Re: [PATCH] tracing: Generate undef symbols allowlist for simple_ring_buffer

Vincent Donnefort
2026-03-13T10:23:40Z

On Thu, Mar 12, 2026 at 04:51:53PM -0700, Nathan Chancellor wrote:
> 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 <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 '}' >> $@
>
> 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?

That looks good, thanks for having a look. I'll spin a v2 with your comments.

>
> 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