Subject: [RFC PATCH v1 7/9] arm64: dwarf: Implement unwind hints

From: [email-masked]
Date: Thu, 7 Apr 2022 15:25:16 -0500

From: "Madhavan T. Venkataraman" <[email-masked]>

DWARF information is not generated for assembly functions. However, we
would like to unwind through some of these functions as they may occur
frequently in a stacktrace. Implement unwind hints for this purpose.

An unwind hint placed after a desired call instruction creates a
DWARF rule for that instruction address in a special section called
".discard.unwind_hints". objtool merges these DWARF rules along with
the ones generated by the compiler for C functions.

Add unwind hints for the following cases:

Exception handlers
==================

When an exception is taken in the kernel, an exception frame gets
pushed on the stack. To be able to unwind through the special frame,
place an unwind hint in the exception handler macro.

Interrupt handlers
==================

When an interrupt happens, the exception handler switches to an IRQ
stack and calls the interrupt handler. Place an unwind hint after
the call to the interrupt handler so the unwinder can unwind through
the switched stacks.

FTrace stub
===========

ftrace_common() calls ftrace_stub(). The stub is patched with tracer
functions when tracing is enabled. Place an unwind hint right after the
call to the stub function so that stack traces taken from within a
tracer function can correctly unwind to the caller.

FTrace Graph Caller
===================

ftrace_graph_caller() calls a function, prepare_ftrace_return(), to prepare
for graph tracing. Place an unwind hint after the call so a stack trace
taken from within the prepare function can correctly unwind to the caller.

FTrace callsite
===============

ftrace_regs_entry() sets up two stackframes - one for the callsite and
one for the ftrace entry code. Unwind hints have been placed for the
ftrace entry code above. We need an unwind hint for the callsite. Callsites
are numerous. But the unwind hint required for all the callsites is the
same. Define a dummy function with the callsite unwind hint like this:

SYM_CODE_START(ftrace_callsite)
        unwind_hint 4, 16, -16                  // for the callsite
        ret
SYM_CODE_END(ftrace_callsite)

When the unwinder comes across an ftrace entry, it will change the PC
that it needs to lookup to ftrace_callsite() to obtain the unwind
hint for the callsite like this:

#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
	if (is_ftrace_entry(frame->prev_pc))
		lookup_pc = (unsigned long)&ftrace_callsite;
#endif

This way, the unwinder can unwind through an ftrace callsite.

Kretprobe Trampoline
====================

This trampoline sets up pt_regs on the stack and sets up a synthetic
frame in the pt_regs. Place an unwind hint where trampoline_probe_handler()
returns to __kretprobe_trampoline to unwind through the synthetic frame.

is_ftrace_entry()
=================

The code for this function has been borrowed from Suraj Jitindar Singh.

Signed-off-by: Madhavan T. Venkataraman <[email-masked]>
Signed-off-by: Suraj Jitindar Singh <[email-masked]>
---
 arch/arm64/include/asm/stacktrace.h           |  3 +
 arch/arm64/include/asm/unwind_hints.h         | 28 ++++++++
 arch/arm64/kernel/entry-ftrace.S              | 23 ++++++
 arch/arm64/kernel/entry.S                     |  3 +
 arch/arm64/kernel/ftrace.c                    | 16 +++++
 arch/arm64/kernel/probes/kprobes_trampoline.S |  2 +
 arch/arm64/kernel/stacktrace.c                | 70 +++++++++++++++++--
 include/linux/dwarf.h                         | 10 ++-
 include/linux/ftrace.h                        |  4 ++
 tools/include/linux/dwarf.h                   | 10 ++-
 tools/objtool/dwarf_parse.c                   | 59 +++++++++++++++-
 tools/objtool/dwarf_rules.c                   | 65 ++++++++++++++++-
 tools/objtool/include/objtool/dwarf_def.h     | 10 +++
 13 files changed, 294 insertions(+), 9 deletions(-)
 create mode 100644 arch/arm64/include/asm/unwind_hints.h

