linux-kernel - [PATCH 2/2] kbuild: Disable GCOV for *.mod.o

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Message-ID: <20230623001140.1111494-6-samitolvanen@google.com>
Date:   Fri, 23 Jun 2023 00:11:43 +0000
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>, Tom Rix <trix@...hat.com>,
        linux-kbuild@...r.kernel.org, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Joe Fradley <joefradley@...gle.com>
Subject: [PATCH 2/2] kbuild: Disable GCOV for *.mod.o

With GCOV_PROFILE_ALL, Clang injects __llvm_gcov_* functions to each
object file, including the *.mod.o. As we filter out CC_FLAGS_CFI
for *.mod.o, the compiler won't generate type hashes for the
injected functions, and therefore indirectly calling them during
module loading trips indirect call checking.

Enabling CFI for *.mod.o isn't sufficient to fix this issue after
commit 0c3e806ec0f9 ("x86/cfi: Add boot time hash randomization"),
as *.mod.o aren't processed by objtool, which means any hashes
emitted there won't be randomized. Therefore, in addition to
disabling CFI for *.mod.o, also disable GCOV, as the object files
don't otherwise contain any executable code.

Fixes: cf68fffb66d6 ("add support for Clang CFI")
Reported-by: Joe Fradley <joefradley@...gle.com>
Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
---
 scripts/Makefile.modfinal | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 4703f652c009..fc19f67039bd 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -23,7 +23,7 @@ modname = $(notdir $(@:.mod.o=))
 part-of-module = y

 quiet_cmd_cc_o_c = CC [M]  $@
-      cmd_cc_o_c = $(CC) $(filter-out $(CC_FLAGS_CFI), $(c_flags)) -c -o $@ $<
+      cmd_cc_o_c = $(CC) $(filter-out $(CC_FLAGS_CFI) $(CFLAGS_GCOV), $(c_flags)) -c -o $@ $<

 %.mod.o: %.mod.c FORCE
 	$(call if_changed_dep,cc_o_c)
-- 
2.41.0.162.gfafddb0af9-goog