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
convert_ip_to_linear(struct task_struct * child,struct pt_regs * regs)10 unsigned 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
is_setting_trap_flag(struct task_struct * child,struct pt_regs * regs)52 static 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 */
enable_single_step(struct task_struct * child)106 static 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
set_task_blockstep(struct task_struct * task,bool on)162 void 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 */
enable_step(struct task_struct * child,bool block)193 static 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
user_enable_single_step(struct task_struct * child)208 void user_enable_single_step(struct task_struct *child)
209 {
210 enable_step(child, 0);
211 }
212
user_enable_block_step(struct task_struct * child)213 void user_enable_block_step(struct task_struct *child)
214 {
215 enable_step(child, 1);
216 }
217
user_disable_single_step(struct task_struct * child)218 void 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