[PATCH] q800: fix segfault with invalid MacROM

[Laurent Vivier]

"qemu-system-m68k -M q800 -bios /dev/null" crahses with a segfault
in q800_init().
This happens because the code doesn't check that rom_ptr() returned
a non-NULL pointer .

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/756
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
---
 hw/m68k/q800.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/hw/m68k/q800.c b/hw/m68k/q800.c
index e4c7c9b88ad0..6261716c8f7e 100644
--- a/hw/m68k/q800.c
+++ b/hw/m68k/q800.c
@@ -672,10 +672,16 @@ static void q800_init(MachineState *machine)
 
         /* Remove qtest_enabled() check once firmware files are in the tree */
         if (!qtest_enabled()) {
-            if (bios_size < 0 || bios_size > MACROM_SIZE) {
+            if (bios_size == -1) {
                 error_report("could not load MacROM '%s'", bios_name);
                 exit(1);
             }
+            if (bios_size != MACROM_SIZE) {
+                error_report("Invalid size for MacROM '%s': %d bytes,"
+                             " expected %d bytes", bios_name, bios_size,
+                             MACROM_SIZE);
+                exit(1);
+            }
 
             ptr = rom_ptr(MACROM_ADDR, MACROM_SIZE);
             stl_phys(cs->as, 0, ldl_p(ptr));    /* reset initial SP */
-- 
2.33.1