On 2021/08/30 22:47, Dan Carpenter wrote:
On Mon, Aug 30, 2021 at 10:37:31PM +0900, Tetsuo Handa wrote:
On 2021/08/30 22:00, Dan Carpenter wrote:
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.
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.
---------- #include <stdio.h> #include <limits.h>
int main(int argc, char *argv[]) { unsigned int w = 0x600; unsigned int h = 0x10000000; if ((unsigned long long) w * h > UINT_MAX) printf("Overflowed\n"); else printf("No overflow\n"); return 0; } ----------