arm64: emulate the swp/swpb instruction

The swp and spwb instructions were deprecated in ARMv6. ARMv8
obsoleted the instruction. Despite this, many applications rely on
these instruuctions.

This patch starts with the version present in the arm architecture.
However, it uses the ldx*()/stx*() functions to implement the handler
in C code. It also removes a lot of code that is not needed.

Bug 1438816

Change-Id: I6882fbe5f71bfa8f9e9a75d067b2111188c6f2fa
Signed-off-by: Alex Van Brunt <avanbrunt@nvidia.com>
Reviewed-on: http://git-master/r/366097
Reviewed-by: Richard Wiley <rwiley@nvidia.com>
Tested-by: Oskari Jaaskelainen <oskarij@nvidia.com>

3 files changed, 182 insertions, 0 deletions

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index aeecfdac027b..d680ed628c2f 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -205,6 +205,31 @@ config HOTPLUG_CPU
 
 	  If you don't know what to do here, say N.
 
+config SWP_EMULATE
+	bool "Emulate SWP/SWPB instructions"
+	help
+	  ARMv6 architecture deprecates use of the SWP/SWPB instructions. ARMv8
+	  oblosetes the use of SWP/SWPB instructions. ARMv7 multiprocessing
+	  extensions introduce the ability to disable these instructions,
+	  triggering an undefined instruction exception when executed. Say Y
+	  here to enable software emulation of these instructions for userspace
+	  (not kernel) using LDREX/STREX. Also creates /proc/cpu/swp_emulation
+	  for statistics.
+
+	  In some older versions of glibc [<=2.8] SWP is used during futex
+	  trylock() operations with the assumption that the code will not
+	  be preempted. This invalid assumption may be more likely to fail
+	  with SWP emulation enabled, leading to deadlock of the user
+	  application.
+
+	  NOTE: when accessing uncached shared regions, LDREX/STREX rely
+	  on an external transaction monitoring block called a global
+	  monitor to maintain update atomicity. If your system does not
+	  implement a global monitor, this option can cause programs that
+	  perform SWP operations to uncached memory to deadlock.
+
+	  If unsure, say Y.
+
 source kernel/Kconfig.preempt
 
 config ARCH_NR_GPIO
@@ -247,6 +272,7 @@ config HW_PERF_EVENTS
 config ARMV7_COMPAT
 	bool "Kernel support for ARMv7 applications"
 	depends on COMPAT
+	select SWP_EMULATE
 	help
 	 This option enables features that allow that ran on an ARMv7 or older
 	 processor to continue functioning.
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index dd35492ee8e3..88260a018fdd 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -21,6 +21,7 @@ arm64-obj-$(CONFIG_EARLY_PRINTK)	+= early_printk.o
 arm64-obj-$(CONFIG_ARM_CPU_SUSPEND)	+= sleep.o suspend.o
 
 obj-$(CONFIG_PCI)               += bios32.o
+obj-$(CONFIG_SWP_EMULATE)	+= swp_emulate.o
 
