Re: [Qemu-devel] [PATCH 1/4] hostmem-memfd: enable seals only if supported

[Ilya Maximets]

On 27.11.2018 14:49, Marc-André Lureau wrote:
> Hi
> On Tue, Nov 27, 2018 at 3:11 PM Ilya Maximets <address@hidden> wrote:
>>
>> If seals are not supported, memfd_create() will fail.
>> Furthermore, there is no way to disable it in this case because
>> '.seal' property is not registered.
>>
>> This issue leads to vhost-user-test failures on RHEL 7.2:
>>
>>   qemu-system-x86_64: -object memory-backend-memfd,id=mem,size=2M,: \
>>                       failed to create memfd: Invalid argument
>>
>> Signed-off-by: Ilya Maximets <address@hidden>
> 
> 
> This will change the default behaviour of memfd backend, and may thus
> me considered a break.

This will change the default behaviour only on systems without sealing
support. But current implementation is broken there anyway and does not
work.

> 
> Instead, memfd vhost-user-test should skipped (or tuned with
> sealed=false if unsupported)

vhost-user-test is just an example here. In fact memfd could not be
used at all on system without sealing support. And there is no way
to disable seals.

> 
>> ---
>>  backends/hostmem-memfd.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
>> index b6836b28e5..ee39bdbde6 100644
>> --- a/backends/hostmem-memfd.c
>> +++ b/backends/hostmem-memfd.c
>> @@ -129,8 +129,8 @@ memfd_backend_instance_init(Object *obj)
>>  {
>>      HostMemoryBackendMemfd *m = MEMORY_BACKEND_MEMFD(obj);
>>
>> -    /* default to sealed file */
>> -    m->seal = true;
>> +    /* default to sealed file if supported */
>> +    m->seal = qemu_memfd_check(MFD_ALLOW_SEALING);
>>  }
>>
>>  static void
>> --
>> 2.17.1
>>
> 
>