[PATCH] tracing: Clamp the printed end of a histogram bucket at U64_MAX

[Donggeun Yoo <donggeunyoo.kernel@gmail.com>]

A key with the ".buckets=N" modifier is stored as the start of its bucket,
and hist_trigger_print_key() prints the range it covers as start to
start + N - 1. That addition overflows once the start is within N of
U64_MAX:

  # echo 'hist:keys=offset.buckets=10' >> \
      events/syscalls/sys_enter_lseek/trigger
  # ./seeker 5 -1                      # lseek(0, 5) and lseek(0, -1)
  # cat events/syscalls/sys_enter_lseek/hist
  ...
  { offset: ~ 18446744073709551610-3 } hitcount:          1
  { offset: ~ 0-9 } hitcount:          1
  ...

18446744073709551610 + 10 - 1 does not fit in 64 bits, so the range ends
below where it starts.

The bucket does end at U64_MAX: it holds every value hist_field_bucket()
maps onto its start, and there is no value above U64_MAX. Clamp the printed
end there. Printed honestly the group is shorter than the size asked for,
which histogram.rst does not mention, so say it there.

Fixes: de9a48a360b7 ("tracing: Add linear buckets to histogram logic")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Assisted-by: Claude:claude-fable-5
---
Tested under QEMU/KVM on 704340f1cd0d, x86_64, unfixed arm first.  Keys come
from lseek(2) offsets.

Nine cases over buckets=1, 10 and 16.  One printed range differs:

  buckets=10, key 18446744073709551610
    before  ~ 18446744073709551610-3
    after   ~ 18446744073709551610-18446744073709551615

The other eight are identical on both arms.

ftracetest test.d/trigger: 42 pass, 2 xfail, 1 fail, per-test verdicts
identical on both arms.  The failure is trigger-synthetic-event-dynstring.tc,
which needs a ping(8) that is not a busybox applet.  No case in the suite
uses .buckets.

 Documentation/trace/histogram.rst | 3 +++
 kernel/trace/trace_events_hist.c  | 7 ++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/Documentation/trace/histogram.rst b/Documentation/trace/histogram.rst
index 340bcb5099e7..97b67f05f5ea 100644
--- a/Documentation/trace/histogram.rst
+++ b/Documentation/trace/histogram.rst
@@ -1943,6 +1943,9 @@ the ".buckets" modifier and specify a size (in this case groups of 10)::
       Entries: 16
       Dropped: 0
 
+Keys are 64-bit, so the group holding the largest values is truncated where
+that range ends and can be smaller than the size asked for.
+
 To save stacktraces, create a synthetic event with a field of type "unsigned long[]"
 or even just "long[]". For example, to see how long a task is blocked in an
 uninterruptible state::
diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 8af97fd4ee2d..86df1a29d8d5 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -5560,9 +5560,14 @@ static void hist_trigger_print_key(struct seq_file *m,
 				   *(u64 *)(key + key_field->offset));
 		} else if (key_field->flags & HIST_FIELD_FL_BUCKET) {
 			unsigned long buckets = key_field->buckets;
+			u64 end;
+
 			uval = *(u64 *)(key + key_field->offset);
+			end = uval + buckets - 1;
+			if (end < uval)
+				end = U64_MAX;
 			seq_printf(m, "%s: ~ %llu-%llu", field_name,
-				   uval, uval + buckets -1);
+				   uval, end);
 		} else if (key_field->flags & HIST_FIELD_FL_STRING) {
 			seq_printf(m, "%s: %-50s", field_name,
 				   (char *)(key + key_field->offset));
-- 
2.53.0