 obj-y					+= $(arm64-obj-y) vdso/
 obj-m					+= $(arm64-obj-m)
diff --git a/arch/arm64/kernel/swp_emulate.c b/arch/arm64/kernel/swp_emulate.c
new file mode 100644
index 000000000000..05ffe59b5bc4
--- /dev/null
+++ b/arch/arm64/kernel/swp_emulate.c
@@ -0,0 +1,155 @@
+/*
+ *  Derived from from linux/arch/arm/kernel/swp_emulate.c
+ *
+ *  Copyright (C) 2009 ARM Limited
+ *  Copyright (c) 2014, NVIDIA CORPORATION.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ *  Implements emulation of the SWP/SWPB instructions using load-exclusive and
+ *  store-exclusive for processors that have them disabled (or future ones that
+ *  might not implement them).
+ *
+ *  Syntax of SWP{B} instruction: SWP{B}<c> <Rt>, <Rt2>, [<Rn>]
+ *  Where: Rt  = destination
+ *	   Rt2 = source
+ *	   Rn  = address
+ */
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/proc_fs.h>
+#include <linux/seq_file.h>
+#include <linux/sched.h>
+#include <linux/syscalls.h>
+#include <linux/perf_event.h>
+
+#include <asm/opcodes.h>
+#include <asm/traps.h>
+#include <asm/uaccess.h>
+#include <asm/system_misc.h>
+#include <linux/debugfs.h>
+
+/*
+ * Macros/defines for extracting register numbers from instruction.
+ */
+#define EXTRACT_REG_NUM(instruction, offset) \
+	(((instruction) & (0xf << (offset))) >> (offset))
+#define RN_OFFSET  16
+#define RT_OFFSET  12
+#define RT2_OFFSET  0
+/*
+ * Bit 22 of the instruction encoding distinguishes between
+ * the SWP and SWPB variants (bit set means SWPB).
+ */
+#define TYPE_SWPB (1 << 22)
+
+static pid_t previous_pid;
+
+u64 swpb_count = 0;
+u64 swp_count = 0;
+
+/*
+ * swp_handler logs the id of calling process, dissects the instruction, sanity
+ * checks the memory location, calls emulate_swpX for the actual operation and
+ * deals with fixup/error handling before returning
+ */
+static int swp_handler(struct pt_regs *regs, unsigned int instr)
+{
+	u32 address_reg, destreg, data, type;
+	uintptr_t address;
+	unsigned int res = 0;
+	u32 temp32;
+	u8 temp8;
+
+	perf_sw_event(PERF_COUNT_SW_EMULATION_FAULTS, 1, regs, regs->pc);
+
+	res = arm_check_condition(instr, regs->pstate);
+	switch (res) {
+	case ARM_OPCODE_CONDTEST_PASS:
+		break;
+	case ARM_OPCODE_CONDTEST_FAIL:
+		/* Condition failed - return to next instruction */
+		regs->pc += 4;
+		return 0;
+	case ARM_OPCODE_CONDTEST_UNCOND:
+		/* If unconditional encoding - not a SWP, undef */
+		return -EFAULT;
+	default:
+		return -EINVAL;
+	}
+
+	if (current->pid != previous_pid) {
+		pr_warn("\"%s\" (%ld) uses obsolete SWP{B} instruction\n",
+			 current->comm, (unsigned long)current->pid);
+		previous_pid = current->pid;
+	}
+
+	address = regs->regs[EXTRACT_REG_NUM(instr, RN_OFFSET)] & 0xffffffff;
+	data = regs->regs[EXTRACT_REG_NUM(instr, RT2_OFFSET)];
+	destreg = EXTRACT_REG_NUM(instr, RT_OFFSET);
+
+	type = instr & TYPE_SWPB;
+
+	/* Check access in reasonable access range for both SWP and SWPB */
+	if (!access_ok(VERIFY_WRITE, (address & ~3), 4)) {
+		pr_debug("SWP{B} emulation: access to %p not allowed!\n",
+			 (void *)address);
+		res = -EFAULT;
+	}
+	if (type == TYPE_SWPB) {
+		do {
+			temp8 = ldax8((u8 *) address);
+		} while (stx8((u8 *) address, (u8) data));
+		regs->regs[destreg] = temp8;
+		regs->pc += 4;
+		swpb_count++;
+	} else if (address & 0x3) {
+		/* SWP to unaligned address not permitted */
+		pr_debug("SWP instruction on unaligned pointer!\n");
+		return -EFAULT;
+	} else {
+		do {
+			temp32 = ldax32((u32 *) address);
+		} while (stlx32((u32 *) address, (u32) data));
+		regs->regs[destreg] = temp32;
+		regs->pc += 4;
+		swp_count++;
+	}
+
+	return 0;
+}
+
+/*
+ * Only emulate SWP/SWPB executed in ARM state/User mode.
+ * The kernel must be SWP free and SWP{B} does not exist in Thumb/ThumbEE.
+ */
+static struct undef_hook swp_hook = {
+	.instr_mask	= 0x0fb00ff0,
+	.instr_val	= 0x01000090,
+	.pstate_mask	= COMPAT_PSR_MODE_MASK | COMPAT_PSR_T_BIT,
+	.pstate_val	= COMPAT_PSR_MODE_USR,
+	.fn		= swp_handler
+};
+
+/*
+ * Register handler and create status file in /proc/cpu
+ * Invoked as late_initcall, since not needed before init spawned.
+ */
+static int __init swp_emulation_init(void)
+{
+	struct dentry *dir;
+	dir = debugfs_create_dir("swp_emulate", NULL);
+	debugfs_create_u64("swp_count", S_IRUGO | S_IWUSR, dir, &swp_count);
+	debugfs_create_u64("swpb_count", S_IRUGO | S_IWUSR, dir, &swpb_count);
+
+	pr_notice("Registering SWP/SWPB emulation handler\n");
+	register_undef_hook(&swp_hook);
+
+
+	return 0;
+}
+
+late_initcall(swp_emulation_init);