[PATCH] FS: timerfd: Fix unexpected return value of timerfd_read function.

[Arul Jeniston]

'hrtimer_forward_now()' returns zero due to bigger backward time drift.
This causes timerfd_read to return 0. As per man page, read on timerfd
 is not expected to return 0.
This problem is well explained in https://lkml.org/lkml/2019/7/31/442
. This patch fixes this problem.
Signed-off-by: Arul Jeniston <arul.jeniston@xxxxxxxxx>


---
 fs/timerfd.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/timerfd.c b/fs/timerfd.c
index 6a6fc8aa1de7..f5094e070e9a 100644
--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -284,8 +284,16 @@ static ssize_t timerfd_read(struct file *file,
char __user *buf, size_t count,
                                        &ctx->t.alarm, ctx->tintv) - 1;
                                alarm_restart(&ctx->t.alarm);
                        } else {
-                               ticks += hrtimer_forward_now(&ctx->t.tmr,
-                                                            ctx->tintv) - 1;
+                               u64 nooftimeo = hrtimer_forward_now(&ctx->t.tmr,
+                                                                ctx->tintv);
+                               /*
+                                * ticks shouldn't become zero at this point.
+                                * Ignore if hrtimer_forward_now returns 0
+                                * due to larger backward time drift.
+                                */
+                               if (likely(nooftimeo)) {
+                                       ticks += nooftimeo - 1;
+                               }
                                hrtimer_restart(&ctx->t.tmr);
                        }
                }
--
2.11.0