Support preserve-fds with non-continuous fd numbers

[scanon]

Feature request description

Currently the preserve-fds allows for a single number that specifies how many additional file descriptors to preserve above fd 2. This means the additional file descriptors have to start at 3 and be contiguous. I have a situation where the file descriptor I need to pass is some high number. I have a work around that duplicates the fd of interest as 3 and then can pass --preserve-fds 1. But it would be much better if we could pass a list of FDs.

Suggest potential solution

I would like to be able to pass a list of fds to preserve. Since it needs to preserve backwards compatibility maybe there could be some way to indicate it is a list even if it is just one number.

So something like

podman run --preserve-fds 4,5 ....

or if there was just one something like

podman run --preserve-fds 5, ....

Have you considered any alternatives?

As mentioned I have a workaround but it is a bit of a hack.

Additional context

Add any other context or screenshots about the feature request here.

[eriksjolund]

Previous discussion:

• preserve-fds behavior #13254

[giuseppe]

it is a consequence of the Go API, that expects the FDs to be continuous:

podman/libpod/oci_conmon_common.go

Lines 1193 to 1199 in 6b9221d

	if preserveFDs > 0 {
	for fd := 3; fd < int(3+preserveFDs); fd++ {
	f := os.NewFile(uintptr(fd), fmt.Sprintf("fd-%d", fd))
	filesToClose = append(filesToClose, f)
	cmd.ExtraFiles = append(cmd.ExtraFiles, f)
	}
	}

so for this to work, we'd need to change the OCI runtime, conmon, and use directly fork+exec instead of the Go os/exec package

[Luap99]

@giuseppe I think it is valid to pass nil in the ExtraFiles array to skip the fd. Try

package main

import (
	"fmt"
	"os"
	"os/exec"
)

func main() {
	cmd := exec.Command("ls", "-l", "/proc/self/fd")
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	f1, _ := os.Open(os.DevNull)

	cmd.ExtraFiles = []*os.File{nil, f1, nil, f1, nil, f1}
	fmt.Println(cmd.Run())
}

[giuseppe]

@Luap99 neat! Thanks, that is good to know. In this case, we could probably handle it completely in Podman, passing --preserve-fds=N to be the maximum value in the FDs set

[giuseppe]

opened a PR: #20866