Re: [Qemu-devel] [PATCH] migration: Fix handling fd protocol

[Yury Kotov]

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18.04.2019, 20:46, "Yury Kotov" <address@hidden>:
> 18.04.2019, 20:01, "Dr. David Alan Gilbert" <address@hidden>:
>>  * Yury Kotov (address@hidden) wrote:
>>>   18.04.2019, 19:03, "Dr. David Alan Gilbert" <address@hidden>:
>>>   > * Yury Kotov (address@hidden) wrote:
>>>   >>  18.04.2019, 17:20, "Dr. David Alan Gilbert" <address@hidden>:
>>>   >>  > * Yury Kotov (address@hidden) wrote:
>>>   >>  >>  15.04.2019, 14:30, "Dr. David Alan Gilbert" <address@hidden>:
>>>   >>  >>  > * Daniel P. Berrangé (address@hidden) wrote:
>>>   >>  >>  >>  On Mon, Apr 15, 2019 at 12:15:12PM +0100, Dr. David Alan 
>>> Gilbert wrote:
>>>   >>  >>  >>  > * Daniel P. Berrangé (address@hidden) wrote:
>>>   >>  >>  >>  > > On Mon, Apr 15, 2019 at 01:33:21PM +0300, Yury Kotov 
>>> wrote:
>>>   >>  >>  >>  > > > 15.04.2019, 13:25, "Daniel P. Berrangé" 
>>> <address@hidden>:
>>>   >>  >>  >>  > > > > On Mon, Apr 15, 2019 at 01:17:06PM +0300, Yury Kotov 
>>> wrote:
>>>   >>  >>  >>  > > > >>  15.04.2019, 13:11, "Daniel P. Berrangé" 
>>> <address@hidden>:
>>>   >>  >>  >>  > > > >>  > On Mon, Apr 15, 2019 at 12:50:08PM +0300, Yury 
>>> Kotov wrote:
>>>   >>  >>  >>  > > > >>  >>  Hi,
>>>   >>  >>  >>  > > > >>  >>
>>>   >>  >>  >>  > > > >>  >>  Just to clarify. I see two possible solutions:
>>>   >>  >>  >>  > > > >>  >>
>>>   >>  >>  >>  > > > >>  >>  1) Since the migration code doesn't receive fd, 
>>> it isn't responsible for
>>>   >>  >>  >>  > > > >>  >>  closing it. So, it may be better to use 
>>> migrate_fd_param for both
>>>   >>  >>  >>  > > > >>  >>  incoming/outgoing and add dupping for 
>>> migrate_fd_param. Thus, clients must
>>>   >>  >>  >>  > > > >>  >>  close the fd themselves. But existing clients 
>>> will have a leak.
>>>   >>  >>  >>  > > > >>  >
>>>   >>  >>  >>  > > > >>  > We can't break existing clients in this way as 
>>> they are correctly
>>>   >>  >>  >>  > > > >>  > using the monitor with its current semantics.
>>>   >>  >>  >>  > > > >>  >
>>>   >>  >>  >>  > > > >>  >>  2) If we don't duplicate fd, then at least we 
>>> should remove fd from
>>>   >>  >>  >>  > > > >>  >>  the corresponding list. Therefore, the solution 
>>> is to fix qemu_close to find
>>>   >>  >>  >>  > > > >>  >>  the list and remove fd from it. But qemu_close 
>>> is currently consistent with
>>>   >>  >>  >>  > > > >>  >>  qemu_open (which opens/dups fd), so adding 
>>> additional logic might not be
>>>   >>  >>  >>  > > > >>  >>  a very good idea.
>>>   >>  >>  >>  > > > >>  >
>>>   >>  >>  >>  > > > >>  > qemu_close is not appropriate place to deal with 
>>> something speciifc
>>>   >>  >>  >>  > > > >>  > to the montor.
>>>   >>  >>  >>  > > > >>  >
>>>   >>  >>  >>  > > > >>  >>  I don't see any other solution, but I might 
>>> miss something.
>>>   >>  >>  >>  > > > >>  >>  What do you think?
>>>   >>  >>  >>  > > > >>  >
>>>   >>  >>  >>  > > > >>  > All callers of monitor_get_fd() will close() the 
>>> FD they get back.
>>>   >>  >>  >>  > > > >>  > Thus monitor_get_fd() should remove it from the 
>>> list when it returns
>>>   >>  >>  >>  > > > >>  > it, and we should add API docs to 
>>> monitor_get_fd() to explain this.
>>>   >>  >>  >>  > > > >>  >
>>>   >>  >>  >>  > > > >>  Ok, it sounds reasonable. But monitor_get_fd is 
>>> only about outgoing migration.
>>>   >>  >>  >>  > > > >>  But what about the incoming migration? It doesn't 
>>> use monitor_get_fd but just
>>>   >>  >>  >>  > > > >>  converts input string to int and use it as fd.
>>>   >>  >>  >>  > > > >
>>>   >>  >>  >>  > > > > The incoming migration expects the FD to be passed 
>>> into QEMU by the mgmt
>>>   >>  >>  >>  > > > > app when it is exec'ing the QEMU binary. It doesn't 
>>> interact with the
>>>   >>  >>  >>  > > > > monitor at all AFAIR.
>>>   >>  >>  >>  > > > >
>>>   >>  >>  >>  > > >
>>>   >>  >>  >>  > > > Oh, sorry. This use case is not obvious. We used add-fd 
>>> to pass fd for
>>>   >>  >>  >>  > > > migrate-incoming and such way has described problems.
>>>   >>  >>  >>  > >
>>>   >>  >>  >>  > > That's a bug in your usage of QEMU IMHO, as the incoming 
>>> code is not
>>>   >>  >>  >>  > > designed to use add-fd.
>>>   >>  >>  >>  >
>>>   >>  >>  >>  > Hmm, that's true - although:
>>>   >>  >>  >>  > a) It's very non-obvious
>>>   >>  >>  >>  > b) Unfortunate, since it would go well with -incoming defer
>>>   >>  >>  >>
>>>   >>  >>  >>  Yeah I think this is a screw up on QMEU's part when 
>>> introducing 'defer'.
>>>   >>  >>  >>
>>>   >>  >>  >>  We should have mandated use of 'add-fd' when using 'defer', 
>>> since FD
