mount: can't remount,rw

[Darkness4]

Hello, I've migrated from docker/docker/pkg/mount to moby/sys/mount, and I've run in a small issue.

I'm running this test:

package main

import (
	"os"

	"github.com/moby/sys/mount"
)

func main() {
	_ = os.Mkdir("/tmp/test", 0755)
	err := mount.Mount("tmpfs", "/tmp/test", "tmpfs", "ro")
	if err != nil {
		panic(err)
	}

	err = mount.Mount("", "/tmp/test", "none", "remount,rw")
	if err != nil {
		panic(err)
	}
}

And "remount,rw" doesn't get executed. Not sure if this is expected behavior.

It doesn't enter any case in this function:

sys/mount/mounter_linux.go

Lines 30 to 72 in 638aa7c

	func mount(device, target, mType string, flags uintptr, data string) error {
	oflags := flags &^ ptypes
	if !isremount(device, flags) || data != "" {
	// Initial call applying all non-propagation flags for mount
	// or remount with changed data
	if err := unix.Mount(device, target, mType, oflags, data); err != nil {
	return &mountError{
	op: "mount",
	source: device,
	target: target,
	flags: oflags,
	data: data,
	err: err,
	}
	}
	}
	if flags&ptypes != 0 {
	// Change the propagation type.
	if err := unix.Mount("", target, "", flags&pflags, ""); err != nil {
	return &mountError{
	op: "remount",
	target: target,
	flags: flags & pflags,
	err: err,
	}
	}
	}
	if oflags&broflags == broflags {
	// Remount the bind to apply read only.
	if err := unix.Mount("", target, "", oflags|unix.MS_REMOUNT, ""); err != nil {
	return &mountError{
	op: "remount-ro",
	target: target,
	flags: oflags | unix.MS_REMOUNT,
	err: err,
	}
	}
	}
	return nil
	}

[kolyshkin]

Hello, I've migrated from docker/docker/pkg/mount to moby/sys/mount, and I've run in a small issue.

This implied you've found a regression (IOW your code worked correctly when the docker/docker/pkg/mount was used. It looks like it's not -- I tested with github.com/moby/moby v17.12.0-ce-rc1.0.20190717161051-705d9623b7c1+incompatible and it gives the same result ("remount,rw" is ignored).

Yet it seems the bug is valid, it's just not a regression. Looking...

[kolyshkin]

My conclusion is, this package did not envision such a use case, but hopefully it can be added without breaking anything else.

Can you give us some more details about your use case?

In the meantime, a workaround for your use case is to specify some text-only option, such as size=....

[Darkness4]

Hello, I maintaining a fork of k3os. I'm not the original maintainer so I don't really know the original motivation of using the docker/docker/pkg/mount package, but they used the package to remount /k3os/system with rw during upgrades, which is immutable by default (mounted ro).

As proper "workaround", I simply used the syscall package to remount: Darkness4/k3os@2214405#diff-05da710ec6495dd56e5e44533a90c845f5fe090ad706dea8b496b8b9e6dc084dL62