Re: [RFC, PATCHv1 15/28] x86: detect 5-level paging support

[hpa@zytor.com]

On December 15, 2016 6:39:44 AM PST, Borislav Petkov <bp@alien8.de> 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.
-- 
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