poll/select: avoid arithmetic overflow in __estimate_accuracy()

__estimate_accuracy() was prone to integer overflow, for example if *tv == {2147, 483648000} on a 32 bit computer (or even for delays as small as {429, 500000000} if the task is niced).

Because the result was already forced between 0 and 100ms, the effect of the overflow was not too problematic, but the use of the hrtimer range feature was not optimal in overflow cases.

This patch ensures that there can not be an integer overflow in this function.

Signed-off-by Guillaume Knispel <[email protected]>

---
fs/select.c | 14 ++++++++++----

1 files changed, 10 insertions(+), 4 deletions(-)

fs/select.c +2 -0

index 8084834..a201fc3 100644
@@ -41,22 +41,28 @@
 * better solutions..
 */
 
+#define MAX_SLACK	(100 * NSEC_PER_MSEC)
+
 static long __estimate_accuracy(struct timespec *tv)
 {
 	long slack;
 	int divfactor = 1000;

fs/select.c +2 -5

 	slack = tv->tv_nsec / divfactor;
 	slack += tv->tv_sec * (NSEC_PER_SEC/divfactor);
 
-	if (slack > 100 * NSEC_PER_MSEC)
-		slack =  100 * NSEC_PER_MSEC;
+	if (slack > MAX_SLACK)
+		return MAX_SLACK;
 
-	if (slack < 0)
-		slack = 0;
 	return slack;
 }
 
--

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Quoted 30 lines

__estimate_accuracy() was prone to integer overflow, for example if *tv == {2147, 483648000} on a 32 bit computer (or even for delays as small as {429, 500000000} if the task is niced).

Because the result was already forced between 0 and 100ms, the effect of the overflow was not too problematic, but the use of the hrtimer range feature was not optimal in overflow cases.

This patch ensures that there can not be an integer overflow in this function.

Signed-off-by Guillaume Knispel <[email protected]>

---
fs/select.c | 14 ++++++++++----

1 files changed, 10 insertions(+), 4 deletions(-)

fs/select.c +2 -0

index 8084834..a201fc3 100644
@@ -41,22 +41,28 @@
 * better solutions..
 */
 
+#define MAX_SLACK	(100 * NSEC_PER_MSEC)
+
 static long __estimate_accuracy(struct timespec *tv)
 {
 	long slack;
 	int divfactor = 1000;

fs/select.c line 59

Quoted 10 lines

fs/select.c +6 -0

 
+	if (tv->tv_sec < 0)
+		return 0;
+
 	if (task_nice(current) > 0)
 		divfactor = divfactor / 5;
 
+	if (tv->tv_sec > MAX_SLACK / (NSEC_PER_SEC/divfactor))
+		return MAX_SLACK;
+

Yes? Isn't MAX_SLACK also too big for 'long' on 32bit machine?

Try:

% printf "%d\n" "100*1000000000 > 0x7fffffff"
1

Am I missing something here?