Re: [PATCH 2/2] net/tap: fix error reporting when bridge helper forgets to send an FD

[Daniel P . Berrangé]

On Wed, May 05, 2021 at 02:32:04PM +0200, Philippe Mathieu-Daudé wrote:
> On 5/5/21 2:12 PM, Daniel P. Berrangé wrote:
> > The recv_fd() method returns -1 on error, 0 on end of file, or an FD
> > number on success.
> > 
> > Technically 0 is also a valid FD number, so the return value is
> > ambiguous.
> > 
> > The caller doesn't even consider the possibility of 0 meaning end
> > of file, so just blindly assume it is a valid FD.
> > 
> > IOW if the bridge helper exits with 0 status code and forgets to
> > send an FD, QEMU will accidentally try to use STDIN as a TAP FD.
> > 
> > Fortunately we get an error shortly thereafter
> > 
> >  $ qemu-system-x86_64 -netdev bridge,br=br99,helper=/bin/true,id=ns0
> >  qemu-system-x86_64: -netdev bridge,br=br99,helper=/bin/true,id=ns0:
> >    Unable to query TUNGETIFF on FD 0: Inappropriate ioctl for device
> > 
> > It is better if we correctly diagnose this broken bridge helper
> > though. To do this we need to return the FD in an output parameter
> > to remove the ambiguity and then fix the caller to check for the
> > end of file condition. With this done we now get
> > 
> >   $ qemu-system-x86_64 -netdev bridge,br=br99,helper=/bin/true,id=ns0
> >   qemu-system-x86_64: -netdev bridge,br=br99,helper=/bin/true,id=ns0:
> >     bridge helper did not send a file descriptor
> > 
> > Fixes: https://gitlab.com/qemu-project/qemu/-/issues/166
> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> > ---
> >  net/tap.c | 32 ++++++++++++++++++++++----------
> >  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> > +        /*
> > +         * ret == 0 means EOF, and if status == 0 then helper
> > +         * exited cleanly but forgot to send us an FD. Opps...
> 
> Opps or Oops?

Oops, it should be Oops :-)

> 
> Otherwise:
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> 

Regards,
Daniel
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