LoongArch: Strip guess unwinder out from prologue unwinder

The prolugue unwinder rely on symbol info. When PC is not in kernel text
address, it cannot find relative symbol info and it will be broken. The
guess unwinder will be used in this case. And the guess unwinder code in
prolugue unwinder is redundant. Strip it out and set the unwinder type
in unwind_state. Make guess_unwinder::unwind_next_frame() as default way
when other unwinders cannot unwind in some extreme case.
Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/unwind.h

@@ -27,6 +27,8 @@ struct unwind_state {
 	unsigned long sp, pc, ra;
 };
 
+bool default_next_frame(struct unwind_state *state);
+
 void unwind_start(struct unwind_state *state,
 		  struct task_struct *task, struct pt_regs *regs);
 bool unwind_next_frame(struct unwind_state *state);
@@ -50,4 +52,31 @@ static inline unsigned long unwind_graph_addr(struct unwind_state *state,
 	return ftrace_graph_ret_addr(state->task, &state->graph_idx,
 				     pc, (unsigned long *)(cfa - GRAPH_FAKE_OFFSET));
 }
+
+static __always_inline void __unwind_start(struct unwind_state *state,
+					struct task_struct *task, struct pt_regs *regs)
+{
+	memset(state, 0, sizeof(*state));
+	if (regs) {
+		state->sp = regs->regs[3];
+		state->pc = regs->csr_era;
+		state->ra = regs->regs[1];
+	} else if (task && task != current) {
+		state->sp = thread_saved_fp(task);
+		state->pc = thread_saved_ra(task);
+		state->ra = 0;
+	} else {
+		state->sp = (unsigned long)__builtin_frame_address(0);
+		state->pc = (unsigned long)__builtin_return_address(0);
+		state->ra = 0;
+	}
+	state->task = task;
+	get_stack_info(state->sp, state->task, &state->stack_info);
+	state->pc = unwind_graph_addr(state, state->pc, state->sp);
+}
+
+static __always_inline unsigned long __unwind_get_return_address(struct unwind_state *state)
+{
+	return unwind_done(state) ? 0 : state->pc;
+}
 #endif /* _ASM_UNWIND_H */

arch/loongarch/kernel/Makefile

@@ -8,7 +8,7 @@ extra-y		:= vmlinux.lds
 obj-y		+= head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
 		   traps.o irq.o idle.o process.o dma.o mem.o io.o reset.o switch.o \
 		   elf.o syscall.o signal.o time.o topology.o inst.o ptrace.o vdso.o \
-		   alternative.o unaligned.o
+		   alternative.o unaligned.o unwind.o
 
 obj-$(CONFIG_ACPI)		+= acpi.o
 obj-$(CONFIG_EFI) 		+= efi.o

arch/loongarch/kernel/traps.c

@@ -72,9 +72,6 @@ static void show_backtrace(struct task_struct *task, const struct pt_regs *regs,
 	if (!task)
 		task = current;
 
-	if (user_mode(regs))
-		state.type = UNWINDER_GUESS;
-
 	printk("%sCall Trace:", loglvl);
 	for (unwind_start(&state, task, pregs);
 	      !unwind_done(&state); unwind_next_frame(&state)) {

arch/loongarch/kernel/unwind.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022-2023 Loongson Technology Corporation Limited
+ */
+#include <linux/kernel.h>
+#include <linux/ftrace.h>
+
+#include <asm/unwind.h>
+
+bool default_next_frame(struct unwind_state *state)
+{
+	struct stack_info *info = &state->stack_info;
+	unsigned long addr;
+
+	if (unwind_done(state))
+		return false;
+
+	do {
+		for (state->sp += sizeof(unsigned long);
+		     state->sp < info->end; state->sp += sizeof(unsigned long)) {
+			addr = *(unsigned long *)(state->sp);
+			state->pc = unwind_graph_addr(state, addr, state->sp + 8);
+			if (__kernel_text_address(state->pc))
+				return true;
+		}
+
+		state->sp = info->next_sp;
+
+	} while (!get_stack_info(state->sp, state->task, info));
+
+	return false;
+}

arch/loongarch/kernel/unwind_guess.c

@@ -2,41 +2,18 @@
 /*
  * Copyright (C) 2022 Loongson Technology Corporation Limited
  */
-#include <linux/kernel.h>
-#include <linux/ftrace.h>
-
 #include <asm/unwind.h>
 