>>>   >>  >>  >>  inheritance-over-execve() should only be used for command 
>>> line args,
>>>   >>  >>  >>  not monitor commands.
>>>   >>  >>  >>
>>>   >>  >>  >>  Not sure how to best fix this is QEMU though without breaking 
>>> back
>>>   >>  >>  >>  compat for apps using 'defer' already.
>>>   >>  >>  >
>>>   >>  >>  > We could add mon-fd: transports that has the same behaviour as 
>>> now for
>>>   >>  >>  > outgoing, and for incoming uses the add-fd stash.
>>>   >>  >>  >
>>>   >>  >>
>>>   >>  >>  Oh, I'm sorry again. I think my suggestion about monitor_fd_param 
>>> wasn't
>>>   >>  >>  relevant to this issue. If migrate-incoming + "fd:" + add-fd is 
>>> an invalid use
>>>   >>  >>  case, should we disallow this?
>>>   >>  >>  I may add a check to fd_start_incoming_migration if fd is in mon 
>>> fds list.
>>>   >>  >>  But I'm afraid there are users like me who are already using this 
>>> wrong use case.
>>>   >>  >>  Because currently nothing in QEMU's docs disallow this.
>>>   >>  >>
>>>   >>  >>  So which solution is better in your opinion?
>>>   >>  >>  1) Disallow fd's from mon fds list in fd_start_incoming_migration
>>>   >>  >
>>>   >>  > I'm surprised anything could be doing that - how would a user know 
>>> what
>>>   >>  > the correct fd index was?
>>>   >>  >
>>>   >>
>>>   >>  Hmm, add-fd returns correct fd value. Maybe I din't catch you 
>>> question...
>>>   >
>>>   > I don't understand, where does it return it?
>>>   >
>>>
>>>   From misc.json:
>>>   # Example:
>>>   #
>>>   # -> { "execute": "add-fd", "arguments": { "fdset-id": 1 } }
>>>   # <- { "return": { "fdset-id": 1, "fd": 3 } }
>>>   #
>>>
>>>   "fd": 3 is a valid fd for migrate-incoming(uri = "fd:3")
>>
>>  Ah OK.
>>
>>>   >>  >>  2) Allow these fds, but dup them or close them correctly
>>>   >>  >
>>>   >>  > I think I'd leave the current (confusing) fd: as it is, maybe put a 
>>> note
>>>   >>  > in the manual.
>>>   >>  >
>>>   >>
>>>   >>  So, using fd from fdset will be an undefined behavior, right?
>>>   >
>>>   > For incoming, yes.
>>>   >
>>>   >>  >>  And how to migrate-incoming defer through fd correctly?
>>>   >>  >>  1) Add "mon-fd:" protocol to work with fds passed by 
>>> "add-fd/remove-fd" commands
>>>   >>  >>  as suggested by Dave
>>>   >>  >
>>>   >>  > That's my preference; it's explicitly named and consistent, and it
>>>   >>  > doesn't touch the existing fd code.
>>>   >>  >
>>>   >>
>>>   >>  Ok, but please tell me what you think of my suggestion (2) about 
>>> using fd added
>>>   >>  by the "getfd" command for incoming migration. It doesn't requires 
>>> introducing
>>>   >>  new protocol and will be consistent with outgoing migration through 
>>> fd.
>>>   >
>>>   > I worry how qemu knows whether the command means it comes from the getfd
>>>   > command or is actually a normal fd like now?
>>>   > Can you give an example.
>>>   >
>>>
>>>   getfd manages naming fds list.
>>>   # -> { "execute": "getfd", "arguments": { "fdname": "fd1" } }
>>>   So, for migrate (not incoming) is now valid migrate(uri="fd:fd1")
>>>
>>>   I want the same for migrate-incoming. If fdname is parseable int, then it 
>>> is
>>>   an old format. Otherwise - it is a name of fd added by addfd.
>>>
>>>   There is a function "monitor_fd_param" which do exactly what I mean:
>>>   int monitor_fd_param(Monitor *mon, const char *fdname, Error **errp) {
>>>       ... local vars ...
>>>       if (!qemu_isdigit(fdname[0]) && mon) {
>>>           fd = monitor_get_fd(mon, fdname, &local_err);
>>>       } else {
>>>           fd = qemu_parse_fd(fdname);
>>>       }
>>>       ... report err to errp ...
>>>   }
>>
>>  OK, if we're already using monitor_fd_param everywhere then I think
>>  we're already down the rat-hole of guessing whether we're an add-fd or
>>  fd by whether it's an integer, and I agree with you that we should
>>  just fix incoming to use that.
>>
>>  Now, that means I guess we need to modify monitor_fd_param to tell us
>>  which type of fd it got, so we know whether to close it later?
>>
>>  Dave
>>  P.S. I'm out from tomorrow for a weekish.
>
> I think the right way is to check whether fd is added by add-fd and if so then
> return error. Because IIUC the only valid usage of add-fd is to use
> qemu_open("/dev/fdset/<fdset_id>") which finds suitable fd from fdset.
> Such behavior is incompatible with fd:<fd_num> at all, as such syntax
> doesn't imply the using of particular fd. But if so, why add-fd returns
> the value of added fd?..
>
> But if I'm right it's enough to:
> 1) Modify monitor_fd_param to check where fd comes from and disallow using
>    fd of "add-fd",
> 2) As we discussed earlier, modify monitor_get_fd to remove named fd from its
>    list before return,

