On Mon, Aug 30, 2021 at 11:25:51PM +0900, Tetsuo Handa wrote:

On 2021/08/30 22:47, Dan Carpenter wrote:

quoted

On Mon, Aug 30, 2021 at 10:37:31PM +0900, Tetsuo Handa wrote:

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On 2021/08/30 22:00, Dan Carpenter wrote:

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diff --git a/drivers/video/fbdev/vga16fb.c b/drivers/video/fbdev/vga16fb.c
index e2757ff1c23d..e483a3f5fd47 100644
--- a/drivers/video/fbdev/vga16fb.c
+++ b/drivers/video/fbdev/vga16fb.c

@@ -403,7 +403,7 @@ static int vga16fb_check_var(struct fb_var_screeninfo *var,

        if (yres > vyres)
                vyres = yres;
-       if (vxres * vyres > maxmem) {
+       if ((u64) vxres * vyres > (u64) maxmem) {

Mindlessly changing the sizes is not the solution.
Please use e.g. the array_size() helper from <linux/overflow.h>
instead.

On a 64bit system the array_size() macro is going to do the exact same
casts?  But I do think this code would be easier to understand if the
integer overflow check were pull out separately and done first:

	if (array_size(vxres, vyres) >= UINT_MAX)
		return -EINVAL;

This is wrong. array_size() returns ULONG_MAX on 64bits upon overflow and
returns UINT_MAX on 32bits upon overflow. However, UINT_MAX is a valid
value without overflow (e.g. vxres == UINT_MAX / 15 && vyres == 15).

Huh...  I just assumed we didn't allow resolutions that high.

Of course, we don't allow resolutions that high. ;-)

Since I don't know possible max resolutions, I chose UINT_MAX + 1 as a common
limit for returning -EINVAL. Unless overflow happens, vga16fb_check_var() will
return -ENOMEM on such high resolutions.

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Comparing like "> (u64) UINT_MAX" is to detect only overflow.

Of course, that doesn't work on 32 bit systems.  Also the cast isn't
required because of type promotion.

Indeed, "> UINT_MAX" seems to work on both 32bits and 64bits.

Sorry, for the confusion.  I'm talking about array_size() which is
size_t.  Your approach using unsigned long long works.

regards,
dan carpenter