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index 93adee4219ed..90392097a768 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -46,6 +46,7 @@ struct stack_info {
  * @prev_fp:     The fp that pointed to this frame record, or a synthetic value
  *               of 0. This is used to ensure that within a stack, each
  *               subsequent frame record is at an increasing address.
+ * @prev_pc:     The pc in the previous frame.
  * @prev_type:   The type of stack this frame record was on, or a synthetic
  *               value of STACK_TYPE_UNKNOWN. This is used to detect a
  *               transition from one stack to another.
@@ -58,11 +59,13 @@ struct stackframe {
 	unsigned long pc;
 	DECLARE_BITMAP(stacks_done, __NR_STACK_TYPES);
 	unsigned long prev_fp;
+	unsigned long prev_pc;
 	enum stack_type prev_type;
 #ifdef CONFIG_KRETPROBES
 	struct llist_node *kr_cur;
 #endif
 	bool reliable;
+	bool synthetic_frame;
 };
 
 extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
diff --git a/arch/arm64/include/asm/unwind_hints.h b/arch/arm64/include/asm/unwind_hints.h
new file mode 100644
index 000000000000..b2312bfaf201
--- /dev/null
+++ b/arch/arm64/include/asm/unwind_hints.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_ARM64_DWARF_UNWIND_HINTS_H
+#define _ASM_ARM64_DWARF_UNWIND_HINTS_H
+
+#ifdef __ASSEMBLY__
+
+#ifdef CONFIG_DWARF_FP
+
+.macro dwarf_unwind_hint, size, sp_offset, fp_offset
+.Ldwarf_hint_pc_\@ :	.pushsection .discard.unwind_hints
+			/* struct dwarf_unwind_hint */
+			.long	0, (.Ldwarf_hint_pc_\@ - .)
+			.int	\size
+			.short	\sp_offset
+			.short	\fp_offset
+			.popsection
+.endm
+
+#else
+
+.macro dwarf_unwind_hint, size, sp_offset, fp_offset
+.endm
+
+#endif
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_ARM64_DWARF_UNWIND_HINTS_H */
diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
index 8cf970d219f5..f82dad9260ec 100644
--- a/arch/arm64/kernel/entry-ftrace.S
+++ b/arch/arm64/kernel/entry-ftrace.S
@@ -11,6 +11,7 @@
 #include <asm/assembler.h>
 #include <asm/ftrace.h>
 #include <asm/insn.h>
+#include <asm/unwind_hints.h>
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
 /*
@@ -99,7 +100,14 @@ SYM_CODE_START(ftrace_common)
 	mov	x3, sp				// regs
 
 SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
+	/*
+	 * Tracer functions are patched at ftrace_stub. Stack traces
+	 * taken from tracer functions will end up here. Place an
+	 * unwind hint based on the stackframe setup in ftrace_regs_entry.
+	 */
 	bl	ftrace_stub
+SYM_INNER_LABEL(ftrace_call_entry, SYM_L_GLOBAL)
+	dwarf_unwind_hint 4, PT_REGS_SIZE, (S_STACKFRAME - PT_REGS_SIZE)
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 SYM_INNER_LABEL(ftrace_graph_call, SYM_L_GLOBAL) // ftrace_graph_caller();
@@ -138,10 +146,25 @@ SYM_CODE_START(ftrace_graph_caller)
 	add	x1, sp, #S_LR			// parent_ip (callsite's LR)
 	ldr	x2, [sp, #PT_REGS_SIZE]	   	// parent fp (callsite's FP)
 	bl	prepare_ftrace_return
+SYM_INNER_LABEL(ftrace_graph_caller_entry, SYM_L_GLOBAL)
+	dwarf_unwind_hint 4, PT_REGS_SIZE, (S_STACKFRAME - PT_REGS_SIZE)
 	b	ftrace_common_return
 SYM_CODE_END(ftrace_graph_caller)
 #endif
 
+/*
+ * ftrace_regs_entry() sets up two stackframes - one for the callsite and
+ * one for the ftrace entry code. Unwind hints have been placed for the
+ * ftrace entry code above. We need an unwind hint for the callsite. Callsites
+ * are numerous. But the unwind hint required for all the callsites is the
+ * same. Define a dummy function here with the callsite unwind hint for the
+ * benefit of the unwinder.
+ */
+SYM_CODE_START(ftrace_callsite)
+	dwarf_unwind_hint 4, 16, -16			// for the callsite
+	ret
+SYM_CODE_END(ftrace_callsite)
+
 #else /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
 
 /*
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 2f69ae43941d..a188e3a3068d 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -28,6 +28,7 @@
 #include <asm/thread_info.h>
 #include <asm/asm-uaccess.h>
 #include <asm/unistd.h>
+#include <asm/unwind_hints.h>
 
