Re: [PATCH bpf-next] tracing: Make per-template BTF id lists file-local

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

On 7/30/26 2:03 PM, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] Using `BTF_ID_LIST()` instead of `BTF_ID_LIST_GLOBAL()` for tracepoint classes introduces significant BSS memory bloat when `CONFIG_DEBUG_INFO_BTF` is disabled.
> --
> 
> --- Patch [1]: [PATCH bpf-next] tracing: Make per-template BTF id lists file-local ---
> commit 49d7faf72ef8695ae45768a193f5a09e2cfe872d
> Author: Mykyta Yatsenko <yatsenko@meta.com>
> 
> tracing: Make per-template BTF id lists file-local
> 
> This commit converts trace event BTF id lists from global to file-local to
> avoid symbol clashes when multiple translation units instantiate the same
> class. It also explicitly provides local BTF id lists for handcrafted syscall
> events.
> 
> Link: https://lore.kernel.org/all/ff58b01c-3f5e-4d55-be82-609d2faaf12e@sirena.org.uk/
> 
>> diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
>> index 09ad57ac4b73e..45ffbe04b5805 100644
>> --- a/include/trace/trace_events.h
>> +++ b/include/trace/trace_events.h
>> @@ -404,10 +404,12 @@ static inline notrace int trace_event_get_offsets_##call(		\
>>   *   [0] FUNC   __bpf_trace_<call>     (the BPF dispatcher)
>>   *   [1] STRUCT trace_event_raw_<call> (the ring-buffer record)
>>   * Exposed via the events/<sys>/<name>/btf_ids tracefs file.
>> + *
>> + * File-local: the class name is not unique, so a global symbol would
>> + * clash across translation units.
>>   */
>>  #define _TRACE_BTF_IDS_DECLARE(call)					\
>> -	extern u32 __bpf_trace_btf_ids_##call[];			\
>> -	BTF_ID_LIST_GLOBAL(__bpf_trace_btf_ids_##call, 2)		\
>> +	BTF_ID_LIST(__bpf_trace_btf_ids_##call)				\
>>  	BTF_ID(func,   __bpf_trace_##call)				\
>>  	BTF_ID(struct, trace_event_raw_##call)
> 
> [Severity: Medium]
> Does changing this to BTF_ID_LIST() introduce significant BSS memory bloat
> when CONFIG_DEBUG_INFO_BTF is disabled?
> 
> When CONFIG_DEBUG_INFO_BTF is disabled, the fallback for BTF_ID_LIST is
> defined as:
> 
> include/linux/btf_ids.h:
> #define BTF_ID_LIST(name) static u32 __maybe_unused name[128];
> 
> BTF_ID_LIST_GLOBAL(..., 2) only allocated 2 elements (8 bytes), but this
> fallback statically allocates a 128-element u32 array (512 bytes) per
> instance.
> 
> Because _TRACE_BTF_IDS_DECLARE is instantiated via DECLARE_EVENT_CLASS for
> over a thousand trace event classes across the kernel, this seems to silently
> add roughly 750 KB of wasted BSS memory.
> 
I'll follow up on this