Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode()

On 3/18/24 09:11, Roman Smirnov wrote:

On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote:

quoted

On 3/5/24 14:51, Roman Smirnov wrote:

quoted

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, ....

quoted

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 <redacted>
Reviewed-by: Sergey Shtylyov <redacted>
---
   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:

quoted

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