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Re: [Qemu-devel] [PATCH] spapr-vty: Fix bad assert() statement
From: |
David Gibson |
Subject: |
Re: [Qemu-devel] [PATCH] spapr-vty: Fix bad assert() statement |
Date: |
Fri, 11 Nov 2016 10:01:06 +1100 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
On Thu, Nov 10, 2016 at 10:06:37AM +0100, Thomas Huth wrote:
> When using the serial console in the GTK interface of QEMU (and
> QEMU has been compiled with CONFIG_VTE), it is possible to trigger
> the assert() statement in vty_receive() in spapr_vty.c by pasting
> a chunk of text with length > 16 into the QEMU window.
> Most of the other serial backends seem to simply drop characters
> that they can not handle, so I think we should also do the same in
> spapr-vty to fix this issue. And since it is quite ugly when pasted
> text is chopped after 16 bytes, we also increase the size of the
> input buffer here so that we can at least handle a couple of text
> lines.
Adding Paolo to CC, as qemu-char maintainer.
So, vastly increasing the buffer like this doesn't seem right - the
plain 16550 serial driver doesn't maintain a buffer bigger than its
small internal FIFO (32 characters IIRC) - or a single byte if the
FIFO is disabled.
I thought the other side of the char driver was supposed to call
can_receive() first and not deliver more bytes than we can handle -
hence the assert.
That said, I think vty_can_receive() has a bug - looking at other
drivers, I think it's supposed to return the amount of buffer space
available, but we're just returning 0 or 1. Although AFAICT that
should still work, just inefficiently.
If you use a serial port with the same GTK interface, do you get the
same problem with pastes of >16 characters being truncated?
>
> Buglink: https://bugs.launchpad.net/qemu/+bug/1639322
> Signed-off-by: Thomas Huth <address@hidden>
> ---
> hw/char/spapr_vty.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/hw/char/spapr_vty.c b/hw/char/spapr_vty.c
> index 31822fe..bee6c34 100644
> --- a/hw/char/spapr_vty.c
> +++ b/hw/char/spapr_vty.c
> @@ -1,4 +1,5 @@
> #include "qemu/osdep.h"
> +#include "qemu/error-report.h"
> #include "qapi/error.h"
> #include "qemu-common.h"
> #include "cpu.h"
> @@ -7,7 +8,7 @@
> #include "hw/ppc/spapr.h"
> #include "hw/ppc/spapr_vio.h"
>
> -#define VTERM_BUFSIZE 16
> +#define VTERM_BUFSIZE 2048
>
> typedef struct VIOsPAPRVTYDevice {
> VIOsPAPRDevice sdev;
> @@ -37,7 +38,15 @@ static void vty_receive(void *opaque, const uint8_t *buf,
> int size)
> qemu_irq_pulse(spapr_vio_qirq(&dev->sdev));
> }
> for (i = 0; i < size; i++) {
> - assert((dev->in - dev->out) < VTERM_BUFSIZE);
> + if (dev->in - dev->out >= VTERM_BUFSIZE) {
> + static bool reported;
> + if (!reported) {
> + error_report("VTY input buffer exhausted - characters
> dropped."
> + " (input size = %i)", size);
> + reported = true;
> + }
> + break;
> + }
> dev->buf[dev->in++ % VTERM_BUFSIZE] = buf[i];
> }
> }
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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