[PATCH v2] tracing: Make per-template BTF id lists file-local

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.

Fixes: eadc0725ab8d3 ("tracing: Expose tracepoint BTF ids via tracefs")
Reported-by: Mark Brown <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Mykyta Yatsenko <[email protected]>
---
Changes in v2:
- Emit the per-class BTF id list only under CONFIG_DEBUG_INFO_BTF;
  without BTF the ids are unresolved and BTF_ID_LIST falls back to a
  128-entry (512-byte) per-class stub, adding hundreds of KB of BSS
  across the ~1000 event classes (reported in review of v1).
- Link to v1: https://patch.msgid.link/[email protected]
---
Changes in v2:
- Emit the per-class BTF id list only under CONFIG_DEBUG_INFO_BTF;
without BTF the ids are unresolved and BTF_ID_LIST falls back to a
128-entry stub.
- Remove verbose comments (Andrii)
- Link to v1: https://patch.msgid.link/[email protected]
---
 include/trace/trace_events.h  |  7 +++----
 kernel/trace/trace_syscalls.c | 27 ++++++++++++++-------------
 2 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
index 09ad57ac4b73..93011f800d0f 100644
--- a/include/trace/trace_events.h
+++ b/include/trace/trace_events.h
@@ -398,7 +398,7 @@ static inline notrace int trace_event_get_offsets_##call(		\
 #define _TRACE_PERF_INIT(call)
 #endif /* CONFIG_PERF_EVENTS */
 
-#ifdef CONFIG_BPF_EVENTS
+#if defined(CONFIG_BPF_EVENTS) && defined(CONFIG_DEBUG_INFO_BTF)
 /*
  * Per-template BTF id list, populated at link time by resolve_btfids:
  *   [0] FUNC   __bpf_trace_<call>     (the BPF dispatcher)
@@ -406,8 +406,7 @@ static inline notrace int trace_event_get_offsets_##call(		\
  * Exposed via the events/<sys>/<name>/btf_ids tracefs file.
  */
 #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)
 
@@ -417,7 +416,7 @@ static inline notrace int trace_event_get_offsets_##call(		\
 #else
 #define _TRACE_BTF_IDS_DECLARE(call)
 #define _TRACE_BTF_IDS_INIT(call)
-#endif /* CONFIG_BPF_EVENTS */
+#endif /* CONFIG_BPF_EVENTS && CONFIG_DEBUG_INFO_BTF */
 
 #include "stages/stage6_event_callback.h"
 
diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index 48ef1a93e576..e35744049e3f 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"
@@ -1303,15 +1304,15 @@ struct trace_event_functions exit_syscall_print_funcs = {
 	.trace		= print_syscall_exit,
 };
 
-#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.
- */
-extern u32 __bpf_trace_btf_ids_sys_enter[];
-extern u32 __bpf_trace_btf_ids_sys_exit[];
+#if defined(CONFIG_BPF_EVENTS) && defined(CONFIG_DEBUG_INFO_BTF)
+/* BTF id lists for the shared sys_enter/sys_exit dispatcher tracepoints. */
+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 = {
@@ -1320,8 +1321,8 @@ struct trace_event_class __refdata event_class_syscall_enter = {
 	.fields_array	= syscall_enter_fields_array,
 	.get_fields	= syscall_get_enter_fields,
 	.raw_init	= init_syscall_trace,
-#ifdef CONFIG_BPF_EVENTS
-	.btf_ids	= __bpf_trace_btf_ids_sys_enter,
+#if defined(CONFIG_BPF_EVENTS) && defined(CONFIG_DEBUG_INFO_BTF)
+	.btf_ids	= syscall_enter_btf_ids,
 #endif
 };
 
@@ -1335,8 +1336,8 @@ struct trace_event_class __refdata event_class_syscall_exit = {
 	},
 	.fields		= LIST_HEAD_INIT(event_class_syscall_exit.fields),
 	.raw_init	= init_syscall_trace,
-#ifdef CONFIG_BPF_EVENTS
-	.btf_ids	= __bpf_trace_btf_ids_sys_exit,
+#if defined(CONFIG_BPF_EVENTS) && defined(CONFIG_DEBUG_INFO_BTF)
+	.btf_ids	= syscall_exit_btf_ids,
 #endif
 };
 

---
base-commit: eadc0725ab8d38c05148fce49f8d5c6a0e01f062
change-id: 20260730-b4-fix_btf_tracefs-fdafda5fdb5b

Best regards,
--  
Mykyta Yatsenko <[email protected]>