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

On Thu, Jul 30, 2026 at 5:52=E2=80=AFAM Mykyta Yatsenko
<[email protected]> wrote:
>
> From: Mykyta Yatsenko <[email protected]>
>
> DECLARE_EVENT_CLASS emitted __bpf_trace_btf_ids_<call> through
> BTF_ID_LIST_GLOBAL, i.e. a global symbol named after the event class.
> The class name is not unique across the kernel, so the symbol
> multiply-defines whenever two translation units instantiate the same
> class.
>
> Switch to the file-local BTF_ID_LIST: the list is reached
> only through the event_class_<call>.btf_ids pointer, initialised in the
> same unit, so tracefs readers never reference the symbol by name and
> resolve_btfids still fills the now-local .BTF_ids entries.
>
> The handcrafted syscall classes are the one cross-unit consumer: give
> them their own local BTF_ID_LIST rather than importing the generated
> sys_{enter,exit} lists, which are now file-local.
>
> Fixes: eadc0725ab8d3 ("tracing: Expose tracepoint BTF ids via tracefs")
> Reported-by: Mark Brown <[email protected]>
> Closes: https://lore.kernel.org/all/ff58b01c-3f5e-4d55-be82-609d2faaf12e@=
sirena.org.uk/
> Signed-off-by: Mykyta Yatsenko <[email protected]>
> ---
>  include/trace/trace_events.h  |  6 ++++--
>  kernel/trace/trace_syscalls.c | 22 ++++++++++++++--------
>  2 files changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
> index 09ad57ac4b73..45ffbe04b580 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.

drop this comment? not super useful and file-local is default behavior
for BTF_ID_LIST anyways

>   */
>  #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)
>

This should go into Steven's tree, not bpf-next, right?

> diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.=
c
> index 48ef1a93e576..9b5931f2f280 100644
> --- a/kernel/trace/trace_syscalls.c
> +++ b/kernel/trace/trace_syscalls.c
> @@ -9,6 +9,7 @@
>  #include <linux/ftrace.h>
>  #include <linux/perf_event.h>
>  #include <linux/xarray.h>
> +#include <linux/btf_ids.h>
>  #include <asm/syscall.h>
>
>  #include "trace_output.h"
> @@ -1305,13 +1306,18 @@ struct trace_event_functions exit_syscall_print_f=
uncs =3D {
>
>  #ifdef CONFIG_BPF_EVENTS
>  /*
> - * BTF id lists generated by DECLARE_EVENT_CLASS for the sys_enter and
> - * sys_exit tracepoints. The auto-generated event_class_sys_{enter,exit}
> - * is unused (per-syscall events share the handcrafted classes below),
> - * but the id lists themselves are global and reusable.
> + * BTF id lists for the shared sys_enter/sys_exit dispatcher tracepoints=
,
> + * resolved by resolve_btfids. Defined locally rather than importing the
> + * DECLARE_EVENT_CLASS-generated lists, which are now file-local to the
> + * translation unit that instantiates the tracepoint.

drop "resolved by resolve_btfids", and drop the part about defined
locally because file-local, too much Claude style

>   */
> -extern u32 __bpf_trace_btf_ids_sys_enter[];
> -extern u32 __bpf_trace_btf_ids_sys_exit[];
> +BTF_ID_LIST(syscall_enter_btf_ids)
> +BTF_ID(func,   __bpf_trace_sys_enter)
> +BTF_ID(struct, trace_event_raw_sys_enter)
> +
> +BTF_ID_LIST(syscall_exit_btf_ids)
> +BTF_ID(func,   __bpf_trace_sys_exit)
> +BTF_ID(struct, trace_event_raw_sys_exit)
>  #endif
>
>  struct trace_event_class __refdata event_class_syscall_enter =3D {
> @@ -1321,7 +1327,7 @@ struct trace_event_class __refdata event_class_sysc=
all_enter =3D {
>         .get_fields     =3D syscall_get_enter_fields,
>         .raw_init       =3D init_syscall_trace,
>  #ifdef CONFIG_BPF_EVENTS
> -       .btf_ids        =3D __bpf_trace_btf_ids_sys_enter,
> +       .btf_ids        =3D syscall_enter_btf_ids,
>  #endif
>  };
>
> @@ -1336,7 +1342,7 @@ struct trace_event_class __refdata event_class_sysc=
all_exit =3D {
>         .fields         =3D LIST_HEAD_INIT(event_class_syscall_exit.field=
s),
>         .raw_init       =3D init_syscall_trace,
>  #ifdef CONFIG_BPF_EVENTS
> -       .btf_ids        =3D __bpf_trace_btf_ids_sys_exit,
> +       .btf_ids        =3D syscall_exit_btf_ids,
>  #endif
>  };
>
>
> ---
> base-commit: eadc0725ab8d38c05148fce49f8d5c6a0e01f062
> change-id: 20260730-b4-fix_btf_tracefs-fdafda5fdb5b
>
> Best regards,
> --
> Mykyta Yatsenko <[email protected]>
>