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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/sched/task_stack.h>
  6 #include <linux/mm.h>
  7 #include <linux/ptrace.h>
  8 #include <asm/desc.h>
  9 #include <asm/mmu_context.h>
 10 
 11 unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs)
 12 {
 13         unsigned long addr, seg;
 14 
 15         addr = regs->ip;
 16         seg = regs->cs & 0xffff;
 17         if (v8086_mode(regs)) {
 18                 addr = (addr & 0xffff) + (seg << 4);
 19                 return addr;
 20         }
 21 
 22 #ifdef CONFIG_MODIFY_LDT_SYSCALL
 23         /*
 24          * We'll assume that the code segments in the GDT
 25          * are all zero-based. That is largely true: the
 26          * TLS segments are used for data, and the PNPBIOS
 27          * and APM bios ones we just ignore here.
 28          */
 29         if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
 30                 struct desc_struct *desc;
 31                 unsigned long base;
 32 
 33                 seg >>= 3;
 34 
 35                 mutex_lock(&child->mm->context.lock);
 36                 if (unlikely(!child->mm->context.ldt ||
 37                              seg >= child->mm->context.ldt->nr_entries))
 38                         addr = -1L; /* bogus selector, access would fault */
 39                 else {
 40                         desc = &child->mm->context.ldt->entries[seg];
 41                         base = get_desc_base(desc);
 42 
 43                         /* 16-bit code segment? */
 44                         if (!desc->d)
 45                                 addr &= 0xffff;
 46                         addr += base;
 47                 }
 48                 mutex_unlock(&child->mm->context.lock);
 49         }
 50 #endif
 51 
 52         return addr;
 53 }
 54 
 55 static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
 56 {
 57         int i, copied;
 58         unsigned char opcode[15];
 59         unsigned long addr = convert_ip_to_linear(child, regs);
 60 
 61         copied = access_process_vm(child, addr, opcode, sizeof(opcode),
 62                         FOLL_FORCE);
 63         for (i = 0; i < copied; i++) {
 64                 switch (opcode[i]) {
 65                 /* popf and iret */
 66                 case 0x9d: case 0xcf:
 67                         return 1;
 68 
 69                         /* CHECKME: 64 65 */
 70 
 71                 /* opcode and address size prefixes */
 72                 case 0x66: case 0x67:
 73                         continue;
 74                 /* irrelevant prefixes (segment overrides and repeats) */
 75                 case 0x26: case 0x2e:
 76                 case 0x36: case 0x3e:
 77                 case 0x64: case 0x65:
 78                 case 0xf0: case 0xf2: case 0xf3:
 79                         continue;
 80 
 81 #ifdef CONFIG_X86_64
 82                 case 0x40 ... 0x4f:
 83                         if (!user_64bit_mode(regs))
 84                                 /* 32-bit mode: register increment */
 85                                 return 0;
 86                         /* 64-bit mode: REX prefix */
 87                         continue;
 88 #endif
 89 
 90                         /* CHECKME: f2, f3 */
 91 
 92                 /*
 93                  * pushf: NOTE! We should probably not let
 94                  * the user see the TF bit being set. But
 95                  * it's more pain than it's worth to avoid
 96                  * it, and a debugger could emulate this
 97                  * all in user space if it _really_ cares.
 98                  */
 99                 case 0x9c:
100                 default:
101                         return 0;
102                 }
103         }
104         return 0;
105 }
106 
107 /*
108  * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
109  */
110 static int enable_single_step(struct task_struct *child)
111 {
112         struct pt_regs *regs = task_pt_regs(child);
113         unsigned long oflags;
114 
115         /*
116          * If we stepped into a sysenter/syscall insn, it trapped in
117          * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
118          * If user-mode had set TF itself, then it's still clear from
119          * do_debug() and we need to set it again to restore the user
120          * state so we don't wrongly set TIF_FORCED_TF below.
121          * If enable_single_step() was used last and that is what
122          * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
123          * already set and our bookkeeping is fine.
124          */
125         if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
126                 regs->flags |= X86_EFLAGS_TF;
127 
128         /*
129          * Always set TIF_SINGLESTEP - this guarantees that
130          * we single-step system calls etc..  This will also
131          * cause us to set TF when returning to user mode.
132          */
133         set_tsk_thread_flag(child, TIF_SINGLESTEP);
134 
135         oflags = regs->flags;
136 
137         /* Set TF on the kernel stack.. */
138         regs->flags |= X86_EFLAGS_TF;
139 
140         /*
141          * ..but if TF is changed by the instruction we will trace,
142          * don't mark it as being "us" that set it, so that we
143          * won't clear it by hand later.
144          *
145          * Note that if we don't actually execute the popf because
146          * of a signal arriving right now or suchlike, we will lose
147          * track of the fact that it really was "us" that set it.
148          */
149         if (is_setting_trap_flag(child, regs)) {
150                 clear_tsk_thread_flag(child, TIF_FORCED_TF);
151                 return 0;
152         }
153 
154         /*
155          * If TF was already set, check whether it was us who set it.
156          * If not, we should never attempt a block step.
157          */
158         if (oflags & X86_EFLAGS_TF)
159                 return test_tsk_thread_flag(child, TIF_FORCED_TF);
160 
161         set_tsk_thread_flag(child, TIF_FORCED_TF);
162 
163         return 1;
164 }
165 
166 void set_task_blockstep(struct task_struct *task, bool on)
167 {
168         unsigned long debugctl;
169 
170         /*
171          * Ensure irq/preemption can't change debugctl in between.
172          * Note also that both TIF_BLOCKSTEP and debugctl should
173          * be changed atomically wrt preemption.
174          *
175          * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
176          * task is current or it can't be running, otherwise we can race
177          * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
178          * PTRACE_KILL is not safe.
179          */
180         local_irq_disable();
181         debugctl = get_debugctlmsr();
182         if (on) {
183                 debugctl |= DEBUGCTLMSR_BTF;
184                 set_tsk_thread_flag(task, TIF_BLOCKSTEP);
185         } else {
186                 debugctl &= ~DEBUGCTLMSR_BTF;
187                 clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
188         }
189         if (task == current)
190                 update_debugctlmsr(debugctl);
191         local_irq_enable();
192 }
193 
194 /*
195  * Enable single or block step.
196  */
197 static void enable_step(struct task_struct *child, bool block)
198 {
199         /*
200          * Make sure block stepping (BTF) is not enabled unless it should be.
201          * Note that we don't try to worry about any is_setting_trap_flag()
202          * instructions after the first when using block stepping.
203          * So no one should try to use debugger block stepping in a program
204          * that uses user-mode single stepping itself.
205          */
206         if (enable_single_step(child) && block)
207                 set_task_blockstep(child, true);
208         else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
209                 set_task_blockstep(child, false);
210 }
211 
212 void user_enable_single_step(struct task_struct *child)
213 {
214         enable_step(child, 0);
215 }
216 
217 void user_enable_block_step(struct task_struct *child)
218 {
219         enable_step(child, 1);
220 }
221 
222 void user_disable_single_step(struct task_struct *child)
223 {
224         /*
225          * Make sure block stepping (BTF) is disabled.
226          */
227         if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
228                 set_task_blockstep(child, false);
229 
230         /* Always clear TIF_SINGLESTEP... */
231         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
232 
233         /* But touch TF only if it was set by us.. */
234         if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
235                 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
236 }
237 

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