 	.macro	clear_gp_regs
 	.irp	n,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29
@@ -551,6 +552,7 @@ SYM_CODE_START_LOCAL(el\el\ht\()_\regsize\()_\label)
 	.if \el == 0
 	b	ret_to_user
 	.else
+	dwarf_unwind_hint 4, PT_REGS_SIZE, (S_STACKFRAME - PT_REGS_SIZE)
 	b	ret_to_kernel
 	.endif
 SYM_CODE_END(el\el\ht\()_\regsize\()_\label)
@@ -783,6 +785,7 @@ SYM_FUNC_START(call_on_irq_stack)
 	/* Move to the new stack and call the function there */
 	mov	sp, x16
 	blr	x1
+	dwarf_unwind_hint 4, 0, -16
 
 	/*
 	 * Restore the SP from the FP, and restore the FP and LR from the frame
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index 4506c4a90ac1..ec9a00d714e5 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -299,3 +299,19 @@ int ftrace_disable_ftrace_graph_caller(void)
 }
 #endif /* CONFIG_DYNAMIC_FTRACE */
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+
+bool is_ftrace_entry(unsigned long pc)
+{
+	if (pc == (unsigned long)&ftrace_call_entry)
+		return true;
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+	if (pc == (unsigned long)&ftrace_graph_caller_entry)
+		return true;
+#endif
+	return false;
+}
+
+#endif
diff --git a/arch/arm64/kernel/probes/kprobes_trampoline.S b/arch/arm64/kernel/probes/kprobes_trampoline.S
index 9a6499bed58b..325932b49857 100644
--- a/arch/arm64/kernel/probes/kprobes_trampoline.S
+++ b/arch/arm64/kernel/probes/kprobes_trampoline.S
@@ -6,6 +6,7 @@
 #include <linux/linkage.h>
 #include <asm/asm-offsets.h>
 #include <asm/assembler.h>
+#include <asm/unwind_hints.h>
 
 	.text
 
@@ -71,6 +72,7 @@ SYM_CODE_START(__kretprobe_trampoline)
 
 	mov x0, sp
 	bl trampoline_probe_handler
+	dwarf_unwind_hint 4, PT_REGS_SIZE, (S_FP - PT_REGS_SIZE)
 	/*
 	 * Replace trampoline address in lr with actual orig_ret_addr return
 	 * address.
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index f9ef7a3e7296..e1a9b695b6ae 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -19,6 +19,16 @@
 #include <asm/stack_pointer.h>
 #include <asm/stacktrace.h>
 
+static inline bool is_synthetic_frame(struct dwarf_rule *rule)
+{
+	short regs_size = sizeof(struct pt_regs);
+	short frame_offset = offsetof(struct pt_regs, stackframe);
+
+	return rule->hint &&
+	       rule->sp_offset == regs_size &&
+	       rule->fp_offset == (frame_offset - regs_size);
+}
+
 /*
  * AArch64 PCS assigns the frame pointer to x29.
  *
@@ -40,6 +50,7 @@ void start_backtrace(struct stackframe *frame, unsigned long fp,
 	struct dwarf_rule *rule;
 
 	frame->reliable = true;
+	frame->synthetic_frame = false;
 	frame->fp = fp;
 	frame->pc = pc;
 	frame->sp = 0;
@@ -48,10 +59,12 @@ void start_backtrace(struct stackframe *frame, unsigned long fp,
 	 * based on the frame pointer passed in.
 	 */
 	rule = dwarf_lookup(pc);
-	if (rule)
+	if (rule) {
 		frame->sp = fp - rule->fp_offset;
-	else
+		frame->synthetic_frame = is_synthetic_frame(rule);
+	} else {
 		frame->reliable = false;
+	}
 
 #ifdef CONFIG_KRETPROBES
 	frame->kr_cur = NULL;
@@ -125,6 +138,7 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 	 * Record this frame record's values and location. The prev_fp and
 	 * prev_type are only meaningful to the next unwind_frame() invocation.
 	 */
+	frame->prev_pc = frame->pc;
 	frame->fp = READ_ONCE_NOCHECK(*(unsigned long *)(fp));
 	frame->pc = READ_ONCE_NOCHECK(*(unsigned long *)(fp + 8));
 	frame->prev_fp = fp;
@@ -168,11 +182,59 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 		return 0;
 	}
 	lookup_pc = frame->pc;
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+	if (is_ftrace_entry(frame->prev_pc))
+		lookup_pc = (unsigned long)&ftrace_callsite;
+#endif
 
