polling path,
+	//         which will detect the dead process on its first liveness check.
+	// Other errors (EINVAL, EMFILE, etc.) are unexpected; bail.


Maybe we need to deal with EMFILE, for example in dind.
So I think we should fall through to the polling path anyway once we got an error:

func waitForFifoReady(f *os.File, pid int) error { // TODO: switch to os.Process.WithHandle once go < 1.26 is no longer supported. pidFd, err := unix.PidfdOpen(pid, 0) if err == nil { defer unix.Close(pidFd) return waitForFifoReadyPidfd(f, pidFd) } // fall through to the polling path. return waitForFifoReadyPolling(f, pid) }

those errors wouldn't warrant a hard fail. got it

lifubang · 2026-06-01T15:20:04Z

+		if err != nil {
+			return fmt.Errorf("poll exec fifo: %w", err)
+		}
+		if n > 0 && pfd[0].Revents&unix.POLLIN != 0 {


Maybe we can add a POLLHUP check to reduce a system.Stat call in some situations:

if n > 0 && pfd[0].Revents&(unix.POLLIN|unix.POLLHUP) != 0 {

Good point. I'll update it.

lifubang · 2026-06-02T14:54:37Z

+		if n > 0 && pfd[0].Revents&(unix.POLLIN|unix.POLLHUP) != 0 {
+			return nil
+		}
+		// If init is dead, fall


nit: The line break here splits "fall through", which reads awkwardly. Suggest merging this comment into a single line.

seen it. thanks

lifubang

LGTM, thanks.

Please avoid modifying unrelated lines in your last force push.

Replace the goroutine + channel + 100ms time.After + blocking open in handleFifo with a poll(2) loop on a non-blocking open. Use pidfd_open(2) where available to wait for init exit without timeout, falling back to /proc state checks with 100ms timeout on older kernels. Fixes opencontainers#5251 Signed-off-by: Mohammed Aminu Futa <mohammedfuta2000@gmail.com> Signed-off-by: lifubang <lifubang@acmcoder.com>

lifubang · 2026-06-09T14:37:02Z

@opencontainers/runc-maintainers PTAL

captainmo1 force-pushed the 5251-simplify-exec-fifo-wait branch 2 times, most recently from e91e67c to 1889c08 Compare May 11, 2026 20:37

kolyshkin reviewed May 13, 2026

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Comment thread libcontainer/container_linux.go

kolyshkin reviewed May 13, 2026

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captainmo1 force-pushed the 5251-simplify-exec-fifo-wait branch 3 times, most recently from 73c4cd4 to 460ea7f Compare May 19, 2026 15:28

captainmo1 force-pushed the 5251-simplify-exec-fifo-wait branch from 460ea7f to 72a9bd5 Compare May 22, 2026 22:29

captainmo1 force-pushed the 5251-simplify-exec-fifo-wait branch from 72a9bd5 to 79e2c6d Compare May 31, 2026 19:32

lifubang reviewed Jun 1, 2026

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captainmo1 force-pushed the 5251-simplify-exec-fifo-wait branch from 79e2c6d to 78c6c52 Compare June 2, 2026 14:11

lifubang reviewed Jun 2, 2026

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captainmo1 force-pushed the 5251-simplify-exec-fifo-wait branch from 78c6c52 to 2cc449c Compare June 2, 2026 15:10

lifubang previously approved these changes Jun 2, 2026

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captainmo1 force-pushed the 5251-simplify-exec-fifo-wait branch from 2cc449c to 7785938 Compare June 2, 2026 15:45

lifubang force-pushed the 5251-simplify-exec-fifo-wait branch 2 times, most recently from ed376c2 to 02b8153 Compare June 4, 2026 15:15

lifubang approved these changes Jun 5, 2026

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lifubang requested review from AkihiroSuda, cyphar, kolyshkin and rata June 5, 2026 01:54

kolyshkin reviewed Jun 5, 2026

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Comment thread libcontainer/container_linux.go Outdated

lifubang force-pushed the 5251-simplify-exec-fifo-wait branch from 02b8153 to 5f365bd Compare June 6, 2026 00:52

lifubang force-pushed the 5251-simplify-exec-fifo-wait branch from 5f365bd to 937d887 Compare June 6, 2026 00:55

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