Re: [Xen-devel] [PATCH for-4.12 RFC] xen/console: Handle NUL character in buffer sent via CONSOLEIO_write

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 2/27/19 10:25 AM, Jan Beulich wrote:
> > > > On 27.02.19 at 00:03, <julien.grall@xxxxxxx> wrote:
> > After upgrading Debian to Buster, I started noticing console mangling
> > when using zsh. This is happenning because output sent by zsh to the
> > console may contain NUL character in the middle of the buffer.
> >
> > Linux is sending the buffer as it is to Xen console via CONSOLEIO_write.
> > However, the implementation in Xen considers NUL character is used to
> > terminate the buffer and therefore will ignore anything after it.
> >
> > The actual documentation of CONSOLEIO_write is pretty limited. From the
> > declaration, the hypercall takes a buffer and size. So this could lead
> > to think the NUL character is allowed in the middle of the buffer.
> >
> > This patch updates the console API to pass the size along the buffer
> > down so we can remove the reliance on buffer terminating by a NUL
> > character.
> We don't need the behavior for internal producers, so I think the change
> touches way too much code. I think all you need to do is make the
> hypercall handler sense null characters, and perhaps simply invoke lower
> level handlers multiple times. Or replace them by something else (e.g. a
> blank).
I have to disagree here. If the OS decides to pass a buffer containing 
NUL character, then we should honor it and send the NUL character to the 
serial. Otherwise you may have a different behavior when running on 
baremetal and on Xen. One case I have in mind is debugging over HVC console.
So we need to modify the console API for handling this purpose. Yes, it 
will allow the internal producers to put NUL character in it. But that's 
not a real issue by itself.
Cheers,

--
Julien Grall

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