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

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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: de9a48a360 ("tracing: Add linear buckets to histogram logic")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Assisted-by: Claude:claude-fable-5
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Donggeun Yoo 2026-09-14 22:23:46 +09:00 committed by Steven Rostedt
parent 91c14d6c13
commit 289e2a775d
2 changed files with 9 additions and 1 deletions
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3
Documentation/trace/histogram.rst
View File

@ -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::

7
kernel/trace/trace_events_hist.c
View File

@ -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));