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Date:   Mon, 25 Oct 2021 16:44:44 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Frederic Weisbecker <frederic@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        James Morse <james.morse@arm.com>,
        David Laight <David.Laight@aculab.com>,
        Quentin Perret <qperret@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH 2/4] arm64: implement support for static call trampolines
Message-ID: <YXbC3NRWDDfsW6DG@hirez.programming.kicks-ass.net>
References: <20211025122102.46089-1-frederic@kernel.org>
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On Mon, Oct 25, 2021 at 04:19:16PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 25, 2021 at 04:08:37PM +0200, Ard Biesheuvel wrote:

> > > Ooohh, but what if you go from !func to NOP.
> > >
> > > assuming:
> > >
> > >         .literal = 0
> > >         BTI C
> > >         RET
> > >
> > > Then
> > >
> > >         CPU0                    CPU1
> > >
> > >         [S] literal = func      [I] NOP
> > >         [S] insn[1] = NOP       [L] x16 = literal (NULL)
> > >                                 b x16
> > >                                 *BANG*
> > >
> > > Is that possible? (total lack of memory ordering etc..)
> > >
> > 
> > The CBZ will branch to the RET instruction if x16 == 0x0, so this
> > should not happen.
> 
> Oooh, I missed that :/ I was about to suggest writing the address of a
> bare 'ret' trampoline instead of NULL into the literal.

Perhaps a little something like so.. Shaves 2 instructions off each
trampoline.

--- a/arch/arm64/include/asm/static_call.h
+++ b/arch/arm64/include/asm/static_call.h
@@ -11,9 +11,7 @@
 	    "	hint 	34	/* BTI C */				\n" \
 		insn "							\n" \
 	    "	ldr	x16, 0b						\n" \
-	    "	cbz	x16, 1f						\n" \
 	    "	br	x16						\n" \
-	    "1:	ret							\n" \
 	    "	.popsection						\n")
 
 #define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func)			\
--- a/arch/arm64/kernel/patching.c
+++ b/arch/arm64/kernel/patching.c
@@ -90,6 +90,11 @@ int __kprobes aarch64_insn_write(void *a
 	return __aarch64_insn_write(addr, &i, AARCH64_INSN_SIZE);
 }
 
+asm("__static_call_ret:		\n"
+    "	ret			\n")
+
+extern void __static_call_ret(void);
+
 void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
 {
 	/*
@@ -97,9 +102,7 @@ void arch_static_call_transform(void *si
 	 *  0x0	bti c		<--- trampoline entry point
 	 *  0x4	<branch or nop>
 	 *  0x8	ldr x16, <literal>
-	 *  0xc	cbz x16, 20
-	 * 0x10	br x16
-	 * 0x14	ret
+	 *  0xc	br x16
 	 */
 	struct {
 		u64	literal;
@@ -113,6 +116,7 @@ void arch_static_call_transform(void *si
 	insns.insn[0] = cpu_to_le32(insn);
 
 	if (!func) {
+		insns.literal = (unsigned long)&__static_call_ret;
 		insn = aarch64_insn_gen_branch_reg(AARCH64_INSN_REG_LR,
 						   AARCH64_INSN_BRANCH_RETURN);
 	} else {