 unsigned long unwind_get_return_address(struct unwind_state *state)
 {
-	if (unwind_done(state))
-		return 0;
-
-	return state->pc;
+	return __unwind_get_return_address(state);
 }
 EXPORT_SYMBOL_GPL(unwind_get_return_address);
 
 void unwind_start(struct unwind_state *state, struct task_struct *task,
 		    struct pt_regs *regs)
 {
-	memset(state, 0, sizeof(*state));
-
-	if (regs) {
-		state->sp = regs->regs[3];
-		state->pc = regs->csr_era;
-	} else if (task && task != current) {
-		state->sp = thread_saved_fp(task);
-		state->pc = thread_saved_ra(task);
-	} else {
-		state->sp = (unsigned long)__builtin_frame_address(0);
-		state->pc = (unsigned long)__builtin_return_address(0);
-	}
-
-	state->task = task;
-	state->first = true;
-	state->pc = unwind_graph_addr(state, state->pc, state->sp);
-	get_stack_info(state->sp, state->task, &state->stack_info);
-
+	__unwind_start(state, task, regs);
 	if (!unwind_done(state) && !__kernel_text_address(state->pc))
 		unwind_next_frame(state);
 }
@@ -44,29 +21,6 @@ EXPORT_SYMBOL_GPL(unwind_start);
 
 bool unwind_next_frame(struct unwind_state *state)
 {
-	struct stack_info *info = &state->stack_info;
-	unsigned long addr;
-
-	if (unwind_done(state))
-		return false;
-
-	if (state->first)
-		state->first = false;
-
-	do {
-		for (state->sp += sizeof(unsigned long);
-		     state->sp < info->end;
-		     state->sp += sizeof(unsigned long)) {
-			addr = *(unsigned long *)(state->sp);
-			state->pc = unwind_graph_addr(state, addr, state->sp + 8);
-			if (__kernel_text_address(state->pc))
-				return true;
-		}
-
-		state->sp = info->next_sp;
-
-	} while (!get_stack_info(state->sp, state->task, info));
-
-	return false;
+	return default_next_frame(state);
 }
 EXPORT_SYMBOL_GPL(unwind_next_frame);

arch/loongarch/kernel/unwind_prologue.c

@@ -19,32 +19,14 @@ static inline void unwind_state_fixup(struct unwind_state *state)
 #endif
 }
 
-unsigned long unwind_get_return_address(struct unwind_state *state)
-{
-	if (unwind_done(state))
-		return 0;
-
-	return state->pc;
-}
-EXPORT_SYMBOL_GPL(unwind_get_return_address);
-
-static bool unwind_by_guess(struct unwind_state *state)
-{
-	struct stack_info *info = &state->stack_info;
-	unsigned long addr;
-
-	for (state->sp += sizeof(unsigned long);
-	     state->sp < info->end;
-	     state->sp += sizeof(unsigned long)) {
-		addr = *(unsigned long *)(state->sp);
-		state->pc = unwind_graph_addr(state, addr, state->sp + 8);
-		if (__kernel_text_address(state->pc))
-			return true;
-	}
-
-	return false;
-}
-
+/*
+ * LoongArch function prologue is like follows,
+ *     [instructions not use stack var]
+ *     addi.d sp, sp, -imm
+ *     st.d   xx, sp, offset <- save callee saved regs and
+ *     st.d   yy, sp, offset    save ra if function is nest.
+ *     [others instructions]
+ */
 static bool unwind_by_prologue(struct unwind_state *state)
 {
 	long frame_ra = -1;
@@ -91,6 +73,10 @@ static bool unwind_by_prologue(struct unwind_state *state)
 		ip++;
 	}
 
+	/*
+	 * Can't find stack alloc action, PC may be in a leaf function. Only the
+	 * first being true is reasonable, otherwise indicate analysis is broken.
+	 */
 	if (!frame_size) {
 		if (state->first)
 			goto first;
@@ -108,6 +94,7 @@ static bool unwind_by_prologue(struct unwind_state *state)
 		ip++;
 	}
 
+	/* Can't find save $ra action, PC may be in a leaf function, too. */
 	if (frame_ra < 0) {
 		if (state->first) {
 			state->sp = state->sp + frame_size;
@@ -116,96 +103,48 @@ static bool unwind_by_prologue(struct unwind_state *state)
 		return false;
 	}
 
-	if (state->first)
-		state->first = false;
-
 	state->pc = *(unsigned long *)(state->sp + frame_ra);
 	state->sp = state->sp + frame_size;
 	goto out;
 
 first:
-	state->first = false;
-	if (state->pc == state->ra)
-		return false;
-
 	state->pc = state->ra;
 
 out:
+	state->first = false;
 	unwind_state_fixup(state);
 	return !!__kernel_text_address(state->pc);
 }
 
