[spice-streaming-agent] handle_stream_error: add comment for inheriting struct

Submitted by Frediano Ziglio on March 8, 2018, 4:32 p.m.

Details

Message ID 68230830.10251966.1520526768180.JavaMail.zimbra@redhat.com
State New
Headers show
Series "handle_stream_error: add comment for inheriting struct" ( rev: 2 ) in Spice

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Commit Message

Frediano Ziglio March 8, 2018, 4:32 p.m.
> On Wed, 2018-03-07 at 13:56 +0100, Christophe de Dinechin wrote:
> > > On 7 Mar 2018, at 07:40, Frediano Ziglio <fziglio@redhat.com> wrote:
> > > 
> > > > 
> > > > Introduced in 548577dc8adae1a558
> > > > 
> > > > Signed-off-by: Uri Lublin <uril@redhat.com>
> > > > ---
> > > > src/spice-streaming-agent.cpp | 5 +++++
> > > > 1 file changed, 5 insertions(+)
> > > > 
> > > > diff --git a/src/spice-streaming-agent.cpp
> > > > b/src/spice-streaming-agent.cpp
> > > > index b39782c..e25d47a 100644
> > > > --- a/src/spice-streaming-agent.cpp
> > > > +++ b/src/spice-streaming-agent.cpp
> > > > @@ -145,6 +145,11 @@ static void handle_stream_error(size_t len)
> > > >                                  std::to_string(sizeof(StreamMsgNotifyError))
> > > >                                  + ")");
> > > >     }
> > > > 
> > > > +    // This struct inherits StreamMsgNotifyError. Its memory layout
> > > > is:
> > > > +    // offset 0: StreamMsgNotifyError.error_code (a uint32_t)
> > > > +    // offset 4: StreamMsgNotifyError.msg and also msg.msg (a
> > > > uint8_t[1024]).
> > > > +    // Both StreamMsgNotifyError.msg and msg.msg point to the same
> > > > +    // memory location (practically local msg overrides inherited
> > > > msg).
> > > >     struct : StreamMsgNotifyError {
> > > >         uint8_t msg[1024];
> > > >     } msg;
> > > 
> > > When I sent my suggestion I didn't wanted to clash with inherited field,
> > > just didn't invented a new name.
> > > Maybe would be easier and more clear to rename to msg_buffer or similar
> > > to avoid the confusion?
> > 
> > Would fix the issue I pointed out. But as I mentioned there, gcc emkits a
> > warning that we access the array out of bounds (msg has zero length). You
> > can silence that using a temporary char * pointer.
> 
> Are you talking about the same thing as Frediano? IIUC:
> - Frediano says rename msg.msg to msg.msg_buffer and use that
> 
> - You talk about out of bounds warning for StreamMsgNotifyError::msg
> and casting it to a pointer to avoid that (so that we wouldn't need to
> inherit the struct at all)
> 
> To sum it up, what changes are requested?
> 
> - Fix msg.msg in a TBD way.
> 
> - If the message is too long, read it out instead of throwing an
> exception to prevent a DoS. (We can still get DoSed though if the
> server sends a neverending message or whatever?)
> 
> - If the inherited struct stays, name it? (I have to say I liked the
> unnamed struct in local scope as it limited it's usage exactly to the
> one 'msg' variable...)
> 
> Cheers,
> Lukas
> 

I think something like



But the terminating like is relatively hackish.
Also Christophe asked for a name for the "msg" structure but didn't
come with something sensible.

Frediano

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--- a/src/spice-streaming-agent.cpp
+++ b/src/spice-streaming-agent.cpp
@@ -159,13 +159,13 @@  static void handle_stream_error(size_t len)
     }

     struct : StreamMsgNotifyError {
-        uint8_t msg[1024];
+        uint8_t msg_buffer[1024];
     } msg;

     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';
+    ((uint8_t *) &msg)[len_to_read] = '\0';

     syslog(LOG_ERR, "Received NotifyError message from the server: %d - %s\n",
         msg.error_code, msg.msg);