 	rule = dwarf_lookup(lookup_pc);
 	if (!rule) {
-		frame->reliable = false;
-		return 0;
+		if (!frame->synthetic_frame) {
+			/*
+			 * If the last instruction in a function happens to be
+			 * a call instruction, the return address would fall
+			 * outside of the function. This could be that case.
+			 * This can happen, for instance, if the called function
+			 * is a "noreturn" function. The compiler can optimize
+			 * away the instructions after the call. So, adjust the
+			 * PC so it falls inside the function and retry.
+			 *
+			 * However, if the previous frame was a synthetic frame
+			 * (e.g., interrupt/exception), the return PC in the
+			 * synthetic frame may not be the location after a call
+			 * instruction at all. In such cases, we don't want to
+			 * adjust the PC and retry.
+			 *
+			 * If this succeeds, adjust the frame pc below.
+			 */
+			lookup_pc -= 4;
+			rule = dwarf_lookup(lookup_pc);
+		}
+		if (!rule) {
+			frame->reliable = false;
+			return 0;
+		}
+		frame->pc = lookup_pc;
+	}
+	frame->synthetic_frame = false;
+
+	if (rule->hint) {
+		if (!rule->sp_offset && !rule->fp_offset) {
+			frame->reliable = false;
+			return 0;
+		}
+		frame->synthetic_frame = is_synthetic_frame(rule);
+		if (!rule->sp_offset) {
+			/*
+			 * This is the unwind hint in call_on_irq_stack(). The
+			 * SP at this point is in the IRQ stack. The CFA and
+			 * the FP are on the normal stack. To compute the CFA,
+			 * rely on the unwind hint, assume that the FP is good
+			 * and just compute the CFA from it.
+			 */
+			frame->sp = frame->fp - rule->fp_offset;
+			return 0;
+		}
 	}
 
 	frame->sp += rule->sp_offset;
diff --git a/include/linux/dwarf.h b/include/linux/dwarf.h
index aa44a414b0b6..fea42feb48a4 100644
--- a/include/linux/dwarf.h
+++ b/include/linux/dwarf.h
@@ -34,9 +34,17 @@
  *	This contains an array of starting PCs, one for each rule.
  */
 struct dwarf_rule {
-	unsigned int	size:30;
+	unsigned int	size:29;
 	unsigned int	sp_saved:1;
 	unsigned int	fp_saved:1;
+	unsigned int	hint:1;
+	short		sp_offset;
+	short		fp_offset;
+};
+
+struct dwarf_unwind_hint {
+	unsigned long	pc;
+	unsigned int	size;
 	short		sp_offset;
 	short		fp_offset;
 };
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 9999e29187de..dbcd95053425 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -604,6 +604,10 @@ extern int ftrace_update_ftrace_func(ftrace_func_t func);
 extern void ftrace_caller(void);
 extern void ftrace_regs_caller(void);
 extern void ftrace_call(void);
+extern void ftrace_call_entry(void);
+extern void ftrace_graph_caller_entry(void);
+extern void ftrace_callsite(void);
+extern bool is_ftrace_entry(unsigned long pc);
 extern void ftrace_regs_call(void);
 extern void mcount_call(void);
 
diff --git a/tools/include/linux/dwarf.h b/tools/include/linux/dwarf.h
index aa44a414b0b6..fea42feb48a4 100644
--- a/tools/include/linux/dwarf.h
+++ b/tools/include/linux/dwarf.h
@@ -34,9 +34,17 @@
  *	This contains an array of starting PCs, one for each rule.
  */
 struct dwarf_rule {
-	unsigned int	size:30;
+	unsigned int	size:29;
 	unsigned int	sp_saved:1;
 	unsigned int	fp_saved:1;
+	unsigned int	hint:1;
+	short		sp_offset;
+	short		fp_offset;
+};
+
+struct dwarf_unwind_hint {
+	unsigned long	pc;
+	unsigned int	size;
 	short		sp_offset;
 	short		fp_offset;
 };
diff --git a/tools/objtool/dwarf_parse.c b/tools/objtool/dwarf_parse.c
index d5ac5630fbba..7a5af7d5c2be 100644
--- a/tools/objtool/dwarf_parse.c
+++ b/tools/objtool/dwarf_parse.c
@@ -18,6 +18,10 @@
 struct objtool_file		*dwarf_file;
 struct section			*debug_frame;
 
