MIPS Kprobes: Refactor branch emulation

This patch refactors MIPS branch emulation code so as to allow skipping
delay slot instruction in case of branch likely instructions when branch is
not taken. This is useful for keeping the code common for use cases like
kprobes where one would like to handle the branch instructions keeping the
delay slot instuction also in picture for branch likely instructions. Also
allow emulation when instruction to be decoded is not at pt_regs->cp0_epc
as in case of kprobes where pt_regs->cp0_epc points to the breakpoint
instruction.

The patch also exports the function for modules.

Signed-off-by: Maneesh Soni <[e-mail]>
Signed-off-by: Victor Kamensky <[e-mail]>
Cc: David Daney <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/2913/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/include/asm/branch.h

···
 #define _ASM_BRANCH_H
 
 #include <asm/ptrace.h>
+#include <asm/inst.h>
 
 static inline int delay_slot(struct pt_regs *regs)
 {
···
 	return regs->cp0_epc + 4;
 }
 
+#define BRANCH_LIKELY_TAKEN 0x0001
+
 extern int __compute_return_epc(struct pt_regs *regs);
+extern int __compute_return_epc_for_insn(struct pt_regs *regs,
+					 union mips_instruction insn);
 
 static inline int compute_return_epc(struct pt_regs *regs)
 {

arch/mips/kernel/branch.c

···
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/signal.h>
+#include <linux/module.h>
 #include <asm/branch.h>
 #include <asm/cpu.h>
 #include <asm/cpu-features.h>
···
 #include <asm/ptrace.h>
 #include <asm/uaccess.h>
 
-/*
- * Compute the return address and do emulate branch simulation, if required.
+/**
+ * __compute_return_epc_for_insn - Computes the return address and do emulate
+ *				    branch simulation, if required.
+ *
+ * @regs:	Pointer to pt_regs
+ * @insn:	branch instruction to decode
+ * @returns:	-EFAULT on error and forces SIGBUS, and on success
+ *		returns 0 or BRANCH_LIKELY_TAKEN as appropriate after
+ *		evaluating the branch.
  */
-int __compute_return_epc(struct pt_regs *regs)
+int __compute_return_epc_for_insn(struct pt_regs *regs,
+				   union mips_instruction insn)
 {
-	unsigned int __user *addr;
 	unsigned int bit, fcr31, dspcontrol;
-	long epc;
-	union mips_instruction insn;
-
-	epc = regs->cp0_epc;
-	if (epc & 3)
-		goto unaligned;
-
-	/*
-	 * Read the instruction
-	 */
-	addr = (unsigned int __user *) epc;
-	if (__get_user(insn.word, addr)) {
-		force_sig(SIGSEGV, current);
-		return -EFAULT;
-	}
+	long epc = regs->cp0_epc;
+	int ret = 0;
 
 	switch (insn.i_format.opcode) {
 	/*
···
 		switch (insn.i_format.rt) {
 	 	case bltz_op:
 		case bltzl_op:
-			if ((long)regs->regs[insn.i_format.rs] < 0)
+			if ((long)regs->regs[insn.i_format.rs] < 0) {
 				epc = epc + 4 + (insn.i_format.simmediate << 2);
-			else
+				if (insn.i_format.rt == bltzl_op)
+					ret = BRANCH_LIKELY_TAKEN;
+			} else
 				epc += 8;
 			regs->cp0_epc = epc;
 			break;
 
