From 289e2a775de434ccab2d107076bc78362ce288bd Mon Sep 17 00:00:00 2001 From: Donggeun Yoo Date: Mon, 14 Sep 2026 22:23:46 +0900 Subject: [PATCH] tracing: Clamp the printed end of a histogram bucket at U64_MAX 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. Link: https://patch.msgid.link/20260914132346.2234731-1-donggeunyoo.kernel@gmail.com Fixes: de9a48a360b7 ("tracing: Add linear buckets to histogram logic") Signed-off-by: Donggeun Yoo Assisted-by: Claude:claude-fable-5 Signed-off-by: Steven Rostedt --- 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 972804709b14..c150e1130a5f 100644 --- a/kernel/trace/trace_events_hist.c +++ b/kernel/trace/trace_events_hist.c @@ -5561,9 +5561,14 @@ 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));