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

[Jürgen Groß <jgross@xxxxxxxx>]

On 05.03.21 15:49, Ian Jackson wrote:
> Ian Jackson writes ("Re: [PATCH for-4.15] tools/xenstored: liveupdate: Increase the 
> maximum number of parameters"):
> > 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.
> Also, this use of max_pars to do job 2 seems very inconsistent.  It is
> applied only to "live-update".
>
> If it is necessary for "live-update", which is it not necessary for
> "check" or whatever ?
live-update is the only command with binary data. The other commands are
checking all parameters to be valid, similar to normal parameter parsing
in a main() function of a user program.


Juergen

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