linux-hardening - [PATCH v5 17/32] ARM: assembler: introduce bl

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Date:   Mon, 24 Jan 2022 18:47:29 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux@...linux.org.uk, linux-arm-kernel@...ts.infradead.org
Cc:     linux-hardening@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Nicolas Pitre <nico@...xnic.net>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Keith Packard <keithpac@...zon.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tony Lindgren <tony@...mide.com>,
        Marc Zyngier <maz@...nel.org>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Jesse Taube <mr.bossman075@...il.com>
Subject: [PATCH v5 17/32] ARM: assembler: introduce bl_r macro

Add a bl_r macro that abstract the difference between the ways indirect
calls are performed on older and newer ARM architecture revisions.

The main difference is to prefer blx instructions over explicit LR
assignments when possible, as these tend to confuse the prediction logic
in out-of-order cores when speculating across a function return.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
Reviewed-by: Arnd Bergmann <arnd@...db.de>
Acked-by: Linus Walleij <linus.walleij@...aro.org>
Tested-by: Keith Packard <keithpac@...zon.com>
Tested-by: Marc Zyngier <maz@...nel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@....com> # ARMv7M
---
 arch/arm/include/asm/assembler.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index bf304596f87e..7242e9a56650 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h
@@ -744,4 +744,19 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
 	.endif
 	.endm
 
+	/*
+	 * bl_r - branch and link to register
+	 *
+	 * @dst: target to branch to
+	 * @c: conditional opcode suffix
+	 */
+	.macro		bl_r, dst:req, c
+	.if		__LINUX_ARM_ARCH__ < 6
+	mov\c		lr, pc
+	mov\c		pc, \dst
+	.else
+	blx\c		\dst
+	.endif
+	.endm
+
 #endif /* __ASM_ASSEMBLER_H__ */
-- 
2.30.2