[PATCH] hw/misc/npcm_clk: Don't divide by zero when calculating frequency

[PATCH] hw/misc/npcm_clk: Don't divide by zero when calculating frequency

[Peter Maydell]

If the guest misprograms the PLL registers to request a zero
divisor, we currently fall over with a division by zero:

../../hw/misc/npcm_clk.c:221:14: runtime error: division by zero
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../hw/misc/npcm_clk.c:221:14

Thread 1 "qemu-system-aar" received signal SIGFPE, Arithmetic exception.
0x00005555584d8f6d in npcm7xx_clk_update_pll (opaque=0x7fffed159a20) at ../../hw/misc/npcm_clk.c:221
221             freq /= PLLCON_INDV(con) * PLLCON_OTDV1(con) * PLLCON_OTDV2(con);

Avoid this by treating this invalid setting like a stopped clock
(setting freq to 0).

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/549
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 hw/misc/npcm_clk.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/hw/misc/npcm_clk.c b/hw/misc/npcm_clk.c
index c48d40b4468..e202a8a2998 100644
--- a/hw/misc/npcm_clk.c
+++ b/hw/misc/npcm_clk.c
@@ -212,13 +212,14 @@ static void npcm7xx_clk_update_pll(void *opaque)
 {
     NPCM7xxClockPLLState *s = opaque;
     uint32_t con = s->clk->regs[s->reg];
-    uint64_t freq;
+    uint64_t freq, freq_div;
 
     /* The PLL is grounded if it is not locked yet. */
     if (con & PLLCON_LOKI) {
         freq = clock_get_hz(s->clock_in);
         freq *= PLLCON_FBDV(con);
-        freq /= PLLCON_INDV(con) * PLLCON_OTDV1(con) * PLLCON_OTDV2(con);
+        freq_div = PLLCON_INDV(con) * PLLCON_OTDV1(con) * PLLCON_OTDV2(con);
+        freq = freq_div ? freq / freq_div : 0;
     } else {
         freq = 0;
     }
-- 
2.43.0

Re: [PATCH] hw/misc/npcm_clk: Don't divide by zero when calculating frequency

[Philippe Mathieu-Daudé]

On 7/11/25 16:01, Peter Maydell wrote:
> If the guest misprograms the PLL registers to request a zero
> divisor, we currently fall over with a division by zero:
> 
> ../../hw/misc/npcm_clk.c:221:14: runtime error: division by zero
> SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../hw/misc/npcm_clk.c:221:14
> 
> Thread 1 "qemu-system-aar" received signal SIGFPE, Arithmetic exception.
> 0x00005555584d8f6d in npcm7xx_clk_update_pll (opaque=0x7fffed159a20) at ../../hw/misc/npcm_clk.c:221
> 221             freq /= PLLCON_INDV(con) * PLLCON_OTDV1(con) * PLLCON_OTDV2(con);
> 
> Avoid this by treating this invalid setting like a stopped clock
> (setting freq to 0).
> 
> Cc: qemu-stable@nongnu.org
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/549
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>   hw/misc/npcm_clk.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>