1/*
2 * x86 single-step support code, common to 32-bit and 64-bit.
3 */
4#include <linux/sched.h>
5#include <linux/mm.h>
6#include <linux/ptrace.h>
7#include <asm/desc.h>
8#include <asm/mmu_context.h>
9
10unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs)
11{
12	unsigned long addr, seg;
13
14	addr = regs->ip;
15	seg = regs->cs & 0xffff;
16	if (v8086_mode(regs)) {
17		addr = (addr & 0xffff) + (seg << 4);
18		return addr;
19	}
20
21	/*
22	 * We'll assume that the code segments in the GDT
23	 * are all zero-based. That is largely true: the
24	 * TLS segments are used for data, and the PNPBIOS
25	 * and APM bios ones we just ignore here.
26	 */
27	if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
28		struct desc_struct *desc;
29		unsigned long base;
30
31		seg >>= 3;
32
33		mutex_lock(&child->mm->context.lock);
34		if (unlikely(!child->mm->context.ldt ||
35			     seg >= child->mm->context.ldt->size))
36			addr = -1L; /* bogus selector, access would fault */
37		else {
38			desc = &child->mm->context.ldt->entries[seg];
39			base = get_desc_base(desc);
40
41			/* 16-bit code segment? */
42			if (!desc->d)
43				addr &= 0xffff;
44			addr += base;
45		}
46		mutex_unlock(&child->mm->context.lock);
47	}
48
49	return addr;
50}
51
52static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
53{
54	int i, copied;
55	unsigned char opcode[15];
56	unsigned long addr = convert_ip_to_linear(child, regs);
57
58	copied = access_process_vm(child, addr, opcode, sizeof(opcode), 0);
59	for (i = 0; i < copied; i++) {
60		switch (opcode[i]) {
61		/* popf and iret */
62		case 0x9d: case 0xcf:
63			return 1;
64
65			/* CHECKME: 64 65 */
66
67		/* opcode and address size prefixes */
68		case 0x66: case 0x67:
69			continue;
70		/* irrelevant prefixes (segment overrides and repeats) */
71		case 0x26: case 0x2e:
72		case 0x36: case 0x3e:
73		case 0x64: case 0x65:
74		case 0xf0: case 0xf2: case 0xf3:
75			continue;
76
77#ifdef CONFIG_X86_64
78		case 0x40 ... 0x4f:
79			if (!user_64bit_mode(regs))
80				/* 32-bit mode: register increment */
81				return 0;
82			/* 64-bit mode: REX prefix */
83			continue;
84#endif
85
86			/* CHECKME: f2, f3 */
87
88		/*
89		 * pushf: NOTE! We should probably not let
90		 * the user see the TF bit being set. But
91		 * it's more pain than it's worth to avoid
92		 * it, and a debugger could emulate this
93		 * all in user space if it _really_ cares.
94		 */
95		case 0x9c:
96		default:
97			return 0;
98		}
99	}
100	return 0;
101}
102
103/*
104 * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
105 */
106static int enable_single_step(struct task_struct *child)
107{
108	struct pt_regs *regs = task_pt_regs(child);
109	unsigned long oflags;
110
111	/*
112	 * If we stepped into a sysenter/syscall insn, it trapped in
113	 * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
114	 * If user-mode had set TF itself, then it's still clear from
115	 * do_debug() and we need to set it again to restore the user
116	 * state so we don't wrongly set TIF_FORCED_TF below.
117	 * If enable_single_step() was used last and that is what
118	 * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
119	 * already set and our bookkeeping is fine.
120	 */
121	if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
122		regs->flags |= X86_EFLAGS_TF;
123
124	/*
125	 * Always set TIF_SINGLESTEP - this guarantees that
126	 * we single-step system calls etc..  This will also
127	 * cause us to set TF when returning to user mode.
128	 */
129	set_tsk_thread_flag(child, TIF_SINGLESTEP);
130
131	oflags = regs->flags;
132
133	/* Set TF on the kernel stack.. */
134	regs->flags |= X86_EFLAGS_TF;
135
136	/*
137	 * ..but if TF is changed by the instruction we will trace,
138	 * don't mark it as being "us" that set it, so that we
139	 * won't clear it by hand later.
140	 *
141	 * Note that if we don't actually execute the popf because
142	 * of a signal arriving right now or suchlike, we will lose
143	 * track of the fact that it really was "us" that set it.
144	 */
145	if (is_setting_trap_flag(child, regs)) {
146		clear_tsk_thread_flag(child, TIF_FORCED_TF);
147		return 0;
148	}
149
150	/*
151	 * If TF was already set, check whether it was us who set it.
152	 * If not, we should never attempt a block step.
153	 */
154	if (oflags & X86_EFLAGS_TF)
155		return test_tsk_thread_flag(child, TIF_FORCED_TF);
156
157	set_tsk_thread_flag(child, TIF_FORCED_TF);
158
159	return 1;
160}
161
162void set_task_blockstep(struct task_struct *task, bool on)
163{
164	unsigned long debugctl;
165
166	/*
167	 * Ensure irq/preemption can't change debugctl in between.
168	 * Note also that both TIF_BLOCKSTEP and debugctl should
169	 * be changed atomically wrt preemption.
170	 *
171	 * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
172	 * task is current or it can't be running, otherwise we can race
173	 * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
174	 * PTRACE_KILL is not safe.
175	 */
176	local_irq_disable();
177	debugctl = get_debugctlmsr();
178	if (on) {
179		debugctl |= DEBUGCTLMSR_BTF;
180		set_tsk_thread_flag(task, TIF_BLOCKSTEP);
181	} else {
182		debugctl &= ~DEBUGCTLMSR_BTF;
183		clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
184	}
185	if (task == current)
186		update_debugctlmsr(debugctl);
187	local_irq_enable();
188}
189
190/*
191 * Enable single or block step.
192 */
193static void enable_step(struct task_struct *child, bool block)
194{
195	/*
196	 * Make sure block stepping (BTF) is not enabled unless it should be.
197	 * Note that we don't try to worry about any is_setting_trap_flag()
198	 * instructions after the first when using block stepping.
199	 * So no one should try to use debugger block stepping in a program
200	 * that uses user-mode single stepping itself.
201	 */
202	if (enable_single_step(child) && block)
203		set_task_blockstep(child, true);
204	else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
205		set_task_blockstep(child, false);
206}
207
208void user_enable_single_step(struct task_struct *child)
209{
210	enable_step(child, 0);
211}
212
213void user_enable_block_step(struct task_struct *child)
214{
215	enable_step(child, 1);
216}
217
218void user_disable_single_step(struct task_struct *child)
219{
220	/*
221	 * Make sure block stepping (BTF) is disabled.
222	 */
223	if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
224		set_task_blockstep(child, false);
225
226	/* Always clear TIF_SINGLESTEP... */
227	clear_tsk_thread_flag(child, TIF_SINGLESTEP);
228
229	/* But touch TF only if it was set by us.. */
230	if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
231		task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
232}
233