arm64: Port SWP/SWPB emulation support from arm

The SWP instruction was deprecated in the ARMv6 architecture. The
ARMv7 multiprocessing extensions mandate that SWP/SWPB instructions
are treated as undefined from reset, with the ability to enable them
through the System Control Register SW bit. With ARMv8, the option to
enable these instructions through System Control Register was dropped
as well.

To support legacy applications using these instructions, port the
emulation of the SWP and SWPB instructions from the arm port to arm64.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/arm64/legacy_instructions.txt

@@ -31,3 +31,10 @@ behaviours and the corresponding values of the sysctl nodes -
 The default mode depends on the status of the instruction in the
 architecture. Deprecated instructions should default to emulation
 while obsolete instructions must be undefined by default.
+
+Supported legacy instructions
+-----------------------------
+* SWP{B}
+Node: /proc/sys/abi/swp
+Status: Obsolete
+Default: Undef (0)

arch/arm64/Kconfig

@@ -180,6 +180,27 @@ menuconfig ARMV8_DEPRECATED
 
 if ARMV8_DEPRECATED
 
+config SWP_EMULATION
+	bool "Emulate SWP/SWPB instructions"
+	help
+	  ARMv8 obsoletes the use of A32 SWP/SWPB instructions such that
+	  they are always undefined. Say Y here to enable software
+	  emulation of these instructions for userspace using LDXR/STXR.
+
+	  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, LDXR/STXR 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
+
 endif
 
 endmenu

arch/arm64/include/asm/insn.h

@@ -356,6 +356,12 @@ int aarch64_insn_patch_text_sync(void *addrs[], u32 insns[], int cnt);
 int aarch64_insn_patch_text(void *addrs[], u32 insns[], int cnt);
 
 bool aarch32_insn_is_wide(u32 insn);
+
+#define A32_RN_OFFSET	16
+#define A32_RT_OFFSET	12
+#define A32_RT2_OFFSET	 0
+
+u32 aarch32_insn_extract_reg_num(u32 insn, int offset);
 #endif /* __ASSEMBLY__ */
 
 #endif	/* __ASM_INSN_H */

arch/arm64/kernel/armv8_deprecated.c

@@ -8,10 +8,16 @@
 
 #include <linux/init.h>
 #include <linux/list.h>
+#include <linux/perf_event.h>
+#include <linux/sched.h>
 #include <linux/slab.h>
 #include <linux/sysctl.h>
 
+#include <asm/insn.h>
+#include <asm/opcodes.h>
+#include <asm/system_misc.h>
 #include <asm/traps.h>
+#include <asm/uaccess.h>
 
 /*
  * The runtime support for deprecated instruction support can be in one of
@@ -203,11 +209,196 @@ static void register_insn_emulation_sysctl(struct ctl_table *table)
 	register_sysctl_table(table);
 }
 
