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