Re: [PATCH v14 18/30] tracing: Check for undefined symbols in simple_ring_buffer
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From: Nathan Chancellor <nathan@kernel.org>
Date: 2026-03-11 22:18:22
Also in:
kvmarm, linux-arm-kernel, lkml
Subsystem:
the rest, tracing · Maintainers:
Linus Torvalds, Steven Rostedt, Masami Hiramatsu
Hi Vincent, On Mon, Mar 09, 2026 at 04:25:04PM +0000, Vincent Donnefort wrote:
quoted hunk ↗ jump to hunk
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 <redacted>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