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 #ifdef CONFIG_MODIFY_LDT_SYSCALL
22 /*
23 * We'll assume that the code segments in the GDT
24 * are all zero-based. That is largely true: the
25 * TLS segments are used for data, and the PNPBIOS
26 * and APM bios ones we just ignore here.
27 */
28 if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
29 struct desc_struct *desc;
30 unsigned long base;
31
32 seg >>= 3;
33
34 mutex_lock(&child->mm->context.lock);
35 if (unlikely(!child->mm->context.ldt ||
36 seg >= child->mm->context.ldt->size))
37 addr = -1L; /* bogus selector, access would fault */
38 else {
39 desc = &child->mm->context.ldt->entries[seg];
40 base = get_desc_base(desc);
41
42 /* 16-bit code segment? */
43 if (!desc->d)
44 addr &= 0xffff;
45 addr += base;
46 }
47 mutex_unlock(&child->mm->context.lock);
48 }
49 #endif
50
51 return addr;
52 }
53
is_setting_trap_flag(struct task_struct * child,struct pt_regs * regs)54 static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
55 {
56 int i, copied;
57 unsigned char opcode[15];
58 unsigned long addr = convert_ip_to_linear(child, regs);
59
60 copied = access_process_vm(child, addr, opcode, sizeof(opcode),
61 FOLL_FORCE);
62 for (i = 0; i < copied; i++) {
63 switch (opcode[i]) {
64 /* popf and iret */
65 case 0x9d: case 0xcf:
66 return 1;
67
68 /* CHECKME: 64 65 */
69
70 /* opcode and address size prefixes */
71 case 0x66: case 0x67:
72 continue;
73 /* irrelevant prefixes (segment overrides and repeats) */
74 case 0x26: case 0x2e:
75 case 0x36: case 0x3e:
76 case 0x64: case 0x65:
77 case 0xf0: case 0xf2: case 0xf3:
78 continue;
79
80 #ifdef CONFIG_X86_64
81 case 0x40 ... 0x4f:
82 if (!user_64bit_mode(regs))
83 /* 32-bit mode: register increment */
84 return 0;
85 /* 64-bit mode: REX prefix */
86 continue;
87 #endif
88
89 /* CHECKME: f2, f3 */
90
91 /*
92 * pushf: NOTE! We should probably not let
93 * the user see the TF bit being set. But
94 * it's more pain than it's worth to avoid
95 * it, and a debugger could emulate this
96 * all in user space if it _really_ cares.
97 */
98 case 0x9c:
99 default:
100 return 0;
101 }
102 }
103 return 0;
104 }
105
106 /*
107 * Enable single-stepping. Return nonzero if user mode is not using TF itself.
108 */
enable_single_step(struct task_struct * child)109 static int enable_single_step(struct task_struct *child)
110 {
111 struct pt_regs *regs = task_pt_regs(child);
112 unsigned long oflags;
113
114 /*
115 * If we stepped into a sysenter/syscall insn, it trapped in
116 * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
117 * If user-mode had set TF itself, then it's still clear from
118 * do_debug() and we need to set it again to restore the user
119 * state so we don't wrongly set TIF_FORCED_TF below.
120 * If enable_single_step() was used last and that is what
121 * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
122 * already set and our bookkeeping is fine.
123 */
124 if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
125 regs->flags |= X86_EFLAGS_TF;
126
127 /*
128 * Always set TIF_SINGLESTEP - this guarantees that
129 * we single-step system calls etc.. This will also
130 * cause us to set TF when returning to user mode.
131 */
132 set_tsk_thread_flag(child, TIF_SINGLESTEP);
133
134 oflags = regs->flags;
135
136 /* Set TF on the kernel stack.. */
137 regs->flags |= X86_EFLAGS_TF;
138
139 /*
140 * ..but if TF is changed by the instruction we will trace,
141 * don't mark it as being "us" that set it, so that we
142 * won't clear it by hand later.
143 *
144 * Note that if we don't actually execute the popf because
145 * of a signal arriving right now or suchlike, we will lose
146 * track of the fact that it really was "us" that set it.
147 */
148 if (is_setting_trap_flag(child, regs)) {
149 clear_tsk_thread_flag(child, TIF_FORCED_TF);
150 return 0;
151 }
152
153 /*
154 * If TF was already set, check whether it was us who set it.
155 * If not, we should never attempt a block step.
156 */
157 if (oflags & X86_EFLAGS_TF)
158 return test_tsk_thread_flag(child, TIF_FORCED_TF);
159
160 set_tsk_thread_flag(child, TIF_FORCED_TF);
161
162 return 1;
163 }
164
set_task_blockstep(struct task_struct * task,bool on)165 void set_task_blockstep(struct task_struct *task, bool on)
166 {
167 unsigned long debugctl;
168
169 /*
170 * Ensure irq/preemption can't change debugctl in between.
171 * Note also that both TIF_BLOCKSTEP and debugctl should
172 * be changed atomically wrt preemption.
173 *
174 * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
175 * task is current or it can't be running, otherwise we can race
176 * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
177 * PTRACE_KILL is not safe.
178 */
179 local_irq_disable();
180 debugctl = get_debugctlmsr();
181 if (on) {
182 debugctl |= DEBUGCTLMSR_BTF;
183 set_tsk_thread_flag(task, TIF_BLOCKSTEP);
184 } else {
185 debugctl &= ~DEBUGCTLMSR_BTF;
186 clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
187 }
188 if (task == current)
189 update_debugctlmsr(debugctl);
190 local_irq_enable();
191 }
192
193 /*
194 * Enable single or block step.
195 */
enable_step(struct task_struct * child,bool block)196 static void enable_step(struct task_struct *child, bool block)
197 {
198 /*
199 * Make sure block stepping (BTF) is not enabled unless it should be.
200 * Note that we don't try to worry about any is_setting_trap_flag()
201 * instructions after the first when using block stepping.
202 * So no one should try to use debugger block stepping in a program
203 * that uses user-mode single stepping itself.
204 */
205 if (enable_single_step(child) && block)
206 set_task_blockstep(child, true);
207 else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
208 set_task_blockstep(child, false);
209 }
210
user_enable_single_step(struct task_struct * child)211 void user_enable_single_step(struct task_struct *child)
212 {
213 enable_step(child, 0);
214 }
215
user_enable_block_step(struct task_struct * child)216 void user_enable_block_step(struct task_struct *child)
217 {
218 enable_step(child, 1);
219 }
220
user_disable_single_step(struct task_struct * child)221 void user_disable_single_step(struct task_struct *child)
222 {
223 /*
224 * Make sure block stepping (BTF) is disabled.
225 */
226 if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
227 set_task_blockstep(child, false);
228
229 /* Always clear TIF_SINGLESTEP... */
230 clear_tsk_thread_flag(child, TIF_SINGLESTEP);
231
232 /* But touch TF only if it was set by us.. */
233 if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
234 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
235 }
236