1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Copyright (C) 2009  Matt Fleming
4 *
5 * Based, in part, on kernel/time/clocksource.c.
6 *
7 * This file provides arbitration code for stack unwinders.
8 *
9 * Multiple stack unwinders can be available on a system, usually with
10 * the most accurate unwinder being the currently active one.
11 */
12#include <linux/errno.h>
13#include <linux/list.h>
14#include <linux/spinlock.h>
15#include <linux/module.h>
16#include <asm/unwinder.h>
17#include <linux/atomic.h>
18
19/*
20 * This is the most basic stack unwinder an architecture can
21 * provide. For architectures without reliable frame pointers, e.g.
22 * RISC CPUs, it can be implemented by looking through the stack for
23 * addresses that lie within the kernel text section.
24 *
25 * Other CPUs, e.g. x86, can use their frame pointer register to
26 * construct more accurate stack traces.
27 */
28static struct list_head unwinder_list;
29static struct unwinder stack_reader = {
30	.name = "stack-reader",
31	.dump = stack_reader_dump,
32	.rating = 50,
33	.list = {
34		.next = &unwinder_list,
35		.prev = &unwinder_list,
36	},
37};
38
39/*
40 * "curr_unwinder" points to the stack unwinder currently in use. This
41 * is the unwinder with the highest rating.
42 *
43 * "unwinder_list" is a linked-list of all available unwinders, sorted
44 * by rating.
45 *
46 * All modifications of "curr_unwinder" and "unwinder_list" must be
47 * performed whilst holding "unwinder_lock".
48 */
49static struct unwinder *curr_unwinder = &stack_reader;
50
51static struct list_head unwinder_list = {
52	.next = &stack_reader.list,
53	.prev = &stack_reader.list,
54};
55
56static DEFINE_SPINLOCK(unwinder_lock);
57
58/**
59 * select_unwinder - Select the best registered stack unwinder.
60 *
61 * Private function. Must hold unwinder_lock when called.
62 *
63 * Select the stack unwinder with the best rating. This is useful for
64 * setting up curr_unwinder.
65 */
66static struct unwinder *select_unwinder(void)
67{
68	struct unwinder *best;
69
70	if (list_empty(&unwinder_list))
71		return NULL;
72
73	best = list_entry(unwinder_list.next, struct unwinder, list);
74	if (best == curr_unwinder)
75		return NULL;
76
77	return best;
78}
79
80/*
81 * Enqueue the stack unwinder sorted by rating.
82 */
83static int unwinder_enqueue(struct unwinder *ops)
84{
85	struct list_head *tmp, *entry = &unwinder_list;
86
87	list_for_each(tmp, &unwinder_list) {
88		struct unwinder *o;
89
90		o = list_entry(tmp, struct unwinder, list);
91		if (o == ops)
92			return -EBUSY;
93		/* Keep track of the place, where to insert */
94		if (o->rating >= ops->rating)
95			entry = tmp;
96	}
97	list_add(&ops->list, entry);
98
99	return 0;
100}
101
102/**
103 * unwinder_register - Used to install new stack unwinder
104 * @u: unwinder to be registered
105 *
106 * Install the new stack unwinder on the unwinder list, which is sorted
107 * by rating.
108 *
109 * Returns -EBUSY if registration fails, zero otherwise.
110 */
111int unwinder_register(struct unwinder *u)
112{
113	unsigned long flags;
114	int ret;
115
116	spin_lock_irqsave(&unwinder_lock, flags);
117	ret = unwinder_enqueue(u);
118	if (!ret)
119		curr_unwinder = select_unwinder();
120	spin_unlock_irqrestore(&unwinder_lock, flags);
121
122	return ret;
123}
124
125int unwinder_faulted = 0;
126
127/*
128 * Unwind the call stack and pass information to the stacktrace_ops
129 * functions. Also handle the case where we need to switch to a new
130 * stack dumper because the current one faulted unexpectedly.
131 */
132void unwind_stack(struct task_struct *task, struct pt_regs *regs,
133		  unsigned long *sp, const struct stacktrace_ops *ops,
134		  void *data)
135{
136	unsigned long flags;
137
138	/*
139	 * The problem with unwinders with high ratings is that they are
140	 * inherently more complicated than the simple ones with lower
141	 * ratings. We are therefore more likely to fault in the
142	 * complicated ones, e.g. hitting BUG()s. If we fault in the
143	 * code for the current stack unwinder we try to downgrade to
144	 * one with a lower rating.
145	 *
146	 * Hopefully this will give us a semi-reliable stacktrace so we
147	 * can diagnose why curr_unwinder->dump() faulted.
148	 */
149	if (unwinder_faulted) {
150		spin_lock_irqsave(&unwinder_lock, flags);
151
152		/* Make sure no one beat us to changing the unwinder */
153		if (unwinder_faulted && !list_is_singular(&unwinder_list)) {
154			list_del(&curr_unwinder->list);
155			curr_unwinder = select_unwinder();
156
157			unwinder_faulted = 0;
158		}
159
160		spin_unlock_irqrestore(&unwinder_lock, flags);
161	}
162
163	curr_unwinder->dump(task, regs, sp, ops, data);
164}
165EXPORT_SYMBOL_GPL(unwind_stack);
166