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Re: [Libguestfs] [xml] Why does libxml2 limit port numbers to 999, 999, 999?



Hi Richard,

On Sat, Oct 17, 2020 at 07:03:22PM +0100, Richard W.M. Jones wrote:
> On Sat, Oct 17, 2020 at 06:32:18PM +0200, Nick Wellnhofer wrote:
> > On Oct 17, 2020, at 12:24 , Richard W.M. Jones via xml <xml gnome org> wrote:
> > > It seems like libxml2 chose to do this for convenience rather than
> > > correctness.
> > 
> > Yes, this is an arbitrary limit introduced to avoid integer overflow.
> >  
> > > I think it should accept port numbers at least up to
> > > signed int (the type used to store port numbers), and give an error if
> > > the port number overflows.
> > 
> > This is fixed now: https://gitlab.gnome.org/GNOME/libxml2/-/commit/b46016b8705b041c0678dd45e445dc73674b75d0
> 
> Oh that's great thanks.  Can confirm it works for me (up to INT_MAX).
> 
> > > Also could the uri->port field be changed to unsigned int without
> > > breaking ABI?
> >
> > It’s a public struct member, so strictly speaking, no. But the risk
> > to break stuff seems low.
> 
> This would allow us to go to 2^32-1 which is the full range of
> port numbers for AF_VSOCK.
> 
> ** Stefano ** Do you think this is worth it for the vsock protocol?
> I'm not sure how often huge port numbers are used - I only hit this
> bug because I was choosing random port numbers in a test case.

In the 99% of cases the remote port is usually chosen by the user and very
often they are similar to TCP/UDP ones, so I guess it's not a problem.

The problem can happen if the remote service is listen on a random port
chosen by the kernel (if the user do bind() with VMADDR_PORT_ANY).
This port is randomly generated in interval [1024, UINT32_MAX].

>From the vsock code seems that in this case 2^32 is the maximum random port
generated, instead in case the port is chosen by the user, the maximum
is 2^32 - 1.

I'll check if the interval for random port is right.

Stefano


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