linux-kernel - [PATCH] lkdtm: Keep the rodata test from causing retbleed WARNINGS

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Date:   Wed, 20 Jul 2022 15:55:07 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Juergen Gross <jgross@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kees Cook <keescook@...omium.org>
Subject: [PATCH] lkdtm: Keep the rodata test from causing retbleed WARNINGS

From: "Steven Rostedt (Google)" <rostedt@...dmis.org>

The rodata test checks if executable code in the .rodata section will not
execute. But this also means that fix ups that the return hook code does
will not be able to change this code at boot up, and this causes a
warning.

By removing the RETHOOK_CFLAGS from the compilation of the rodata function
makes it hidden from objtool and it will not add its return hook into the
.returns section. This keeps the fix up code from trying and failing on
modifying the rodata return call.

Link: https://lore.kernel.org/all/20220720125736.48164a14@gandalf.local.home/

Fixes: ee88d363d1561 ("x86,static_call: Use alternative RET encoding")
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---

 [ Note, my tests are still running with this, but it passed the test that
   caused the failure, so I decided to post this now. ]

 arch/x86/Makefile           | 1 +
 drivers/misc/lkdtm/Makefile | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 1f40dad30d50..2dd61d8594f4 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -25,6 +25,7 @@ endif
 ifdef CONFIG_RETHUNK
 RETHUNK_CFLAGS		:= -mfunction-return=thunk-extern
 RETPOLINE_CFLAGS	+= $(RETHUNK_CFLAGS)
+export RETHUNK_CFLAGS
 endif
 
 export RETPOLINE_CFLAGS
diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
index 2e0aa74ac185..fd96ac1617f7 100644
--- a/drivers/misc/lkdtm/Makefile
+++ b/drivers/misc/lkdtm/Makefile
@@ -16,7 +16,7 @@ lkdtm-$(CONFIG_PPC_64S_HASH_MMU)	+= powerpc.o
 KASAN_SANITIZE_rodata.o		:= n
 KASAN_SANITIZE_stackleak.o	:= n
 KCOV_INSTRUMENT_rodata.o	:= n
-CFLAGS_REMOVE_rodata.o		+= $(CC_FLAGS_LTO)
+CFLAGS_REMOVE_rodata.o		+= $(CC_FLAGS_LTO) $(RETHUNK_CFLAGS)
 
 OBJCOPYFLAGS :=
 OBJCOPYFLAGS_rodata_objcopy.o	:= \
-- 
2.35.1