[PATCH] tracing: Generate undef symbols allowlist for simple_ring_buffer

Vincent Donnefort
2026-03-12T18:20:24Z

Compiler and tooling-generated symbols are difficult to maintain
across all supported architectures. Make the allowlist more robust by
replacing the harcoded list with a mechanism that automatically detects
these symbols.

This mechanism generates a C function designed to trigger common
compiler-inserted symbols.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index beb15936829d..3b427b76434a 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -136,17 +136,37 @@ obj-$(CONFIG_TRACE_REMOTE_TEST) += remote_test.o
# simple_ring_buffer is used by the pKVM hypervisor which does not have access
# to all kernel symbols. Fail the build if forbidden symbols are found.
#
-UNDEFINED_ALLOWLIST := memset alt_cb_patch_nops __x86 __ubsan __asan __kasan __gcov __aeabi_unwind
-UNDEFINED_ALLOWLIST += __stack_chk_fail stackleak_track_stack __ref_stack __sanitizer llvm_gcda llvm_gcov
-UNDEFINED_ALLOWLIST += .TOC\. __clear_pages_unrolled __memmove copy_page warn_slowpath_fmt
-UNDEFINED_ALLOWLIST += ftrace_likely_update __hwasan_load __hwasan_store __hwasan_tag_memory
-UNDEFINED_ALLOWLIST += warn_bogus_irq_restore __stack_chk_guard
-UNDEFINED_ALLOWLIST := $(addprefix -e , $(UNDEFINED_ALLOWLIST))
+# undefsyms_base generates a set of compiler and tooling-generated symbols that can
+# safely be ignored for simple_ring_buffer.
+#
+$(obj)/undefsyms_base.c: FORCE
+ $(Q)echo '#include <asm/page.h>' > $@
+ $(Q)echo '#include <asm/local.h>' >> $@
+ $(Q)echo 'static char page[PAGE_SIZE] __aligned(PAGE_SIZE);' >> $@
+ $(Q)echo 'void undefsyms_base(int n);' >> $@
+ $(Q)echo 'void undefsyms_base(int n) {' >> $@
+ $(Q)echo ' char buffer[256] = { 0 };' >> $@
+ $(Q)echo ' u32 u = 0;' >> $@
+ $(Q)echo ' memset((char * volatile)page, 8, PAGE_SIZE);' >> $@
+ $(Q)echo ' memset((char * volatile)buffer, 8, sizeof(buffer));' >> $@
+ $(Q)echo ' cmpxchg((u32 * volatile)&u, 0, 8);' >> $@
+ $(Q)echo ' WARN_ON(n == 0xdeadbeef);' >> $@
+ $(Q)echo '}' >> $@
+
+clean-files += undefsyms_base.c
+targets += undefsyms_base.c
+
+$(obj)/undefsyms_base.o: $(obj)/undefsyms_base.c
+
+extra-y += undefsyms_base.o
+
+UNDEFINED_ALLOWLIST = __asan __gcov __kasan __kcsan __hwasan __sanitizer __tsan __ubsan __x86_indirect_thunk \
+ $(shell $(NM) -u $(obj)/undefsyms_base.o 2>/dev/null | awk '{print $$2}')

quiet_cmd_check_undefined = NM $<
- cmd_check_undefined = test -z "`$(NM) -u $< | grep -v $(UNDEFINED_ALLOWLIST)`"
+ cmd_check_undefined = test -z "`$(NM) -u $< | grep -v $(addprefix -e ,$(UNDEFINED_ALLOWLIST))`"

-$(obj)/%.o.checked: $(obj)/%.o FORCE
+$(obj)/%.o.checked: $(obj)/%.o $(obj)/undefsyms_base.o FORCE
$(call if_changed,check_undefined)

always-$(CONFIG_SIMPLE_RING_BUFFER) += simple_ring_buffer.o.checked

base-commit: 33f2e266515717c4b2df585dadefa0525557726c
--
2.53.0.851.ga537e3e6e9-goog