Re: 2.6.35-rc4: mount results with and without MS_SILENT differ

[Chuck Ebbert <cebbert@redhat.com>]

On Mon, 14 Mar 2011 05:20:08 +0100
Denys Vlasenko <vda.linux@googlemail.com> wrote:

It looks like this bug has been there forever.

This fails:
> mount -vv --make-rshared mount.shared1               2>&1

This succeeds:
> mount -vv --loud --make-rshared mount.shared2               2>&1  # <-HERE

Failure case uses MS_REC | MS_SHARED | MS_SILENT
> mount: mount('','mount.shared1','',0x0010c000,''):0

It succeeds with MS_REC | MS_SHARED
> mount: mount('','mount.shared2','',0x00104000,''):0

Looking at do_mount() we see some flags get filtered out before checking
namespace flags:

        flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
                   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
                   MS_STRICTATIME);

Then:

        else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                retval = do_change_type(&path, flags);

In do_change_type()

        type = flags_to_propagation_type(flag);
        if (!type)
                return -EINVAL;

And flags_to_propagation_type() filters out MS_REC before making sure one
and only one of the propagation type flags is set:

        int type = flags & ~MS_REC;

        /* Fail if any non-propagation flags are set */
        if (type & ~(MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                return 0;
        /* Only one propagation flag should be set */
        if (!is_power_of_2(type))
                return 0;

Looks like the is_power_of_2() test is failing because MS_SILENT is set.
I'm not sure whether to filter MS_SILENT in the upper or lower function
though.