Re: [PATCH v2 2/2] migration/multifd: Ensure we're not given a socket for file migration

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Wed, Mar 13, 2024 at 06:28:24PM -0300, Fabiano Rosas wrote:
>> When doing migration using the fd: URI, the incoming migration starts
>> before the user has passed the file descriptor to QEMU. This means
>> that the checks at migration_channels_and_transport_compatible()
>> happen too soon and we need to allow a migration channel of type
>> SOCKET_ADDRESS_TYPE_FD even though socket migration is not supported
>> with multifd.
>
> Hmm, bare with me if this is a stupid one.. why the incoming migration can
> start _before_ the user passed in the fd?

It's been a while since I looked at this. Looking into it once more
today, I think the issue is actually that we only fetch the fds from the
monitor at fd_start_outgoing|incoming_migration().

>
> IOW, why can't we rely on a single fd_is_socket() check for
> SOCKET_ADDRESS_TYPE_FD in transport_supports_multi_channels()?
>

There's no fd at that point. Just a string.

I think the right fix here would be to move the
monitor_fd_get/monitor_fd_param (why two different functions?) earlier
into migrate_uri_parse. And possibly also extend FileMigrationArgs to
contain an fd. Not sure how easy would that be.

>> 
>> The commit decdc76772 ("migration/multifd: Add mapped-ram support to
>> fd: URI") was supposed to add a second check prior to starting
>> migration to make sure a socket fd is not passed instead of a file fd,
>> but failed to do so.
>> 
>> Add the missing verification.
>> 
>> Fixes: decdc76772 ("migration/multifd: Add mapped-ram support to fd: URI")
>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> ---
>>  migration/fd.c   | 8 ++++++++
>>  migration/file.c | 7 +++++++
>>  2 files changed, 15 insertions(+)
>> 
>> diff --git a/migration/fd.c b/migration/fd.c
>> index 39a52e5c90..c07030f715 100644
>> --- a/migration/fd.c
>> +++ b/migration/fd.c
>> @@ -22,6 +22,7 @@
>>  #include "migration.h"
>>  #include "monitor/monitor.h"
>>  #include "io/channel-file.h"
>> +#include "io/channel-socket.h"
>>  #include "io/channel-util.h"
>>  #include "options.h"
>>  #include "trace.h"
>> @@ -95,6 +96,13 @@ void fd_start_incoming_migration(const char *fdname, Error **errp)
>>      }
>>  
>>      if (migrate_multifd()) {
>> +        if (fd_is_socket(fd)) {
>> +            error_setg(errp,
>> +                       "Multifd migration to a socket FD is not supported");
>> +            object_unref(ioc);
>> +            return;
>> +        }
>> +
>>          file_create_incoming_channels(ioc, errp);
>>      } else {
>>          qio_channel_set_name(ioc, "migration-fd-incoming");
>> diff --git a/migration/file.c b/migration/file.c
>> index ddde0ca818..b6e8ba13f2 100644
>> --- a/migration/file.c
>> +++ b/migration/file.c
>> @@ -15,6 +15,7 @@
>>  #include "file.h"
>>  #include "migration.h"
>>  #include "io/channel-file.h"
>> +#include "io/channel-socket.h"
>>  #include "io/channel-util.h"
>>  #include "options.h"
>>  #include "trace.h"
>> @@ -58,6 +59,12 @@ bool file_send_channel_create(gpointer opaque, Error **errp)
>>      int fd = fd_args_get_fd();
>>  
>>      if (fd && fd != -1) {
>> +        if (fd_is_socket(fd)) {
>> +            error_setg(errp,
>> +                       "Multifd migration to a socket FD is not supported");
>> +            goto out;
>> +        }
>> +
>>          ioc = qio_channel_file_new_dupfd(fd, errp);
>>      } else {
>>          ioc = qio_channel_file_new_path(outgoing_args.fname, flags, 0, errp);
>> -- 
>> 2.35.3
>>