[PATCH] tracing: Fix the shift out of bounds in the log2 histogram modifier

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

The .log2 key modifier files a value under its base-2 order:

  # echo 'hist:keys=bytes_req.log2' > events/kmem/kmalloc/trigger

hist_field_log2() computes that as ilog2(roundup_pow_of_two(val)). The
out-of-line form of roundup_pow_of_two() is

	return 1UL << fls_long(n - 1);

which shifts by 64 on a 64-bit kernel both when n is 0, where n - 1
wraps to ULONG_MAX, and when n is above 2^63. Both are ordinary keys,
because the field fetch hands every value over as a u64: a syscall
argument of 0 reaches the first, and one above 2^63 reaches the second.

Keying sys_enter_lseek on offset.log2 and passing 0, 1, 2, 3, 1000,
2^63+1 and U64_MAX gives

  { offset: ~ 2^1  } hitcount:          1
  { offset: ~ 2^10 } hitcount:          1
  { offset: ~ 2^2  } hitcount:          1
  { offset: ~ 2^0  } hitcount:          4

Keys above 2^63 land in the lowest bucket. With CONFIG_UBSAN_SHIFT=y
the shift is also reported:

  UBSAN: shift-out-of-bounds in include/linux/log2.h:57:13
  shift exponent 64 is too large for 64-bit type 'long unsigned int'
   __ubsan_handle_shift_out_of_bounds.cold+0xdd/0x1cb
   hist_fn_call.cold+0x8b/0xd9
   event_hist_trigger+0x1cc/0x790
   ftrace_syscall_enter+0x197/0x360
   do_syscall_64+0x402/0x4b0

Use order_base_2(), which is ilog2(n - 1) + 1 for n > 1 and 0 below
that. It returns what the old expression returned on every input the
old one was defined for. Where it was not, 0 stays in bucket 0, which
is where x86_64 had been putting it, and the keys above 2^63 move into
bucket 64.

Cc: stable@vger.kernel.org
Fixes: 4b94f5b7b4a5 ("tracing: Add hist trigger 'log2' modifier")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Assisted-by: Claude:claude-fable-5
---
QEMU x86_64, v7.3-rc3-78-g9b87fdc9af2f, CONFIG_UBSAN_SHIFT=y, one kernel per
arm and one initramfs. The histogram above is the unfixed arm; the same run
with this patch gives

  { offset: ~ 2^2  } hitcount:          1
  { offset: ~ 2^10 } hitcount:          1
  { offset: ~ 2^1  } hitcount:          1
  { offset: ~ 2^64 } hitcount:          2
  { offset: ~ 2^0  } hitcount:          2

and no UBSAN report.

 kernel/trace/trace_events_hist.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 8af97fd4ee2d..1d7169527dcf 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -289,7 +289,7 @@ static u64 hist_field_log2(struct hist_field *hist_field,
 
 	u64 val = hist_fn_call(operand, elt, buffer, rbe, event);
 
-	return (u64) ilog2(roundup_pow_of_two(val));
+	return order_base_2(val);
 }
 
 static u64 hist_field_bucket(struct hist_field *hist_field,
-- 
2.53.0