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

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

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>
---
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