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Mon, 14 Nov 2022 16:23:45 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 5207822361755; Mon, 14 Nov 2022 16:23:45 +0100 (CET) Date: Mon, 14 Nov 2022 16:23:45 +0100 From: Peter Zijlstra To: kernel test robot , Kees Cook Cc: oe-kbuild-all@lists.linux.dev, linux-kernel@vger.kernel.org, Josh Poimboeuf Subject: [PATCH] lkdtm: Avoid objtool/ibt warning Message-ID: References: <202211142027.0E3ISIZr-lkp@intel.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <202211142027.0E3ISIZr-lkp@intel.com> X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED,SPF_HELO_NONE, SPF_NONE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1749493243846275584?= X-GMAIL-MSGID: =?utf-8?q?1749493243846275584?= On Mon, Nov 14, 2022 at 08:26:17PM +0800, kernel test robot wrote: > tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master > head: 094226ad94f471a9f19e8f8e7140a09c2625abaa > commit: 656d054e0a15ec327bd82801ccd58201e59f6896 jump_label,noinstr: Avoid instrumentation for JUMP_LABEL=n builds > date: 6 months ago > config: x86_64-buildonly-randconfig-r001-20221114 > compiler: gcc-11 (Debian 11.3.0-8) 11.3.0 > reproduce (this is a W=1 build): > # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=656d054e0a15ec327bd82801ccd58201e59f6896 > git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git > git fetch --no-tags linus master > git checkout 656d054e0a15ec327bd82801ccd58201e59f6896 > # save the config file > mkdir build_dir && cp config build_dir/.config > make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash > >> vmlinux.o: warning: objtool: lkdtm_UNSET_SMEP+0x1c3: relocation to !ENDBR: native_write_cr4+0x41 Acked-by: Kees Cook --- Subject: lkdtm: Avoid objtool/ibt warning From: Peter Zijlstra Date: Mon Nov 14 16:17:50 CET 2022 For certain configs objtool will complain like: vmlinux.o: warning: objtool: lkdtm_UNSET_SMEP+0x1c3: relocation to !ENDBR: native_write_cr4+0x41 What happens is that GCC optimizes the loop: insn = (unsigned char *)native_write_cr4; for (i = 0; i < MOV_CR4_DEPTH; i++) to read something like: for (insn = (unsigned char *)native_write_cr4; insn < (unsigned char *)native_write_cr4 + MOV_CR4_DEPTH; insn++) Which then obviously generates the text reference native_write_cr4+041. Since none of this is a fast path, simply confuse GCC enough to inhibit this optimization. Reported-by: kernel test robot Signed-off-by: Peter Zijlstra (Intel) --- drivers/misc/lkdtm/bugs.c | 1 + 1 file changed, 1 insertion(+) --- a/drivers/misc/lkdtm/bugs.c +++ b/drivers/misc/lkdtm/bugs.c @@ -487,6 +487,7 @@ static void lkdtm_UNSET_SMEP(void) * the cr4 writing instruction. */ insn = (unsigned char *)native_write_cr4; + OPTIMIZER_HIDE_VAR(insn); for (i = 0; i < MOV_CR4_DEPTH; i++) { /* mov %rdi, %cr4 */ if (insn[i] == 0x0f && insn[i+1] == 0x22 && insn[i+2] == 0xe7)