+struct section			*unwind_hints;
+struct section			*unwind_hints_reloc;
+static int			nhints;
+
 struct cie			*cies, *cur_cie;
 struct fde			*fdes, *cur_fde;
 
@@ -31,6 +35,8 @@ static unsigned int		offset_size;
 static u64			entry_length;
 static unsigned char		*saved_start;
 
+static int	dwarf_add_hints(void);
+
 /*
  * Parse and create a new CIE.
  */
@@ -270,7 +276,7 @@ int dwarf_parse(struct objtool_file *file)
 	 */
 	debug_frame = find_section_by_name(file->elf, ".debug_frame");
 	if (!debug_frame)
-		return 0;
+		goto hints;
 
 	dwarf_alloc_init();
 
@@ -290,5 +296,56 @@ int dwarf_parse(struct objtool_file *file)
 	 * Run all the DWARF instructions in the CIEs and FDEs.
 	 */
 	dwarf_parse_instructions();
+hints:
+	unwind_hints = find_section_by_name(file->elf, ".discard.unwind_hints");
+	if (!unwind_hints)
+		return 0;
+
+	unwind_hints_reloc = unwind_hints->reloc;
+
+	if (unwind_hints->sh.sh_size % sizeof(struct dwarf_unwind_hint)) {
+		WARN("struct dwarf_unwind_hint size mismatch");
+		return -1;
+	}
+	nhints = unwind_hints->sh.sh_size / sizeof(struct dwarf_unwind_hint);
+
+	return dwarf_add_hints();
+}
+
+/*
+ * Errors in this function are non-fatal. The worst case is that the
+ * unwind hints will not be part of the dwarf rules. That is all.
+ */
+static int dwarf_add_hints(void)
+{
+	struct dwarf_unwind_hint	*hints, *hint;
+	struct reloc			*reloc;
+	int				i;
+	struct section			*sec;
+
+	if (!nhints)
+		return 0;
+
+	hints = (struct dwarf_unwind_hint *) unwind_hints->data->d_buf;
+	for (i = 0; i < nhints; i++) {
+		hint = &hints[i];
+		if (unwind_hints_reloc) {
+			reloc = find_reloc_by_dest_range(dwarf_file->elf,
+							 unwind_hints,
+							 i * sizeof(*hint),
+							 sizeof(*hint));
+			if (!reloc) {
+				WARN("can't find reloc for hint %d", i);
+				return 0;
+			}
+			hint->pc = reloc->addend;
+			sec = reloc->sym->sec;
+		} else {
+			sec = find_section_by_name(dwarf_file->elf, ".text");
+			if (!sec)
+				return 0;
+		}
+		dwarf_hint_add(sec, hint);
+	}
 	return 0;
 }
diff --git a/tools/objtool/dwarf_rules.c b/tools/objtool/dwarf_rules.c
index a118b392aac8..632776463c23 100644
--- a/tools/objtool/dwarf_rules.c
+++ b/tools/objtool/dwarf_rules.c
@@ -19,6 +19,9 @@ struct section			*dwarf_pcs_sec;
 static struct fde_entry		*cur_entry;
 static int			nentries;
 
+static struct hint_entry	*hint_list;
+static int			nhints;
+
 static int dwarf_rule_insert(struct fde *fde, unsigned long addr,
 			     struct rule *sp_rule, struct rule *fp_rule);
 
@@ -51,6 +54,25 @@ int dwarf_rule_add(struct fde *fde, unsigned long addr,
 	return dwarf_rule_insert(fde, addr, sp_rule, fp_rule);
 }
 
+int dwarf_hint_add(struct section *section, struct dwarf_unwind_hint *hint)
+{
+	struct hint_entry	*entry;
+
+	entry = dwarf_alloc(sizeof(*entry));
+	if (!entry)
+		return -1;
+
+	/* Add the entry to the hints list. */
+	entry->next = hint_list;
+	hint_list = entry;
+
+	entry->section = section;
+	entry->hint = hint;
+	nhints++;
+
+	return 0;
+}
+
 void dwarf_rule_next(struct fde *fde, unsigned long addr)
 {
 	if (cur_entry) {
@@ -119,6 +141,7 @@ static int dwarf_rule_write(struct elf *elf, struct fde *fde,
 	rule.size = entry->size;
 	rule.sp_saved = entry->sp_rule.saved;
 	rule.fp_saved = entry->fp_rule.saved;
+	rule.hint = 0;
 	rule.sp_offset = entry->sp_rule.offset;
 	rule.fp_offset = entry->fp_rule.offset;
 
