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 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