Re: [PATCH v14 18/30] tracing: Check for undefined symbols in simple_ring_buffer

[Nathan Chancellor <nathan@kernel.org>]

Hi Vincent,

On Mon, Mar 09, 2026 at 04:25:04PM +0000, Vincent Donnefort wrote:
> The simple_ring_buffer implementation must remain simple enough to be
> used by the pKVM hypervisor. Prevent the object build if unresolved
> symbols are found.
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> 
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index d106beca8d7f..3182e1bc1cf7 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -132,4 +132,20 @@ obj-$(CONFIG_TRACE_REMOTE) += trace_remote.o
>  obj-$(CONFIG_SIMPLE_RING_BUFFER) += simple_ring_buffer.o
>  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
> +UNDEFINED_ALLOWLIST := $(addprefix -e , $(UNDEFINED_ALLOWLIST))
> +
> +quiet_cmd_check_undefined = NM      $<
> +      cmd_check_undefined = test -z "`$(NM) -u $< | grep -v $(UNDEFINED_ALLOWLIST)`"

This check triggers when building allmodconfig targeting arm, arm64,
powerpc, and x86_64 (at least, I did not test more at the moment) with
clang. If this is a hard failure, this really needs to print something
out to the developer/user to help them debug off the bat, versus having
to manually dig the $(NM) command out from the .cmd file or V=1. I came
up with

diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 3182e1bc1cf7..c725b06876bc 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -141,7 +141,13 @@ UNDEFINED_ALLOWLIST += __stack_chk_fail stackleak_track_stack __ref_stack __sani
 UNDEFINED_ALLOWLIST := $(addprefix -e , $(UNDEFINED_ALLOWLIST))
 
 quiet_cmd_check_undefined = NM      $<
-      cmd_check_undefined = test -z "`$(NM) -u $< | grep -v $(UNDEFINED_ALLOWLIST)`"
+      cmd_check_undefined = \
+          undefsyms=$$($(NM) -u $< | grep -v $(UNDEFINED_ALLOWLIST) || true); \
+          if [ -n "$$undefsyms" ]; then \
+              echo "Unexpected symbols in $<:" >&2; \
+              echo "$$undefsyms" >&2; \
+              false; \
+          fi
 
 $(obj)/%.o.checked: $(obj)/%.o FORCE
 	$(call if_changed,check_undefined)
--

which prints

  Unexpected symbols in kernel/trace/simple_ring_buffer.o:
                   U llvm_gcda_emit_arcs
                   U llvm_gcda_emit_function
                   U llvm_gcda_end_file
                   U llvm_gcda_start_file
                   U llvm_gcda_summary_info
                   U llvm_gcov_init

for arm64, which makes sense since these are LLVM specific GCOV symbols,
so they should probably get the same treatment as the other ones:

diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index c725b06876bc..d464e3aa5bdd 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -136,8 +136,8 @@ 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
+UNDEFINED_ALLOWLIST := memset alt_cb_patch_nops __x86 __ubsan __asan __kasan __gcov llvm_gcda llvm_gcov
+UNDEFINED_ALLOWLIST += __aeabi_unwind __stack_chk_fail stackleak_track_stack __ref_stack __sanitizer
 UNDEFINED_ALLOWLIST := $(addprefix -e , $(UNDEFINED_ALLOWLIST))
 
 quiet_cmd_check_undefined = NM      $<
--

For x86_64, I see

  Unexpected symbols in kernel/trace/simple_ring_buffer.o:
                   U __clear_pages_unrolled
                   U __memmove
                   U copy_page

which comes from the use of KCFI_ADDRESSABLE(), since allmodconfig has
CONFIG_CFI=y.

For powerpc (with both clang and GCC), I see

  Unexpected symbols in kernel/trace/simple_ring_buffer.o:
                   U .TOC.

For arm (with both clang and GCC), I see

  Unexpected symbols in kernel/trace/simple_ring_buffer.o:
           U __stack_chk_guard
           U warn_slowpath_fmt

Presumably adding all of those should be fine as well?

diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index d464e3aa5bdd..4f120cb8c79c 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -137,7 +137,8 @@ obj-$(CONFIG_TRACE_REMOTE_TEST) += remote_test.o
 # to all kernel symbols. Fail the build if forbidden symbols are found.
 #
 UNDEFINED_ALLOWLIST := memset alt_cb_patch_nops __x86 __ubsan __asan __kasan __gcov llvm_gcda llvm_gcov
-UNDEFINED_ALLOWLIST += __aeabi_unwind __stack_chk_fail stackleak_track_stack __ref_stack __sanitizer
+UNDEFINED_ALLOWLIST += __aeabi_unwind __stack_chk_fail __stack_chk_guard stackleak_track_stack __ref_stack __sanitizer
+UNDEFINED_ALLOWLIST += \.TOC\. __clear_pages_unrolled __memmove copy_page warn_slowpath_fmt
 UNDEFINED_ALLOWLIST := $(addprefix -e , $(UNDEFINED_ALLOWLIST))
 
 quiet_cmd_check_undefined = NM      $<
--

I don't mind sending a series for these, I just wanted to make sure I
was reasoning about everything correctly.

Cheers,
Nathan