From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754359AbcLORxY (ORCPT ); Thu, 15 Dec 2016 12:53:24 -0500 Received: from terminus.zytor.com ([198.137.202.10]:57242 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752020AbcLORxW (ORCPT ); Thu, 15 Dec 2016 12:53:22 -0500 User-Agent: K-9 Mail for Android In-Reply-To: <20161215143944.ruxr6r3b2atg4tnf@pd.tnic> References: <20161208162150.148763-1-kirill.shutemov@linux.intel.com> <20161208162150.148763-17-kirill.shutemov@linux.intel.com> <20161208200505.c6xiy56oufg6d24m@pd.tnic> <20161208202013.uutsny6avn5gimwq@pd.tnic> <20161215143944.ruxr6r3b2atg4tnf@pd.tnic> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Subject: Re: [RFC, PATCHv1 15/28] x86: detect 5-level paging support From: hpa@zytor.com Date: Thu, 15 Dec 2016 09:52:12 -0800 To: Borislav Petkov , Linus Torvalds CC: "Kirill A. Shutemov" , Andrew Morton , the arch/x86 maintainers , Thomas Gleixner , Ingo Molnar , Arnd Bergmann , Andi Kleen , Dave Hansen , Andy Lutomirski , "linux-arch@vger.kernel.org" , linux-mm , Linux Kernel Mailing List Message-ID: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On December 15, 2016 6:39:44 AM PST, Borislav Petkov wrote: >On Wed, Dec 14, 2016 at 12:07:54AM +0100, Boris Petkov wrote: >> Thus I was thinking of adding a build-time check for the gcc version >> but that might turn out to be more code in the end than those ugly >> ifnc clauses. > >IOW, something like this. I did this just to try to see whether it is >doable. And it does work - gcc 4.8 and 4.9 -m32 cannot preserve the PIC >register - actually the inline asm fails building due to impossible >constraints. > >However, so many lines changed just to save the ifnc, meh, I dunno... > >--- > arch/x86/boot/compressed/Makefile | 8 ++++++ > arch/x86/boot/cpuflags.c | 14 ++++++++-- >scripts/gcc-clobber-pic.sh | 58 >+++++++++++++++++++++++++++++++++++++++ > 3 files changed, 77 insertions(+), 3 deletions(-) > create mode 100755 scripts/gcc-clobber-pic.sh > >diff --git a/arch/x86/boot/compressed/Makefile >b/arch/x86/boot/compressed/Makefile >index 34d9e15857c3..705fc2ab3fd6 100644 >--- a/arch/x86/boot/compressed/Makefile >+++ b/arch/x86/boot/compressed/Makefile >@@ -35,6 +35,14 @@ KBUILD_CFLAGS += -mno-mmx -mno-sse > KBUILD_CFLAGS += $(call cc-option,-ffreestanding) > KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector) > >+# check whether inline asm clobbers the PIC register >+ifeq ($(CONFIG_X86_32),y) >+ifeq ($(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-clobber-pic.sh >$(CC) -m32),n) >+ KBUILD_CFLAGS += -DCC_PRESERVES_PIC >+ KBUILD_AFLAGS += -DCC_PRESERVES_PIC >+endif >+endif >+ > KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__ > GCOV_PROFILE := n > UBSAN_SANITIZE :=n >diff --git a/arch/x86/boot/cpuflags.c b/arch/x86/boot/cpuflags.c >index 6687ab953257..913c3f5ab3a0 100644 >--- a/arch/x86/boot/cpuflags.c >+++ b/arch/x86/boot/cpuflags.c >@@ -70,11 +70,19 @@ int has_eflag(unsigned long mask) > # define EBX_REG "=b" > #endif > >+#if defined(__i386__) && defined(__PIC__) && >!defined(CC_PRESERVES_PIC) >+# define SAVE_PIC ".ifnc %%ebx, %3; movl %%ebx, %3; .endif\n\t" >+# define SWAP_PIC ".ifnc %%ebx, %3; xchgl %%ebx, %3; .endif\n\t" >+#else >+# define SAVE_PIC >+# define SWAP_PIC >+#endif >+ > static inline void cpuid(u32 id, u32 *a, u32 *b, u32 *c, u32 *d) > { >- asm volatile(".ifnc %%ebx,%3 ; movl %%ebx,%3 ; .endif \n\t" >- "cpuid \n\t" >- ".ifnc %%ebx,%3 ; xchgl %%ebx,%3 ; .endif \n\t" >+ asm volatile(SAVE_PIC >+ "cpuid\n\t" >+ SWAP_PIC > : "=a" (*a), "=c" (*c), "=d" (*d), EBX_REG (*b) > : "a" (id) > ); >diff --git a/scripts/gcc-clobber-pic.sh b/scripts/gcc-clobber-pic.sh >new file mode 100755 >index 000000000000..7ff10edf9b08 >--- /dev/null >+++ b/scripts/gcc-clobber-pic.sh >@@ -0,0 +1,58 @@ >+#!/bin/bash -x >+err=0 >+O=$(mktemp) >+cat << "END" | $@ -fPIC -x c - -o $O >/dev/null 2>&1 || err=1 >+int some_global_var, some_other_global_var; >+ >+typedef unsigned int u32; >+ >+void __attribute__((noinline)) foo(void) >+{ >+ asm volatile("# some crap just so that we don't get optimized away"); >+ >+ some_other_global_var = 43; >+} >+ >+static inline void cpuid(u32 id, u32 *a, u32 *b, u32 *c, u32 *d) >+{ >+ asm volatile("cpuid" >+ : "=a" (*a), "=b" (*b), "=c" (*c), "=d" (*d) >+ : "a" (id), "2" (*c) >+ : "si", "di" >+ ); >+ >+ some_global_var = 42; >+ foo(); >+} >+ >+int main(void) >+{ >+ u32 a, b, c = 0, d; >+ >+ cpuid(0x1, &a, &b, &c, &d); >+ >+ /* >+ * Make sure foo() gets actually called and not optimized away due to >+ * miscompilation. >+ */ >+ if (some_global_var == 42 && some_other_global_var == 43) >+ return 0; >+ else >+ return 1; >+} >+END >+ >+if (( $err )); >+then >+ exit 1 >+fi >+ >+chmod u+x $O >+$O >+ >+if ! (( $? )); >+then >+ echo "n" >+fi >+ >+rm -f $O This really is only worthwhile if it ends up producing better code, but I doubt it. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.