[PATCH 20/82] fpga: dfl: Refactor intentional wrap-around calculation

[Kees Cook]

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded unsigned wrap-around addition test to use
check_add_overflow(), retaining the result for later usage (which removes
the redundant open-coded addition). This paves the way to enabling the
wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Wu Hao <hao.wu@xxxxxxxxx>
Cc: Tom Rix <trix@xxxxxxxxxx>
Cc: Moritz Fischer <mdf@xxxxxxxxxx>
Cc: Xu Yilun <yilun.xu@xxxxxxxxx>
Cc: linux-fpga@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/fpga/dfl.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/fpga/dfl.c b/drivers/fpga/dfl.c
index e6d12fbab653..7d10780e3a98 100644
--- a/drivers/fpga/dfl.c
+++ b/drivers/fpga/dfl.c
@@ -1939,15 +1939,16 @@ static int do_set_irq_trigger(struct dfl_feature *feature, unsigned int idx,
 int dfl_fpga_set_irq_triggers(struct dfl_feature *feature, unsigned int start,
 			      unsigned int count, int32_t *fds)
 {
+	unsigned int sum;
 	unsigned int i;
 	int ret = 0;
 
 	/* overflow */
-	if (unlikely(start + count < start))
+	if (unlikely(check_add_overflow(start, count, &sum)))
 		return -EINVAL;
 
 	/* exceeds nr_irqs */
-	if (start + count > feature->nr_irqs)
+	if (sum > feature->nr_irqs)
 		return -EINVAL;
 
 	for (i = 0; i < count; i++) {
-- 
2.34.1