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[34.140.115.198]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48541b6f708sm433813545e9.11.2026.03.13.03.23.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Mar 2026 03:23:37 -0700 (PDT) Date: Fri, 13 Mar 2026 10:23:34 +0000 From: Vincent Donnefort To: Nathan Chancellor Cc: maz@kernel.org, rostedt@goodmis.org, arnd@arndb.de, linux-trace-kernel@vger.kernel.org, kvmarm@lists.linux.dev, kernel-team@android.com Subject: Re: [PATCH] tracing: Generate undef symbols allowlist for simple_ring_buffer Message-ID: References: <20260312182010.111013-1-vdonnefort@google.com> <20260312235153.GA1147071@ax162> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260312235153.GA1147071@ax162> 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 > > > > 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 ' > $@ > > + $(Q)echo '#include ' >> $@ > > + $(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 '; \ > echo '$(pound)include '; \ > 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 '; \ > echo '$(pound)include '; \ > + echo '$(pound)include '; \ > 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