[PATCH v2 1/2] timekeeping: Read getboottime64() offsets under the sequence counter

[Hungyu Lin <dennylin0707@gmail.com>]

getboottime64() computes the wall-clock boot timestamp from offs_real
minus offs_boot without checking tk_core.seq. A concurrent timekeeping
update can leave the reader with an inconsistent pair of offsets or,
on 32-bit systems, a torn 64-bit offset, producing an incorrect boot
timestamp.

Read both offsets under tk_core.seq and retry if an update overlaps
the sample. Convert the resulting local value to a timespec64 afterward.

Fixes: 7c3f1a573237 ("Introduce boot based time")
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
New prerequisite in v2, addressing Andrew Morton's review.

 kernel/time/timekeeping.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index dcf47f1ffdcf..7d7a7372f3c8 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2700,7 +2700,13 @@ void update_wall_time(void)
 void getboottime64(struct timespec64 *ts)
 {
 	struct timekeeper *tk = &tk_core.timekeeper;
-	ktime_t t = ktime_sub(tk->offs_real, tk->offs_boot);
+	unsigned int seq;
+	ktime_t t;
+
+	do {
+		seq = read_seqcount_begin(&tk_core.seq);
+		t = ktime_sub(tk->offs_real, tk->offs_boot);
+	} while (read_seqcount_retry(&tk_core.seq, seq));
 
 	*ts = ktime_to_timespec64(t);
 }
-- 
2.43.0