[PATCH] tracing: Adjust cmd_check_undefined to show unexpected undefined symbols

[Nathan Chancellor <nathan@kernel.org>]

When the check_undefined command in kernel/trace/Makefile fails, there
is no output, making it hard to understand why the build failed. Capture
the output of the $(NM) + grep command and print it when failing to make
it clearer what the problem is.

Fixes: a717943d8ecc ("tracing: Check for undefined symbols in simple_ring_buffer")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
Commit a717943d8ecc ("tracing: Check for undefined symbols in
simple_ring_buffer") and its follow up fixes are in the kvmarm tree so
this should go there as well. This is the rebased version of my
suggestion in the original thread:

https://lore.kernel.org/20260311221816.GA316631@ax162/
---
 kernel/trace/Makefile | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index c5e14ffd36ee..d662c1a64cd5 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -174,7 +174,13 @@ UNDEFINED_ALLOWLIST = __asan __gcov __kasan __kcsan __hwasan __sancov __sanitize
 		      $(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 $(addprefix -e , $(UNDEFINED_ALLOWLIST))`"
+      cmd_check_undefined = \
+          undefsyms=$$($(NM) -u $< | grep -v $(addprefix -e , $(UNDEFINED_ALLOWLIST)) || true); \
+          if [ -n "$$undefsyms" ]; then \
+              echo "Unexpected symbols in $<:" >&2; \
+              echo "$$undefsyms" >&2; \
+              false; \
+          fi
 
 $(obj)/%.o.checked: $(obj)/%.o $(obj)/undefsyms_base.o FORCE
 	$(call if_changed,check_undefined)

---
base-commit: e3d585ed3ff891a00c2284fef4be9cf8581735ab
change-id: 20260320-cmd_check_undefined-verbose-7d15f13f615d

Best regards,
--  
Nathan Chancellor <nathan@kernel.org>