Re: [PATCH] virtiofsd: Show submounts

[Max Reitz]

On 28.04.20 11:59, Dr. David Alan Gilbert wrote:
> * Max Reitz (address@hidden) wrote:
>> On 27.04.20 19:59, Dr. David Alan Gilbert wrote:
>>> * Max Reitz (address@hidden) 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 ?
>>
>> Well, I bisected it and landed at 3ca8a2b1.  So while the problematic
>> line may have been introduced by 5baa3b8e, it wasn’t used until 3ca8a2b1.
> 
> OK, I'd rather stick with the Fixes: for the patch that was actually
> wrong.

Why not both? :)

>>>> Signed-off-by: Max Reitz <address@hidden>
>>>> ---
>>>>  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?
>>
>> Hm.  So first it looks to me from the man page like one shouldn’t give
>> MS_SLAVE on the first mount() call but kind of only use it for remounts
>> (in the list at the start, “Create a bind mount” is separate from
>> “Change the propagation type of an existing mount”, and the man page
>> later says “The only other flags that can be specified while changing
>> the propagation type are MS_REC (described below) and MS_SILENT (which
>> is ignored).”).
>>
>> Second, even if I do change the propagation type to MS_SLAVE in a second
>> call, mounts done after qemu has been started don’t show up in the guest
>> (for me).
>>
>> So while it sounds correct, I can’t see it having an effect, actually.
> 
> That's unfortunate; but I guess we can debug that separately
> 
>>> What's the interaction between this and the MS_REC|MS_SLAVE that we have
>>> a few lines above for / ?
>>
>> Good question.  It would seem to me that there isn’t any.  That previous
>> mount call just sets MS_REC | MS_SLAVE for the whole mount namespace,
>> and then we do a new mount here (by default from / to /) that needs its
>> own flags.
>>
>> (More interesting is perhaps why we have that other mount() call below,
>> which again sets MS_REC | MS_SLAVE for the old (not-yet-bind-mounted) /.
>>  I can’t imagine that to have any effect.)
> 
> Is that just trying to be careful before the umount2 so it doesn't try
> to unmount something useful?

Perhaps, but still, it shouldn’t matter.  I rather suspect that
setup_namespaces() and setup_mounts() were developed (or taken from
elsewhere) independently, so they both have to work independently, and
thus they do overlapping stuff.

Max

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