Messages in this thread | | | Date | Mon, 18 Mar 2024 20:15:55 +0100 | Subject | Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode() | From | Helge Deller <> |
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On 3/18/24 09:11, Roman Smirnov wrote: > On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote: >> On 3/5/24 14:51, Roman Smirnov wrote: >>> The expression htotal * vtotal can have a zero value on >>> overflow. >> >> I'm not sure if those always results in zero in kernel on overflow. >> Might be architecture-depended too, but let's assume it >> can become zero, .... >> >>> It is necessary to prevent division by zero like in >>> fb_var_to_videomode(). >>> >>> Found by Linux Verification Center (linuxtesting.org) with Svace. >>> >>> Signed-off-by: Roman Smirnov <r.smirnov@omp.ru> >>> Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru> >>> --- >>> V1 -> V2: Replaced the code of the first version with a check. >>> >>> drivers/video/fbdev/core/fbmon.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c >>> index 79e5bfbdd34c..b137590386da 100644 >>> --- a/drivers/video/fbdev/core/fbmon.c >>> +++ b/drivers/video/fbdev/core/fbmon.c >>> @@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode *vm, >>> vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch + >>> vm->vsync_len; >>> /* prevent division by zero */ >>> - if (htotal && vtotal) { >>> + if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) { >> >> why don't you then simply check for >> if .. ((htotal * vtotal) == 0) ... >> instead? >> >> Helge > > Thomas Zimmermann from the previous discussion said: > > On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote: >> Maybe use >> >> if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) >> >> for the test. That rules out overflowing multiplication and sets >> refresh to 0 in such cases. > > This prevents overflow, which is also a problematic case.
I don't like adding another division here and I doubt we have a problem with possible overflow. So, I suggest to keep it simple, something like: ... total = htotal * vtotal; if (total) fbmode->refresh = vm->pixelclock / total; else... Helge
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