[Spice-devel] [PATCH spice-streaming-agent v5 2/2] Implement handling of error messages from the server

[Frediano Ziglio]

> 
> > On 28 Feb 2018, at 13:53, Lukáš Hrázký <lhrazky at redhat.com> wrote:
> > 
> > Log error messages from the server to syslog (not aborting the agent).
> > Limits the messages to 1023 bytes, error messages from the server are
> > not expected to be longer. In the unlikely case the message is longer,
> > log the first 1023 bytes and throw an exception (because we don't read
> > out the rest of the message, causing desynchronization between the
> > server and the streaming agent).
> > 
> > Signed-off-by: Lukáš Hrázký <lhrazky at redhat.com>
> > ---
> > src/spice-streaming-agent.cpp | 26 +++++++++++++++++++++++---
> > 1 file changed, 23 insertions(+), 3 deletions(-)
> > 
> > diff --git a/src/spice-streaming-agent.cpp b/src/spice-streaming-agent.cpp
> > index ee57920..954d1b3 100644
> > --- a/src/spice-streaming-agent.cpp
> > +++ b/src/spice-streaming-agent.cpp
> > @@ -137,10 +137,30 @@ static void handle_stream_capabilities(uint32_t len)
> >     }
> > }
> > 
> > -static void handle_stream_error(uint32_t len)
> > +static void handle_stream_error(size_t len)
> > {
> > -    // TODO read message and use it
> > -    throw std::runtime_error("got an error message from server");
> > +    if (len < sizeof(StreamMsgNotifyError)) {
> > +        throw std::runtime_error("Received NotifyError message size " +
> > std::to_string(len) +
> > +                                 " is too small (smaller than " +
> > +
> > std::to_string(sizeof(StreamMsgNotifyError))
> > + ")");
> > +    }
> > +
> > +    struct : StreamMsgNotifyError {
> 
> :-O
> 
> I had t read that twice to make sure that was actually what was written…
> Barf.
> 
> 
> > +        uint8_t msg[1024];
> 
> Hard-coded magic value. There should be a constant for it in stream-device.h.
> Please add it.
> 

Currently the protocol does not define a limit, this is a guest limit but
probably a safe and reasonable limit for the protocol can be decided.

> > +    } msg;
> 
> So I was aggravated very recently regarding padding (having to initialize
> it…), but this patch gets nary a peep and the series is acked and merged
> right away?
> 

Perhaps you lost the mails saying that the protocol structure don't and
won't have internal padding.

> When you inherit, the derived members are aligned, and there is potential
> padding. However, the code as merged relies on msg.msg being identical to
> msg.StreamMsgNotifyError::msg. In other words, this code implicitly relies
> on the lack of any padding. Add for example a ‘char c’ or ‘bool b’ after
> error_code in StreamMsgNotifyError, and suddenly,

Adding bool or char to StreamMsgNotifyError or a internal padding is
a bug writing protocol (as already discussed) and also being an ABI now
we can't change it so there isn't and there won't be any such field.

> msg.StreamMsgNotifyError::msg is at offset 5 while msg.msg is at offset 8 on
> my machine, and all your messages will “mysteriously” be off by three bytes.
> 

Yes, if you don't know how to write a protocol structure, ignore the ABI
and ignore the notes on the protocol file describing it you have this problem.

> Please fix it.
> 

Fix what ?

> > +
> > +    size_t len_to_read = std::min(len, sizeof(msg) - 1);
> > +
> > +    read_all(&msg, len_to_read);
> > +    msg.msg[len_to_read - sizeof(StreamMsgNotifyError)] = '\0';
> > +
> > +    syslog(LOG_ERR, "Received NotifyError message from the server: %d -
> > %s\n",
> > +        msg.error_code, msg.msg);
> > +
> > +    if (len_to_read < len) {
> > +        throw std::runtime_error("Received NotifyError message size " +
> > std::to_string(len) +
> > +                                 " is too big (bigger than " +
> > std::to_string(sizeof(msg)) + ")");
> > +    }
> > }
> > 
> > static void read_command_from_device(void)

Frediano