Re: [PATCH for-4.15] tools/xenstored: liveupdate: Increase the maximum number of parameters

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Jürgen Groß writes ("Re: [PATCH for-4.15] tools/xenstored: liveupdate: Increase 
the maximum number of parameters"):
> On 05.03.21 15:37, Ian Jackson wrote:
> > Jürgen Groß writes ("Re: [PATCH for-4.15] tools/xenstored: liveupdate: 
> > Increase the maximum number of parameters"):
> >> This is the max number of 0 delimited string parameters. Especially the
> >> stubdom case needs a binary blob (with length, of course) as parameter,
> >> and the number of 0 bytes in this data is just limited by the allowed
> >> payload length.
> >>
> >> See the comment in line 111 of xenstored_control.c.
> > 
> > AFAICT this "live-update" command is variadic.  So why is this
> > parameter set here it all then ?
> 
> In order to avoid allocating an array for 4000 arguments when there
> are only 5 which need to be treated as strings.

So this parameter is doing two jobs: 1. enabling non-variadic commands
to take binary input; 2. preventing variadic commands from allocating
unbounded memory.

The problem with this is that in case 1 exceeding the value is normal
and the final argument is binary data; whereas in case 2 glomming
together the arguments together with zeroes is wrong and potentially
hazrdous.

I suggest we solve problem 2 by imposing a higher fixed (for all
commands, variadic or not) limit (20 or something) which causes errors
when exceeded, rather than silent argument misinterpretation.

Ian.