[PATCH v3 0/4] tracing: Fix NULL dereference when copying keys for a field variable

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

A hist trigger with an onmatch() action copies the key list of the
compatible histogram it finds on the matched event.  Reading each key's
name straight out of key_field->field->name faults on any pseudo-field
key.  4/4 renders the key with expr_field_str() instead.  The three before
it make that renderer produce something parse_field() accepts.

Link: https://lore.kernel.org/linux-trace-kernel/20260913203129.941270-1-donggeunyoo.kernel@gmail.com/

Changes since v2:
 - Rebased onto v7.3-rc4.  v2 was built on 2f0c1cf72f46, before the tracing
   fixes merged, and that turns out to matter -- see the next entry.
 - Reinstated the stacktrace patch, now 3/4.  v2 dropped it after measuring
   that common_stacktrace.stacktrace parsed fine.  That was true of
   2f0c1cf72f46 and is no longer true: a5e70ba87ca8 now refuses the
   modifier unless the field is a real one with FILTER_STACKTRACE, so
   without 3/4 a common_stacktrace key renders into a command that is
   rejected.
 - 1/4 is new: hist_field_print() prints the bucket size with %ld, so a
   size above LONG_MAX reads back negative.  Reported by sashiko-bot.
 - 2/4 uses %lu for the same reason.

Patches 1/4 through 3/4 have no effect on their own -- nothing reaches
expr_field_str() with a bucketed or stacktrace field until 4/4 renders keys
with it -- but each is needed before 4/4, and in this order no bisection
point regresses.

The command create_field_var_hist() generates, for each kind of key the
copied histogram can carry:

  WK key                  unpatched   patched
  pid                     keys=pid    keys=pid
  pid.log2                keys=pid    keys=pid.log2
  pid.buckets=10          keys=pid    keys=pid.buckets=10
  common_cpu              oops        keys=common_cpu
  common_comm             oops        keys=common_comm
  common_timestamp        oops        keys=common_timestamp
  common_timestamp.usecs  oops        keys=common_timestamp.usecs
  common_stacktrace       oops        keys=common_stacktrace
  hitcount                oops        keys=hitcount

Unpatched, the .log2 and .buckets rows drop their modifier, so the
generated histogram does not bucket the way the one it mirrors does.  Each
oops is a null-ptr-deref at create_field_var_hist+0x771, taken in its own
boot; the three real-field rows run the same loop to completion without
faulting, so the six are the loop reaching the faulting line rather than a
boot failure.

And the bucket size a key can carry, read back from the trigger:

  .buckets=                     unpatched                patched
  0                             rejected                 rejected
  -5                            rejected                 rejected
  18446744073709551616          rejected                 rejected
  9223372036854775807           9223372036854775807      9223372036854775807
  9223372036854775808           -9223372036854775808     9223372036854775808
  18446744073709551615          -1                       18446744073709551615

x86_64 under QEMU, CONFIG_KASAN=y, 4 CPUs, base 704340f1cd0d.  A compatible
histogram on sched_waking keyed on WK, an onmatch() target on sched_switch
keyed on SK, my_synth($wakeup_lat,prio) forcing a field variable.

Donggeun Yoo (4):
  tracing: Print the bucket size as unsigned
  tracing: Add the bucket size to expr_field_str()
  tracing: Only report the stacktrace modifier on a real field
  tracing: Fix NULL dereference when copying keys for a field variable

 kernel/trace/trace_events_hist.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

-- 
2.53.0