Omg, monitor_fd_param is already do so... Sorry, so the problem only in
incoming case.

> 3) Use monitor_fd_param in migrate_incoming for "fd:" proto.
>
> I'm not insist. May be it's ok to use fd from fdset directly and so qemu_close
> should be modifyed.
>
> Just to clarify what I mean:
> fdset is a struct:
> struct MonFdset {
>     int64_t id;
>     QLIST_HEAD(, MonFdsetFd) fds;
>     QLIST_HEAD(, MonFdsetFd) dup_fds;
>     QLIST_ENTRY(MonFdset) next;
> };
>
> * add-fd appends new fd to "->fds" list.
> * qemu_open("/dev/fdset/X", int perms) is looking for suitable (by perms) fd
>   from fdset with id X, dup it and append "->dup_fds" list.
> * qemu_close(int fd) tryes to find the fd in all "->dup_fds" lists
>   of all fdsets and remove it. And closes fd anyway.
>
> If not to disallow using fds added by add-fd then there are three
> possible solutions:
> 1) dup fd in monitor_fd_param it the fd is from some fdset,
> 2) remove the fd from "->fds" list in qemu_close
> 3) don't close it in qemu_close, so client is responsible to close it by
>    remove-fd.
>
>>>   >>  >
>>>   >>  >>  2) My suggestion about monitor_fd_param and make "fd:" for
>>>   >>  >>  migrate/migrate-incoming consistent. So user will be able to use
>>>   >>  >>  getfd + migrate-incoming
>>>   >>  >>  3) Both of them or something else
>>>   >>  >>
>>>   >>

Regards,
Yury