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

From: Chuck Ebbert
Date: Thu Mar 24 2011 - 23:51:37 EST


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