Hi Vincent, On Thu, Mar 12, 2026 at 06:20:10PM +0000, Vincent Donnefort wrote: > 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.
This certainly seems more robust. > Signed-off-by: Vincent Donnefort <[email protected]> > > 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 '}' >> $@ This should use filechk, otherwise undefsyms_base.c will be regenerated every build, resulting in undefsyms_base.o being rebuilt every time. $ make -skj"$(nproc)" ARCH=x86_64 mrproper allmodconfig kernel/trace/ $ make -skj"$(nproc)" ARCH=x86_64 V=2 kernel/trace/ ... CC kernel/trace/undefsyms_base.o - due to: kernel/trace/undefsyms_base.c NM kernel/trace/simple_ring_buffer.o - due to target missing filechk_undefsyms_base = { \ echo '$(pound)include <asm/page.h>'; \ echo '$(pound)include <asm/local.h>'; \ echo 'static char page[PAGE_SIZE] __aligned(PAGE_SIZE);'; \ echo 'void undefsyms_base(int n);'; \ echo 'void undefsyms_base(int n) {'; \ echo ' char buffer[256] = { 0 };'; \ echo ' u32 u = 0;'; \ echo ' memset((char * volatile)page, 8, PAGE_SIZE);'; \ echo ' memset((char * volatile)buffer, 8, sizeof(buffer));'; \ echo ' cmpxchg((u32 * volatile)&u, 0, 8);'; \ echo ' WARN_ON(n == 0xdeadbeef);'; \ echo '}'; \ } $(obj)/undefsyms_base.c: FORCE $(call filechk,undefsyms_base) $ make -skj"$(nproc)" ARCH=x86_64 mrproper allmodconfig kernel/trace/ $ make -skj"$(nproc)" ARCH=x86_64 V=2 kernel/trace/ GEN Makefile - due to target is PHONY DESCEND objtool CALL scripts/checksyscalls.sh - due to target is PHONY INSTALL libsubcmd_headers NM kernel/trace/simple_ring_buffer.o - due to target missing > +clean-files += undefsyms_base.c > +targets += undefsyms_base.c I don't think this targets addition is necessary. > +$(obj)/undefsyms_base.o: $(obj)/undefsyms_base.c > + > +extra-y += undefsyms_base.o I think this should be targets += undefsyms_base.o as extra-y is deprecated per Documentation/kbuild/makefiles.rst. > +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}') With an allmodconfig + ThinLTO build, I still see: $ cat allmod.config CONFIG_GCOV_KERNEL=n CONFIG_KASAN=n CONFIG_LTO_CLANG_THIN=y $ make -skj"$(nproc)" ARCH=x86_64 KCONFIG_ALLCONFIG=1 LLVM=1 mrproper allmodconfig kernel/trace/ Unexpected symbols in kernel/trace/simple_ring_buffer.o: U __fortify_panic U __write_overflow_field U simple_ring_buffer_commit U simple_ring_buffer_enable_tracing U simple_ring_buffer_init U simple_ring_buffer_reserve U simple_ring_buffer_reset U simple_ring_buffer_swap_reader_page U simple_ring_buffer_unload Something like: diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile index 48c415a0c7e4..0f9a6ce9abd9 100644 --- a/kernel/trace/Makefile +++ b/kernel/trace/Makefile @@ -142,13 +142,15 @@ obj-$(CONFIG_TRACE_REMOTE_TEST) += remote_test.o filechk_undefsyms_base = { \ echo '$(pound)include <asm/page.h>'; \ echo '$(pound)include <asm/local.h>'; \ + echo '$(pound)include <linux/string.h>'; \ echo 'static char page[PAGE_SIZE] __aligned(PAGE_SIZE);'; \ - echo 'void undefsyms_base(int n);'; \ - echo 'void undefsyms_base(int n) {'; \ + echo 'void undefsyms_base(int n, void *ptr);'; \ + echo 'void undefsyms_base(int n, void *ptr) {'; \ echo ' char buffer[256] = { 0 };'; \ echo ' u32 u = 0;'; \ echo ' memset((char * volatile)page, 8, PAGE_SIZE);'; \ echo ' memset((char * volatile)buffer, 8, sizeof(buffer));'; \ + echo ' memcpy((void* volatile)ptr, buffer, sizeof(buffer));'; \ echo ' cmpxchg((u32 * volatile)&u, 0, 8);'; \ echo ' WARN_ON(n == 0xdeadbeef);'; \ echo '}'; \ -- cures the first two. The simple_ring_buffer symbols are very odd... $ llvm-nm kernel/trace/simple_ring_buffer.o | grep simple_ring_buffer ---------------- d __UNIQUE_ID_addressable_simple_ring_buffer_commit_845 ---------------- d __UNIQUE_ID_addressable_simple_ring_buffer_enable_tracing_849 ---------------- d __UNIQUE_ID_addressable_simple_ring_buffer_init_847 ---------------- d __UNIQUE_ID_addressable_simple_ring_buffer_reserve_841 ---------------- d __UNIQUE_ID_addressable_simple_ring_buffer_reset_846 ---------------- d __UNIQUE_ID_addressable_simple_ring_buffer_swap_reader_page_837 ---------------- d __UNIQUE_ID_addressable_simple_ring_buffer_unload_848 ---------------- t __export_symbol_simple_ring_buffer_commit ---------------- t __export_symbol_simple_ring_buffer_enable_tracing ---------------- t __export_symbol_simple_ring_buffer_init ---------------- t __export_symbol_simple_ring_buffer_reserve ---------------- t __export_symbol_simple_ring_buffer_reset ---------------- t __export_symbol_simple_ring_buffer_swap_reader_page ---------------- t __export_symbol_simple_ring_buffer_unload ---------------- T simple_ring_buffer_commit U simple_ring_buffer_commit ---------------- T simple_ring_buffer_enable_tracing U simple_ring_buffer_enable_tracing U simple_ring_buffer_init ---------------- T simple_ring_buffer_init ---------------- T simple_ring_buffer_init_mm ---------------- T simple_ring_buffer_reserve U simple_ring_buffer_reserve ---------------- T simple_ring_buffer_reset U simple_ring_buffer_reset U simple_ring_buffer_swap_reader_page ---------------- T simple_ring_buffer_swap_reader_page U simple_ring_buffer_unload ---------------- T simple_ring_buffer_unload ---------------- T simple_ring_buffer_unload_mm This is LLVM IR bitcode at this stage, which could be messing things up. $ file kernel/trace/simple_ring_buffer.o kernel/trace/simple_ring_buffer.o: LLVM IR bitcode Maybe not worth thinking about too much and just adding it to the allowlist manually? diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile index 0f9a6ce9abd9..cb1ec50a8386 100644 --- a/kernel/trace/Makefile +++ b/kernel/trace/Makefile @@ -166,6 +166,7 @@ $(obj)/undefsyms_base.o: $(obj)/undefsyms_base.c targets += undefsyms_base.o UNDEFINED_ALLOWLIST = __asan __gcov __kasan __kcsan __hwasan __sanitizer __tsan __ubsan __x86_indirect_thunk \ + simple_ring_buffer \ $(shell $(NM) -u $(obj)/undefsyms_base.o 2>/dev/null | awk '{print $$2}') quiet_cmd_check_undefined = NM $< -- Cheers, Nathan
