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Linux/arch/x86/kernel/step.c

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  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 
  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 
 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  */
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 
160 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          *
169          * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
170          * task is current or it can't be running, otherwise we can race
171          * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
172          * PTRACE_KILL is not safe.
173          */
174         local_irq_disable();
175         debugctl = get_debugctlmsr();
176         if (on) {
177                 debugctl |= DEBUGCTLMSR_BTF;
178                 set_tsk_thread_flag(task, TIF_BLOCKSTEP);
179         } else {
180                 debugctl &= ~DEBUGCTLMSR_BTF;
181                 clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
182         }
183         if (task == current)
184                 update_debugctlmsr(debugctl);
185         local_irq_enable();
186 }
187 
188 /*
189  * Enable single or block step.
190  */
191 static void enable_step(struct task_struct *child, bool block)
192 {
193         /*
194          * Make sure block stepping (BTF) is not enabled unless it should be.
195          * Note that we don't try to worry about any is_setting_trap_flag()
196          * instructions after the first when using block stepping.
197          * So no one should try to use debugger block stepping in a program
198          * that uses user-mode single stepping itself.
199          */
200         if (enable_single_step(child) && block)
201                 set_task_blockstep(child, true);
202         else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
203                 set_task_blockstep(child, false);
204 }
205 
206 void user_enable_single_step(struct task_struct *child)
207 {
208         enable_step(child, 0);
209 }
210 
211 void user_enable_block_step(struct task_struct *child)
212 {
213         enable_step(child, 1);
214 }
215 
216 void user_disable_single_step(struct task_struct *child)
217 {
218         /*
219          * Make sure block stepping (BTF) is disabled.
220          */
221         if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
222                 set_task_blockstep(child, false);
223 
224         /* Always clear TIF_SINGLESTEP... */
225         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
226 
227         /* But touch TF only if it was set by us.. */
228         if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
229                 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
230 }
231 

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