Re: [RFC PATCH-for-5.1 v2] hw/ide: Avoid #DIV/0! FPU exception by setting CD-ROM sector count

[Darren Kenny <darren.kenny@oracle.com>]

On Friday, 2020-07-17 at 15:38:47 +02, Philippe Mathieu-Daudé wrote:
> libFuzzer found an undefined behavior (#DIV/0!) in ide_set_sector()
> when using a CD-ROM (reproducer available on the BugLink):
>
>   UndefinedBehaviorSanitizer:DEADLYSIGNAL
>   ==12163==ERROR: UndefinedBehaviorSanitizer: FPE on unknown address 0x5616279cffdc (pc 0x5616279cffdc bp 0x7ffcdaabae90 sp 0x7ffcdaabae30 T12163)
>
> Fix by initializing the CD-ROM number of sectors in ide_dev_initfn().
>
> Reported-by: Alexander Bulekov <alxndr@bu.edu>
> Fixes: b2df431407 ("ide scsi: Mess with geometry only for hard disk devices")
> BugLink: https://bugs.launchpad.net/qemu/+bug/1887309
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

The changes LGTM, presume the 'shrug' is OK to leave in ;)

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>

Thanks,

Darren.

> ---
> Since v1:
> - Allow zero-sized drive images (not sure why we need them)
>   but display a friendly message that this is unsupported
>
> Unrelated but interesting:
> http://www.physics.udel.edu/~watson/scen103/cdsoln.html
> ---
>  hw/ide/qdev.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/hw/ide/qdev.c b/hw/ide/qdev.c
> index 27ff1f7f66..005d73bdb9 100644
> --- a/hw/ide/qdev.c
> +++ b/hw/ide/qdev.c
> @@ -201,6 +201,15 @@ static void ide_dev_initfn(IDEDevice *dev, IDEDriveKind kind, Error **errp)
>                                errp)) {
>              return;
>          }
> +    } else {
> +        uint64_t nb_sectors;
> +
> +        blk_get_geometry(dev->conf.blk, &nb_sectors);
> +        if (!nb_sectors) {
> +            warn_report("Drive image of size zero is unsupported for 'ide-cd', "
> +                        "use at your own risk ¯\\_(ツ)_/¯");
> +        }
> +        dev->conf.secs = nb_sectors;
>      }
>      if (!blkconf_apply_backend_options(&dev->conf, kind == IDE_CD,
>                                         kind != IDE_CD, errp)) {
> -- 
> 2.21.3