* [PATCH v2] tracing: Generate undef symbols allowlist for simple_ring_buffer
@ 2026-03-13 10:58 Vincent Donnefort
2026-03-13 16:37 ` Nathan Chancellor
0 siblings, 1 reply; 3+ messages in thread
From: Vincent Donnefort @ 2026-03-13 10:58 UTC (permalink / raw)
To: maz
Cc: rostedt, arnd, nathan, linux-trace-kernel, kvmarm, kernel-team,
Vincent Donnefort
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>
---
Changes in v2:
- Use filechk (Nathan)
- Removed deprecated extra-y (Nathan)
- Added simple_ring_buffer in allowlist (Nathan)
- Added memcpy() to generate more symbols (Nathan)
- Added __sancov
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index beb15936829d..96627a909ecc 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -136,17 +136,42 @@ 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 , $(UNDEFINED_ALLOWLIST))
+# undefsyms_base generates a set of compiler and tooling-generated symbols that can
+# safely be ignored for simple_ring_buffer.
+#
+filechk_undefsyms_base = \
+ echo '$(pound)include <linux/atomic.h>'; \
+ echo '$(pound)include <linux/string.h>'; \
+ echo '$(pound)include <asm/page.h>'; \
+ echo 'static char page[PAGE_SIZE] __aligned(PAGE_SIZE);'; \
+ echo 'void undefsyms_base(void *p, int n);'; \
+ echo 'void undefsyms_base(void *p, 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 ' memcpy((void * volatile)p, buffer, sizeof(buffer));'; \
+ echo ' cmpxchg((u32 * volatile)&u, 0, 8);'; \
+ echo ' WARN_ON(n == 0xdeadbeef);'; \
+ echo '}'
+
+$(obj)/undefsyms_base.c: FORCE
+ $(call filechk,undefsyms_base)
+
+clean-files += undefsyms_base.c
+
+$(obj)/undefsyms_base.o: $(obj)/undefsyms_base.c
+
+targets += undefsyms_base.o
+
+UNDEFINED_ALLOWLIST = __asan __gcov __kasan __kcsan __hwasan __sancov __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 $<
- 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
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH v2] tracing: Generate undef symbols allowlist for simple_ring_buffer
2026-03-13 10:58 [PATCH v2] tracing: Generate undef symbols allowlist for simple_ring_buffer Vincent Donnefort
@ 2026-03-13 16:37 ` Nathan Chancellor
2026-03-16 9:31 ` Vincent Donnefort
0 siblings, 1 reply; 3+ messages in thread
From: Nathan Chancellor @ 2026-03-13 16:37 UTC (permalink / raw)
To: Vincent Donnefort
Cc: maz, rostedt, arnd, linux-trace-kernel, kvmarm, kernel-team
On Fri, Mar 13, 2026 at 10:58:29AM +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.
>
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
>
> ---
>
> Changes in v2:
>
> - Use filechk (Nathan)
> - Removed deprecated extra-y (Nathan)
> - Added simple_ring_buffer in allowlist (Nathan)
> - Added memcpy() to generate more symbols (Nathan)
> - Added __sancov
>
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index beb15936829d..96627a909ecc 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -136,17 +136,42 @@ 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.
> +#
> +filechk_undefsyms_base = \
> + echo '$(pound)include <linux/atomic.h>'; \
> + echo '$(pound)include <linux/string.h>'; \
> + echo '$(pound)include <asm/page.h>'; \
> + echo 'static char page[PAGE_SIZE] __aligned(PAGE_SIZE);'; \
> + echo 'void undefsyms_base(void *p, int n);'; \
> + echo 'void undefsyms_base(void *p, 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 ' memcpy((void * volatile)p, buffer, sizeof(buffer));'; \
> + echo ' cmpxchg((u32 * volatile)&u, 0, 8);'; \
> + echo ' WARN_ON(n == 0xdeadbeef);'; \
> + echo '}'
> +
> +$(obj)/undefsyms_base.c: FORCE
> + $(call filechk,undefsyms_base)
> +
> +clean-files += undefsyms_base.c
> +
> +$(obj)/undefsyms_base.o: $(obj)/undefsyms_base.c
> +
> +targets += undefsyms_base.o
> +
> +UNDEFINED_ALLOWLIST = __asan __gcov __kasan __kcsan __hwasan __sancov __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 $<
> - cmd_check_undefined = test -z "`$(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
>
Thanks! This is almost perfect for my tests, one final thing that I
noticed as a result of my full overnight builds. For ARCH=riscv (and
some other architectures from a quick grep), there is some logic in
their include/asm/string.h files to avoid FORTIFY_SOURCE when KASAN is
enabled for the entire build but not enabled for the particular file. As
undefsyms_base.o is not linked into vmlinux or modules, it does not
automatically have KASAN enabled.
$ cat allmod.config
CONFIG_GCOV_KERNEL=n
CONFIG_LTO_CLANG_THIN=y
CONFIG_WERROR=n
$ make -skj"$(nproc)" ARCH=riscv 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
...
This cures that for me.
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 260382f62dbf..55af887a90e2 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -164,6 +164,11 @@ $(obj)/undefsyms_base.o: $(obj)/undefsyms_base.c
targets += undefsyms_base.o
+# ensure KASAN is enabled to avoid logic that may disable FORTIFY_SOURCE when
+# KASAN is not enabled. undefsyms_base.o does not automatically get KASAN flags
+# because it is not linked into vmlinux.
