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

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Date:   Mon, 15 Jun 2020 17:20:56 +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>
Subject: Re: [PATCH -tip v3 1/2] kcov: Make runtime functions
 noinstr-compatible

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
:/

---
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index af75109485c26..d74fd6313a4ed 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -686,17 +686,17 @@ struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
 	 * just below the IRET frame) and we want to pretend that the
 	 * exception came from the IRET target.
 	 */
-	struct bad_iret_stack tmp, *new_stack =
+	struct bad_iret_stack tmp = *s, *new_stack =
 		(struct bad_iret_stack *)__this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;
+	unsigned long *p = (unsigned long *)s->regs.sp;
 
-	/* Copy the IRET target to the temporary storage. */
-	memcpy(&tmp.regs.ip, (void *)s->regs.sp, 5*8);
+	tmp.regs.ip	= p[0];
+	tmp.regs.cs	= p[1];
+	tmp.regs.flags	= p[2];
+	tmp.regs.sp	= p[3];
+	tmp.regs.ss	= p[4];
 
-	/* Copy the remainder of the stack from the current stack. */
-	memcpy(&tmp, s, offsetof(struct bad_iret_stack, regs.ip));
-
-	/* Update the entry stack */
-	memcpy(new_stack, &tmp, sizeof(tmp));
+	*new_stack = tmp;
 
 	BUG_ON(!user_mode(&new_stack->regs));
 	return new_stack;