+/*
+ *  Implement emulation of the SWP/SWPB instructions using load-exclusive and
+ *  store-exclusive.
+ *
+ *  Syntax of SWP{B} instruction: SWP{B}<c> <Rt>, <Rt2>, [<Rn>]
+ *  Where: Rt  = destination
+ *	   Rt2 = source
+ *	   Rn  = address
+ */
+
+/*
+ * Error-checking SWP macros implemented using ldxr{b}/stxr{b}
+ */
+#define __user_swpX_asm(data, addr, res, temp, B)		\
+	__asm__ __volatile__(					\
+	"	mov		%w2, %w1\n"			\
+	"0:	ldxr"B"		%w1, [%3]\n"			\
+	"1:	stxr"B"		%w0, %w2, [%3]\n"		\
+	"	cbz		%w0, 2f\n"			\
+	"	mov		%w0, %w4\n"			\
+	"2:\n"							\
+	"	.pushsection	 .fixup,\"ax\"\n"		\
+	"	.align		2\n"				\
+	"3:	mov		%w0, %w5\n"			\
+	"	b		2b\n"				\
+	"	.popsection"					\
+	"	.pushsection	 __ex_table,\"a\"\n"		\
+	"	.align		3\n"				\
+	"	.quad		0b, 3b\n"			\
+	"	.quad		1b, 3b\n"			\
+	"	.popsection"					\
+	: "=&r" (res), "+r" (data), "=&r" (temp)		\
+	: "r" (addr), "i" (-EAGAIN), "i" (-EFAULT)		\
+	: "memory")
+
+#define __user_swp_asm(data, addr, res, temp) \
+	__user_swpX_asm(data, addr, res, temp, "")
+#define __user_swpb_asm(data, addr, res, temp) \
+	__user_swpX_asm(data, addr, res, temp, "b")
+
+/*
+ * Bit 22 of the instruction encoding distinguishes between
+ * the SWP and SWPB variants (bit set means SWPB).
+ */
+#define TYPE_SWPB (1 << 22)
+
+/*
+ * Set up process info to signal segmentation fault - called on access error.
+ */
+static void set_segfault(struct pt_regs *regs, unsigned long addr)
+{
+	siginfo_t info;
+
+	down_read(&current->mm->mmap_sem);
+	if (find_vma(current->mm, addr) == NULL)
+		info.si_code = SEGV_MAPERR;
+	else
+		info.si_code = SEGV_ACCERR;
+	up_read(&current->mm->mmap_sem);
+
+	info.si_signo = SIGSEGV;
+	info.si_errno = 0;
+	info.si_addr  = (void *) instruction_pointer(regs);
+
+	pr_debug("SWP{B} emulation: access caused memory abort!\n");
+	arm64_notify_die("Illegal memory access", regs, &info, 0);
+}
+
+static int emulate_swpX(unsigned int address, unsigned int *data,
+			unsigned int type)
+{
+	unsigned int res = 0;
+
+	if ((type != TYPE_SWPB) && (address & 0x3)) {
+		/* SWP to unaligned address not permitted */
+		pr_debug("SWP instruction on unaligned pointer!\n");
+		return -EFAULT;
+	}
+
+	while (1) {
+		unsigned long temp;
+
+		if (type == TYPE_SWPB)
+			__user_swpb_asm(*data, address, res, temp);
+		else
+			__user_swp_asm(*data, address, res, temp);
+
+		if (likely(res != -EAGAIN) || signal_pending(current))
+			break;
+
+		cond_resched();
+	}
+
+	return res;
+}
+
+/*
+ * 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, u32 instr)
+{
+	u32 destreg, data, type, address = 0;
+	int rn, rt2, res = 0;
+
+	perf_sw_event(PERF_COUNT_SW_EMULATION_FAULTS, 1, regs, regs->pc);
+
+	type = instr & TYPE_SWPB;
+
+	switch (arm_check_condition(instr, regs->pstate)) {
+	case ARM_OPCODE_CONDTEST_PASS:
+		break;
+	case ARM_OPCODE_CONDTEST_FAIL:
+		/* Condition failed - return to next instruction */
+		goto ret;
+	case ARM_OPCODE_CONDTEST_UNCOND:
+		/* If unconditional encoding - not a SWP, undef */
+		return -EFAULT;
+	default:
+		return -EINVAL;
+	}
+
+	rn = aarch32_insn_extract_reg_num(instr, A32_RN_OFFSET);
+	rt2 = aarch32_insn_extract_reg_num(instr, A32_RT2_OFFSET);
+
+	address = (u32)regs->user_regs.regs[rn];
+	data	= (u32)regs->user_regs.regs[rt2];
+	destreg = aarch32_insn_extract_reg_num(instr, A32_RT_OFFSET);
+
+	pr_debug("addr in r%d->0x%08x, dest is r%d, source in r%d->0x%08x)\n",
+		rn, address, destreg,
+		aarch32_insn_extract_reg_num(instr, A32_RT2_OFFSET), data);
+
+	/* 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 0x%08x not allowed!\n",
+			address);
+		goto fault;
+	}
+
+	res = emulate_swpX(address, &data, type);
+	if (res == -EFAULT)
+		goto fault;
+	else if (res == 0)
+		regs->user_regs.regs[destreg] = data;
+
+ret:
+	pr_warn_ratelimited("\"%s\" (%ld) uses obsolete SWP{B} instruction at 0x%llx\n",
+			current->comm, (unsigned long)current->pid, regs->pc);
+
+	regs->pc += 4;
+	return 0;
+
+fault:
+	set_segfault(regs, address);
+
+	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.
+ */
+static struct undef_hook swp_hooks[] = {
+	{
+		.instr_mask	= 0x0fb00ff0,
+		.instr_val	= 0x01000090,
+		.pstate_mask	= COMPAT_PSR_MODE_MASK,
+		.pstate_val	= COMPAT_PSR_MODE_USR,
+		.fn		= swp_handler
+	},
+	{ }
+};
+
+static struct insn_emulation_ops swp_ops = {
+	.name = "swp",
+	.status = INSN_OBSOLETE,
+	.hooks = swp_hooks,
+	.set_hw_mode = NULL,
+};
+
 /*
  * Invoked as late_initcall, since not needed before init spawned.
  */
 static int __init armv8_deprecated_init(void)
 {
+	if (IS_ENABLED(CONFIG_SWP_EMULATION))
+		register_insn_emulation(&swp_ops);
+
 	register_insn_emulation_sysctl(ctl_abi);
 
 	return 0;

arch/arm64/kernel/insn.c

@@ -964,3 +964,11 @@ bool aarch32_insn_is_wide(u32 insn)
 {
 	return insn >= 0xe800;
 }
+
+/*
+ * Macros/defines for extracting register numbers from instruction.
+ */
+u32 aarch32_insn_extract_reg_num(u32 insn, int offset)
+{
+	return (insn & (0xf << offset)) >> offset;
+}