Re: [Virtio-fs] [PATCH] virtiofsd: Show submounts

[Max Reitz <mreitz@redhat.com>]

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On 28.04.20 21:15, Dr. David Alan Gilbert wrote:
> * Miklos Szeredi (mszeredi@redhat.com) wrote:
>> On Tue, Apr 28, 2020 at 4:52 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>>>
>>> On Mon, Apr 27, 2020 at 06:59:02PM +0100, Dr. David Alan Gilbert wrote:
>>>> * Max Reitz (mreitz@redhat.com) wrote:
>>>>> Currently, setup_mounts() bind-mounts the shared directory without
>>>>> MS_REC.  This makes all submounts disappear.
>>>>>
>>>>> Pass MS_REC so that the guest can see submounts again.
>>>>
>>>> Thanks!
>>>>
>>>>> Fixes: 3ca8a2b1c83eb185c232a4e87abbb65495263756
>>>>
>>>> Should this actually be 5baa3b8e95064c2434bd9e2f312edd5e9ae275dc ?
>>>>
>>>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>>>> ---
>>>>>  tools/virtiofsd/passthrough_ll.c | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
>>>>> index 4c35c95b25..9d7f863e66 100644
>>>>> --- a/tools/virtiofsd/passthrough_ll.c
>>>>> +++ b/tools/virtiofsd/passthrough_ll.c
>>>>> @@ -2643,7 +2643,7 @@ static void setup_mounts(const char *source)
>>>>>      int oldroot;
>>>>>      int newroot;
>>>>>
>>>>> -    if (mount(source, source, NULL, MS_BIND, NULL) < 0) {
>>>>> +    if (mount(source, source, NULL, MS_BIND | MS_REC, NULL) < 0) {
>>>>>          fuse_log(FUSE_LOG_ERR, "mount(%s, %s, MS_BIND): %m\n", source, source);
>>>>>          exit(1);
>>>>>      }
>>>>
>>>> Do we want MS_SLAVE to pick up future mounts that might happenf rom the
>>>> host?
>>>
>>> There are two separate concepts:
>>>
>>> 1. Mount namespaces.  The virtiofsd process is sandboxed and lives in
>>>    its own mount namespace.  Therefore it does not share the mounts that
>>>    the rest of the host system sees.
>>>
>>> 2. Propagation type.  This is related to bind mounts so that mount
>>>    operations that happen in one bind-mounted location can also appear
>>>    in other bind-mounted locations.
>>>
>>> Since virtiofsd is in a separate mount namespace, does the propagation
>>> type even have any effect?
>>
>> It's a complicated thing.  Current setup results in propagation
>> happening to the cloned namespace, but not to the bind mounted root.
>>
>> Why?  Because setting mounts "slave" after unshare, results in the
>> propagation being stopped at that point.  To make it propagate
>> further, change it back to "shared".  Note: the result changing to
>> "slave" and then to "shared" results in breaking the backward
>> propagation to the original namespace, but allowing propagation
>> further down the chain.
> 
> Do you mean on the "/" ?
> 
> So our current sequence is:
> 
>    (new namespace)
>  1)    if (mount(NULL, "/", NULL, MS_REC | MS_SLAVE, NULL) < 0) {
>  2)   if (mount("proc", "/proc", "proc",
>            ....
>  3)   if (mount(source, source, NULL, MS_BIND | MS_REC, NULL) < 0) {
>  4)  (chdir newroot, pivot, chdir oldroot)
>  5)   if (mount("", ".", "", MS_SLAVE | MS_REC, NULL) < 0) {
>  6)   if (umount2(".", MNT_DETACH) < 0) {
> 
> So are you saying we need a:
>        if (mount(NULL, "/", NULL, MS_REC | MS_SHARED, NULL) < 0) {
> 
>   and can this go straight after (1) ?

Isn’t MS_SHARED and MS_SLAVE mutually exclusive, that is, both are just
different propagation types?  So shouldn’t putting this after (1) be
effectively the same as replacing (1)?

Max


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