[PATCH] tracing: Avoid undefined shifts for zero histogram map size

[Jaewon Jung <jaewon8868@gmail.com>]

Reject size=0 before calling roundup_pow_of_two().

include/linux/log2.h documents roundup_pow_of_two() as:

  - the result is undefined when n == 0

With CONFIG_UBSAN_SHIFT=y, a size=0 histogram trigger reports:

  UBSAN: shift-out-of-bounds in include/linux/log2.h
  shift exponent 64 is too large for 64-bit type 'long unsigned int'

Tested on x86_64 QEMU: size=0 is rejected without UBSAN warnings,
and the histogram modifier and parser tests pass.

Fixes: 7ef224d1d0e3 ("tracing: Add 'hist' event trigger command")
Cc: stable@vger.kernel.org
Signed-off-by: Jaewon Jung <jaewon8868@gmail.com>
---
 kernel/trace/trace_events_hist.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index f39fa8942673..3de794c7f5b2 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -1430,6 +1430,11 @@ static int parse_map_size(char *str)
 	if (ret)
 		goto out;
 
+	if (!size) {
+		ret = -EINVAL;
+		goto out;
+	}
+
 	map_bits = ilog2(roundup_pow_of_two(size));
 	if (map_bits < TRACING_MAP_BITS_MIN ||
 	    map_bits > TRACING_MAP_BITS_MAX)
-- 
2.43.0