+KASAN_SANITIZE_undefsyms_base.o := y
+
UNDEFINED_ALLOWLIST = __asan __gcov __kasan __kcsan __hwasan __sancov __sanitizer __tsan __ubsan __x86_indirect_thunk \
simple_ring_buffer \
$(shell $(NM) -u $(obj)/undefsyms_base.o 2>/dev/null | awk '{print $$2}')
--
With that addressed:
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Cheers,
Nathan
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH v2] tracing: Generate undef symbols allowlist for simple_ring_buffer
2026-03-13 16:37 ` Nathan Chancellor
@ 2026-03-16 9:31 ` Vincent Donnefort
0 siblings, 0 replies; 3+ messages in thread
From: Vincent Donnefort @ 2026-03-16 9:31 UTC (permalink / raw)
To: Nathan Chancellor
Cc: maz, rostedt, arnd, linux-trace-kernel, kvmarm, kernel-team
On Fri, Mar 13, 2026 at 09:37:24AM -0700, Nathan Chancellor wrote:
> On Fri, Mar 13, 2026 at 10:58:29AM +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.
> >
> > Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> >
> > ---
> >
> > Changes in v2:
> >
> > - Use filechk (Nathan)
> > - Removed deprecated extra-y (Nathan)
> > - Added simple_ring_buffer in allowlist (Nathan)
> > - Added memcpy() to generate more symbols (Nathan)
> > - Added __sancov
> >
> > diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> > index beb15936829d..96627a909ecc 100644
> > --- a/kernel/trace/Makefile
> > +++ b/kernel/trace/Makefile
> > @@ -136,17 +136,42 @@ 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.
> > +#
> > +filechk_undefsyms_base = \
> > + echo '$(pound)include <linux/atomic.h>'; \
> > + echo '$(pound)include <linux/string.h>'; \
> > + echo '$(pound)include <asm/page.h>'; \
> > + echo 'static char page[PAGE_SIZE] __aligned(PAGE_SIZE);'; \
> > + echo 'void undefsyms_base(void *p, int n);'; \
> > + echo 'void undefsyms_base(void *p, 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 ' memcpy((void * volatile)p, buffer, sizeof(buffer));'; \
> > + echo ' cmpxchg((u32 * volatile)&u, 0, 8);'; \
> > + echo ' WARN_ON(n == 0xdeadbeef);'; \
> > + echo '}'
> > +
> > +$(obj)/undefsyms_base.c: FORCE
> > + $(call filechk,undefsyms_base)
> > +
> > +clean-files += undefsyms_base.c
> > +
> > +$(obj)/undefsyms_base.o: $(obj)/undefsyms_base.c
> > +
> > +targets += undefsyms_base.o
> > +
> > +UNDEFINED_ALLOWLIST = __asan __gcov __kasan __kcsan __hwasan __sancov __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 $<
> > - cmd_check_undefined = test -z "`$(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
> >
>
> Thanks! This is almost perfect for my tests, one final thing that I
> noticed as a result of my full overnight builds. For ARCH=riscv (and
> some other architectures from a quick grep), there is some logic in
> their include/asm/string.h files to avoid FORTIFY_SOURCE when KASAN is
> enabled for the entire build but not enabled for the particular file. As
> undefsyms_base.o is not linked into vmlinux or modules, it does not
> automatically have KASAN enabled.
>
> $ cat allmod.config
> CONFIG_GCOV_KERNEL=n
> CONFIG_LTO_CLANG_THIN=y
> CONFIG_WERROR=n
>
> $ make -skj"$(nproc)" ARCH=riscv 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
> ...
>
> This cures that for me.
>
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index 260382f62dbf..55af887a90e2 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -164,6 +164,11 @@ $(obj)/undefsyms_base.o: $(obj)/undefsyms_base.c
>
> targets += undefsyms_base.o
>
> +# ensure KASAN is enabled to avoid logic that may disable FORTIFY_SOURCE when
> +# KASAN is not enabled. undefsyms_base.o does not automatically get KASAN flags
> +# because it is not linked into vmlinux.
> +KASAN_SANITIZE_undefsyms_base.o := y
> +
> UNDEFINED_ALLOWLIST = __asan __gcov __kasan __kcsan __hwasan __sancov __sanitizer __tsan __ubsan __x86_indirect_thunk \
> simple_ring_buffer \
> $(shell $(NM) -u $(obj)/undefsyms_base.o 2>/dev/null | awk '{print $$2}')
> --
>
> With that addressed:
>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> Tested-by: Nathan Chancellor <nathan@kernel.org>
I've just sent a v3 with all that. I have tested locally with allmodconfig and
many architectures with both clang and gcc.
Thanks a lot for your help!
--
Vincent
>
> Cheers,
> Nathan
^ permalink raw reply [flat|nested] 3+ messages in thread
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2026-03-13 10:58 [PATCH v2] tracing: Generate undef symbols allowlist for simple_ring_buffer Vincent Donnefort
2026-03-13 16:37 ` Nathan Chancellor
2026-03-16 9:31 ` Vincent Donnefort
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