linux-kernel - Re: [PATCH -tip v3 1/2] kcov: Make runtime functions noinstr-compatible

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Date:   Wed, 17 Jun 2020 16:49:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, ndesaulniers@...gle.com
Subject: Re: [PATCH -tip v3 1/2] kcov: Make runtime functions
 noinstr-compatible

On Wed, Jun 17, 2020 at 04:32:08PM +0200, Marco Elver wrote:
> On Mon, Jun 15, 2020 at 05:20PM +0200, Peter Zijlstra wrote:
> > On Mon, Jun 15, 2020 at 05:03:27PM +0200, Peter Zijlstra wrote:
> > 
> > > Yes, I think so. x86_64 needs lib/memcpy_64.S in .noinstr.text then. For
> > > i386 it's an __always_inline inline-asm thing.
> > 
> > Bah, I tried writing it without memcpy, but clang inserts memcpy anyway
> > :/
> 
> Hmm, __builtin_memcpy() won't help either.
> 
> Turns out, Clang 11 got __builtin_memcpy_inline(): https://reviews.llvm.org/D73543
> 
> The below works, no more crash on either KASAN or KCSAN with Clang. We
> can test if we have it with __has_feature(__builtin_memcpy_inline)
> (although that's currently not working as expected, trying to fix :-/).
> 
> Would a memcpy_inline() be generally useful? It's not just Clang but
> also GCC that isn't entirely upfront about which memcpy is inlined and
> which isn't. If the compiler has __builtin_memcpy_inline(), we can use
> it, otherwise the arch likely has to provide the implementation.
> 
> Thoughts?

I had the below, except of course that yields another objtool
complaint, and I was still looking at that.

Does GCC (8, as per the new KASAN thing) have that
__builtin_memcpy_inline() ?

---
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index af75109485c26..a7d1570905727 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -690,13 +690,13 @@ struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
 		(struct bad_iret_stack *)__this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;
 
 	/* Copy the IRET target to the temporary storage. */
-	memcpy(&tmp.regs.ip, (void *)s->regs.sp, 5*8);
+	__memcpy(&tmp.regs.ip, (void *)s->regs.sp, 5*8);
 
 	/* Copy the remainder of the stack from the current stack. */
-	memcpy(&tmp, s, offsetof(struct bad_iret_stack, regs.ip));
+	__memcpy(&tmp, s, offsetof(struct bad_iret_stack, regs.ip));
 
 	/* Update the entry stack */
-	memcpy(new_stack, &tmp, sizeof(tmp));
+	__memcpy(new_stack, &tmp, sizeof(tmp));
 
 	BUG_ON(!user_mode(&new_stack->regs));
 	return new_stack;
diff --git a/arch/x86/lib/memcpy_64.S b/arch/x86/lib/memcpy_64.S
index 56b243b14c3a2..bbcc05bcefadb 100644
--- a/arch/x86/lib/memcpy_64.S
+++ b/arch/x86/lib/memcpy_64.S
@@ -8,6 +8,8 @@
 #include <asm/alternative-asm.h>
 #include <asm/export.h>
 
+.pushsection .noinstr.text, "ax"
+
 /*
  * We build a jump to memcpy_orig by default which gets NOPped out on
  * the majority of x86 CPUs which set REP_GOOD. In addition, CPUs which
@@ -184,6 +186,8 @@ SYM_FUNC_START_LOCAL(memcpy_orig)
 	retq
 SYM_FUNC_END(memcpy_orig)
 
+.popsection
+
 #ifndef CONFIG_UML
 
 MCSAFE_TEST_CTL