mm/page-writeback.c: avoid potential division by zero

The variables 'min', 'max' and 'bw' are unsigned long and do_div truncates them to 32 bits, which means it can test non-zero and be truncated to zero for division. Fix this issue by using div64_ul() instead.

For the two variables 'numerator' and 'denominator', though they are declared as long, they should actually be unsigned long (according to the implementation of the fprop_fraction_percpu() function).

---
 mm/page-writeback.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

index 50055d2..2caf780 100644
@@ -201,11 +201,11 @@ static void wb_min_max_ratio(struct bdi_writeback *wb,
 	if (this_bw < tot_bw) {
 		if (min) {
 			min *= this_bw;
-			do_div(min, tot_bw);
+			min = div64_ul(min, tot_bw);
 		}
 		if (max < 100) {
 			max *= this_bw;
-			do_div(max, tot_bw);
+			max = div64_ul(max, tot_bw);
 		}
 	}
 

@@ -766,7 +766,7 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 	struct wb_domain *dom = dtc_dom(dtc);
 	unsigned long thresh = dtc->thresh;
 	u64 wb_thresh;
-	long numerator, denominator;
+	unsigned long numerator, denominator;
 	unsigned long wb_min_ratio, wb_max_ratio;
 
 	/*

@@ -777,7 +777,7 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 
 	wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;
 	wb_thresh *= numerator;
-	do_div(wb_thresh, denominator);
+	wb_thresh = div64_ul(wb_thresh, denominator);
 
 	wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);
 

@@ -1102,7 +1102,7 @@ static void wb_update_write_bandwidth(struct bdi_writeback *wb,
 	bw = written - min(written, wb->written_stamp);
 	bw *= HZ;
 	if (unlikely(elapsed > period)) {
-		do_div(bw, elapsed);
+		bw = div64_ul(bw, elapsed);
 		avg = bw;
 		goto out;
 	}

1.8.3.1

How did you find out the issue? If it is caught by compilers, can you paste the original warnings? Also, can you figure out which commit introduced the issue in the first place, so it could be backported to stable if needed?

Thanks for your comments.
There are no compilation warnings here.

We found this issue by following these steps:
We were first inspired by commit b0ab99e7736a ("sched: Fix possible divide by zero in avg_atom () calculation"), combined with our recently analyzed mm code, we found this suspicious place.

And we also disassembled and confirmed it:

201 if (min) {
202 min *= this_bw;
203 do_div(min, tot_bw);
204 }

/usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 201

0xffffffff811c37da <__wb_calc_thresh+234>:      xor    %r10d,%r10d
0xffffffff811c37dd <__wb_calc_thresh+237>:      test   %rax,%rax
0xffffffff811c37e0 <__wb_calc_thresh+240>:      je 0xffffffff811c3800 <__wb_calc_thresh+272>

/usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 202
0xffffffff811c37e2 <__wb_calc_thresh+242>: imul %r8,%rax
/usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 203
0xffffffff811c37e6 <__wb_calc_thresh+246>: mov %r9d,%r10d
---> truncates it to 32 bits here

0xffffffff811c37e9 <__wb_calc_thresh+249>:      xor    %edx,%edx
0xffffffff811c37eb <__wb_calc_thresh+251>:      div    %r10
0xffffffff811c37ee <__wb_calc_thresh+254>:      imul   %rbx,%rax
0xffffffff811c37f2 <__wb_calc_thresh+258>:      shr    $0x2,%rax
0xffffffff811c37f6 <__wb_calc_thresh+262>:      mul    %rcx
0xffffffff811c37f9 <__wb_calc_thresh+265>:      shr    $0x2,%rdx
0xffffffff811c37fd <__wb_calc_thresh+269>:      mov    %rdx,%r10

This issue was introduced by commit 693108a8a667 (“writeback: make bdi->min/max_ratio handling cgroup writeback aware”).

Finally, we will summarize the above cases and plan to write a general coccinelle rule to check for similar problems.

Okay, this needs a Fixes tag then, and Cc the relevant people, i.e., Tejun and Jens at least.