-void unwind_start(struct unwind_state *state, struct task_struct *task,
-		    struct pt_regs *regs)
-{
-	memset(state, 0, sizeof(*state));
-	state->type = UNWINDER_PROLOGUE;
-
-	if (regs) {
-		state->sp = regs->regs[3];
-		state->pc = regs->csr_era;
-		state->ra = regs->regs[1];
-		if (!__kernel_text_address(state->pc))
-			state->type = UNWINDER_GUESS;
-	} else if (task && task != current) {
-		state->sp = thread_saved_fp(task);
-		state->pc = thread_saved_ra(task);
-		state->ra = 0;
-	} else {
-		state->sp = (unsigned long)__builtin_frame_address(0);
-		state->pc = (unsigned long)__builtin_return_address(0);
-		state->ra = 0;
-	}
-
-	state->task = task;
-	state->first = true;
-	state->pc = unwind_graph_addr(state, state->pc, state->sp);
-	get_stack_info(state->sp, state->task, &state->stack_info);
-
-	if (!unwind_done(state) && !__kernel_text_address(state->pc))
-		unwind_next_frame(state);
-}
-EXPORT_SYMBOL_GPL(unwind_start);
-
-bool unwind_next_frame(struct unwind_state *state)
+static bool next_frame(struct unwind_state *state)
 {
-	struct stack_info *info = &state->stack_info;
-	struct pt_regs *regs;
 	unsigned long pc;
+	struct pt_regs *regs;
+	struct stack_info *info = &state->stack_info;
 
 	if (unwind_done(state))
 		return false;
 
 	do {
-		switch (state->type) {
-		case UNWINDER_GUESS:
-			state->first = false;
-			if (unwind_by_guess(state))
-				return true;
-			break;
-
-		case UNWINDER_PROLOGUE:
-			if (unwind_by_prologue(state)) {
-				state->pc = unwind_graph_addr(state, state->pc, state->sp);
-				return true;
-			}
+		if (unwind_by_prologue(state)) {
+			state->pc = unwind_graph_addr(state, state->pc, state->sp);
+			return true;
+		}
 
-			if (info->type == STACK_TYPE_IRQ &&
-				info->end == state->sp) {
-				regs = (struct pt_regs *)info->next_sp;
-				pc = regs->csr_era;
+		if (info->type == STACK_TYPE_IRQ && info->end == state->sp) {
+			regs = (struct pt_regs *)info->next_sp;
+			pc = regs->csr_era;
 
-				if (user_mode(regs) || !__kernel_text_address(pc))
-					return false;
+			if (user_mode(regs) || !__kernel_text_address(pc))
+				return false;
 
-				state->first = true;
-				state->ra = regs->regs[1];
-				state->sp = regs->regs[3];
-				state->pc = pc;
-				get_stack_info(state->sp, state->task, info);
+			state->first = true;
+			state->pc = pc;
+			state->ra = regs->regs[1];
+			state->sp = regs->regs[3];
+			get_stack_info(state->sp, state->task, info);
 
-				return true;
-			}
+			return true;
 		}
 
 		state->sp = info->next_sp;
@@ -214,4 +153,36 @@ bool unwind_next_frame(struct unwind_state *state)
 
 	return false;
 }
+
+unsigned long unwind_get_return_address(struct unwind_state *state)
+{
+	return __unwind_get_return_address(state);
+}
+EXPORT_SYMBOL_GPL(unwind_get_return_address);
+
+void unwind_start(struct unwind_state *state, struct task_struct *task,
+		    struct pt_regs *regs)
+{
+	__unwind_start(state, task, regs);
+	state->type = UNWINDER_PROLOGUE;
+	state->first = true;
+
+	/*
+	 * The current PC is not kernel text address, we cannot find its
+	 * relative symbol. Thus, prologue analysis will be broken. Luckily,
+	 * we can use the default_next_frame().
+	 */
+	if (!__kernel_text_address(state->pc)) {
+		state->type = UNWINDER_GUESS;
+		if (!unwind_done(state))
+			unwind_next_frame(state);
+	}
+}
+EXPORT_SYMBOL_GPL(unwind_start);
+
+bool unwind_next_frame(struct unwind_state *state)
+{
+	return state->type == UNWINDER_PROLOGUE ?
+			next_frame(state) : default_next_frame(state);
+}
 EXPORT_SYMBOL_GPL(unwind_next_frame);