 		case bgez_op:
 		case bgezl_op:
-			if ((long)regs->regs[insn.i_format.rs] >= 0)
+			if ((long)regs->regs[insn.i_format.rs] >= 0) {
 				epc = epc + 4 + (insn.i_format.simmediate << 2);
-			else
+				if (insn.i_format.rt == bgezl_op)
+					ret = BRANCH_LIKELY_TAKEN;
+			} else
 				epc += 8;
 			regs->cp0_epc = epc;
 			break;
···
 		case bltzal_op:
 		case bltzall_op:
 			regs->regs[31] = epc + 8;
-			if ((long)regs->regs[insn.i_format.rs] < 0)
+			if ((long)regs->regs[insn.i_format.rs] < 0) {
 				epc = epc + 4 + (insn.i_format.simmediate << 2);
-			else
+				if (insn.i_format.rt == bltzall_op)
+					ret = BRANCH_LIKELY_TAKEN;
+			} else
 				epc += 8;
 			regs->cp0_epc = epc;
 			break;
···
 		case bgezal_op:
 		case bgezall_op:
 			regs->regs[31] = epc + 8;
-			if ((long)regs->regs[insn.i_format.rs] >= 0)
+			if ((long)regs->regs[insn.i_format.rs] >= 0) {
 				epc = epc + 4 + (insn.i_format.simmediate << 2);
-			else
+				if (insn.i_format.rt == bgezall_op)
+					ret = BRANCH_LIKELY_TAKEN;
+			} else
 				epc += 8;
 			regs->cp0_epc = epc;
 			break;
+
 		case bposge32_op:
 			if (!cpu_has_dsp)
 				goto sigill;
···
 	case beq_op:
 	case beql_op:
 		if (regs->regs[insn.i_format.rs] ==
-		    regs->regs[insn.i_format.rt])
+		    regs->regs[insn.i_format.rt]) {
 			epc = epc + 4 + (insn.i_format.simmediate << 2);
-		else
+			if (insn.i_format.rt == beql_op)
+				ret = BRANCH_LIKELY_TAKEN;
+		} else
 			epc += 8;
 		regs->cp0_epc = epc;
 		break;
···
 	case bne_op:
 	case bnel_op:
 		if (regs->regs[insn.i_format.rs] !=
-		    regs->regs[insn.i_format.rt])
+		    regs->regs[insn.i_format.rt]) {
 			epc = epc + 4 + (insn.i_format.simmediate << 2);
-		else
+			if (insn.i_format.rt == bnel_op)
+				ret = BRANCH_LIKELY_TAKEN;
+		} else
 			epc += 8;
 		regs->cp0_epc = epc;
 		break;
···
 	case blez_op: /* not really i_format */
 	case blezl_op:
 		/* rt field assumed to be zero */
-		if ((long)regs->regs[insn.i_format.rs] <= 0)
+		if ((long)regs->regs[insn.i_format.rs] <= 0) {
 			epc = epc + 4 + (insn.i_format.simmediate << 2);
-		else
+			if (insn.i_format.rt == bnel_op)
+				ret = BRANCH_LIKELY_TAKEN;
+		} else
 			epc += 8;
 		regs->cp0_epc = epc;
 		break;
···
 	case bgtz_op:
 	case bgtzl_op:
 		/* rt field assumed to be zero */
-		if ((long)regs->regs[insn.i_format.rs] > 0)
+		if ((long)regs->regs[insn.i_format.rs] > 0) {
 			epc = epc + 4 + (insn.i_format.simmediate << 2);
-		else
+			if (insn.i_format.rt == bnel_op)
+				ret = BRANCH_LIKELY_TAKEN;
+		} else
 			epc += 8;
 		regs->cp0_epc = epc;
 		break;
···
 		switch (insn.i_format.rt & 3) {
 		case 0:	/* bc1f */
 		case 2:	/* bc1fl */
-			if (~fcr31 & (1 << bit))
+			if (~fcr31 & (1 << bit)) {
 				epc = epc + 4 + (insn.i_format.simmediate << 2);
-			else
+				if (insn.i_format.rt == 2)
+					ret = BRANCH_LIKELY_TAKEN;
+			} else
 				epc += 8;
 			regs->cp0_epc = epc;
 			break;
 
 		case 1:	/* bc1t */
 		case 3:	/* bc1tl */
-			if (fcr31 & (1 << bit))
+			if (fcr31 & (1 << bit)) {
 				epc = epc + 4 + (insn.i_format.simmediate << 2);
-			else
+				if (insn.i_format.rt == 3)
+					ret = BRANCH_LIKELY_TAKEN;
+			} else
 				epc += 8;
 			regs->cp0_epc = epc;
 			break;
···
 #endif
 	}
 
-	return 0;
+	return ret;
+
+sigill:
+	printk("%s: DSP branch but not DSP ASE - sending SIGBUS.\n", current->comm);
+	force_sig(SIGBUS, current);
+	return -EFAULT;
+}
+EXPORT_SYMBOL_GPL(__compute_return_epc_for_insn);
+
+int __compute_return_epc(struct pt_regs *regs)
+{
+	unsigned int __user *addr;
+	long epc;
+	union mips_instruction insn;
+
+	epc = regs->cp0_epc;
+	if (epc & 3)
+		goto unaligned;
+
+	/*
+	 * Read the instruction
+	 */
+	addr = (unsigned int __user *) epc;
+	if (__get_user(insn.word, addr)) {
+		force_sig(SIGSEGV, current);
+		return -EFAULT;
+	}
+
+	return __compute_return_epc_for_insn(regs, insn);
 
 unaligned:
 	printk("%s: unaligned epc - sending SIGBUS.\n", current->comm);
 	force_sig(SIGBUS, current);
 	return -EFAULT;
 
-sigill:
-	printk("%s: DSP branch but not DSP ASE - sending SIGBUS.\n", current->comm);
-	force_sig(SIGBUS, current);
-	return -EFAULT;
 }

arch/mips/math-emu/cp1emu.c

···
 		 */
 		emulpc = xcp->cp0_epc + 4;	/* Snapshot emulation target */
 
-		if (__compute_return_epc(xcp)) {
+		if (__compute_return_epc(xcp) < 0) {
 #ifdef CP1DBG
 			printk("failed to emulate branch at %p\n",
 				(void *) (xcp->cp0_epc));