@@ -135,11 +158,42 @@ static int dwarf_rule_write(struct elf *elf, struct fde *fde,
 	return 0;
 }
 
+static int dwarf_hint_write(struct elf *elf, struct hint_entry *hentry,
+			    unsigned int index)
+{
+	struct dwarf_rule		rule, *drule;
+	struct dwarf_unwind_hint	*hint = hentry->hint;
+
+	/*
+	 * Encode the SP and FP rules from the entry into a single dwarf_rule
+	 * for the kernel's benefit. Copy it into .dwarf_rules.
+	 */
+	rule.size = hint->size;
+	rule.sp_saved = 0;
+	rule.fp_saved = 1;
+	rule.hint = 1;
+	rule.sp_offset = hint->sp_offset;
+	rule.fp_offset = hint->fp_offset;
+
+	drule = (struct dwarf_rule *) dwarf_rules_sec->data->d_buf + index;
+	memcpy(drule, &rule, sizeof(rule));
+
+	/* Add relocation information for the code range. */
+	if (elf_add_reloc_to_insn(elf, dwarf_pcs_sec,
+				  index * sizeof(unsigned long),
+				  R_AARCH64_ABS64,
+				  hentry->section, hint->pc)) {
+		return -1;
+	}
+	return 0;
+}
+
 int dwarf_write(struct objtool_file *file)
 {
 	struct elf		*elf = file->elf;
 	struct fde		*fde;
 	struct fde_entry	*entry;
+	struct hint_entry	*hentry;
 	int			index;
 
 	/*
@@ -154,7 +208,7 @@ int dwarf_write(struct objtool_file *file)
 	/* Create .dwarf_rules. */
 	dwarf_rules_sec = elf_create_section(elf, ".dwarf_rules", 0,
 					     sizeof(struct dwarf_rule),
-					     nentries);
+					     nentries + nhints);
 	if (!dwarf_rules_sec) {
 		WARN("Unable to create .dwarf_rules");
 		return -1;
@@ -162,7 +216,8 @@ int dwarf_write(struct objtool_file *file)
 
 	/* Create .dwarf_pcs. */
 	dwarf_pcs_sec = elf_create_section(elf, ".dwarf_pcs", 0,
-					   sizeof(unsigned long), nentries);
+					   sizeof(unsigned long),
+					   nentries + nhints);
 	if (!dwarf_pcs_sec) {
 		WARN("Unable to create .dwarf_pcs");
 		return -1;
@@ -178,6 +233,12 @@ int dwarf_write(struct objtool_file *file)
 		}
 	}
 
+	for (hentry = hint_list; hentry; hentry = hentry->next) {
+		if (dwarf_hint_write(elf, hentry, index))
+			return -1;
+		index++;
+	}
+
 	return 0;
 }
 
diff --git a/tools/objtool/include/objtool/dwarf_def.h b/tools/objtool/include/objtool/dwarf_def.h
index af56ccb52fff..afa6db6c3828 100644
--- a/tools/objtool/include/objtool/dwarf_def.h
+++ b/tools/objtool/include/objtool/dwarf_def.h
@@ -304,6 +304,15 @@ struct fde {
 	struct fde_entry	*tail;
 };
 
+/*
+ * Entry for an unwind hint.
+ */
+struct hint_entry {
+	struct hint_entry		*next;
+	struct section			*section;
+	struct dwarf_unwind_hint	*hint;
+};
+
 /*
  * These are identifiers for 32-bit and 64-bit CIE entries respectively.
  * These identifiers distinguish a CIE from an FDE in .debug_frame.
@@ -429,6 +438,7 @@ extern u64		(*get_value)(unsigned char *field, unsigned int size);
 void dwarf_rule_start(struct fde *fde);
 int dwarf_rule_add(struct fde *fde, unsigned long addr,
 		   struct rule *sp_rule, struct rule *fp_rule);
+int dwarf_hint_add(struct section *section, struct dwarf_unwind_hint *hint);
 void dwarf_rule_next(struct fde *fde, unsigned long addr);
 void dwarf_rule_reset(struct fde *fde);
 int arch_dwarf_fde_reloc(struct fde *fde);
-- 
2.25.1

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