arch/x86/kernel/step.c

   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
   8 #ifdef CONFIG_X86_32
   9 static
  10 #endif
  11 unsigned long convert_rip_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         /*
  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                 u32 *desc;
  30                 unsigned long base;
  31
  32                 seg &= ~7UL;
  33
  34                 mutex_lock(&child->mm->context.lock);
  35                 if (unlikely((seg >> 3) >= child->mm->context.size))
  36                         addr = -1L; /* bogus selector, access would fault */
  37                 else {
  38                         desc = child->mm->context.ldt + seg;
  39                         base = ((desc[0] >> 16) |
  40                                 ((desc[1] & 0xff) << 16) |
  41                                 (desc[1] & 0xff000000));
  42
  43                         /* 16-bit code segment? */
  44                         if (!((desc[1] >> 22) & 1))
  45                                 addr &= 0xffff;
  46                         addr += base;
  47                 }
  48                 mutex_unlock(&child->mm->context.lock);
  49         }
  50
  51         return addr;
  52 }
  53
  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_rip_to_linear(child, regs);
  59
  60         copied = access_process_vm(child, addr, opcode, sizeof(opcode), 0);
  61         for (i = 0; i < copied; i++) {
  62                 switch (opcode[i]) {
  63                 /* popf and iret */
  64                 case 0x9d: case 0xcf:
  65                         return 1;
  66
  67                         /* CHECKME: 64 65 */
  68
  69                 /* opcode and address size prefixes */
  70                 case 0x66: case 0x67:
  71                         continue;
  72                 /* irrelevant prefixes (segment overrides and repeats) */
  73                 case 0x26: case 0x2e:
  74                 case 0x36: case 0x3e:
  75                 case 0x64: case 0x65:
  76                 case 0xf0: case 0xf2: case 0xf3:
  77                         continue;
  78
  79 #ifdef CONFIG_X86_64
  80                 case 0x40 ... 0x4f:
  81                         if (regs->cs != __USER_CS)
  82                                 /* 32-bit mode: register increment */
  83                                 return 0;
  84                         /* 64-bit mode: REX prefix */
  85                         continue;
  86 #endif
  87
  88                         /* CHECKME: f2, f3 */
  89
  90                 /*
  91                  * pushf: NOTE! We should probably not let
  92                  * the user see the TF bit being set. But
  93                  * it's more pain than it's worth to avoid
  94                  * it, and a debugger could emulate this
  95                  * all in user space if it _really_ cares.
  96                  */
  97                 case 0x9c:
  98                 default:
  99                         return 0;
 100                 }
 101         }
 102         return 0;
 103 }
 104
 105 /*
 106  * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
 107  */
 108 static int enable_single_step(struct task_struct *child)
 109 {
 110         struct pt_regs *regs = task_pt_regs(child);
 111
 112         /*
 113          * Always set TIF_SINGLESTEP - this guarantees that
 114          * we single-step system calls etc..  This will also
 115          * cause us to set TF when returning to user mode.
 116          */
 117         set_tsk_thread_flag(child, TIF_SINGLESTEP);
 118
 119         /*
 120          * If TF was already set, don't do anything else
 121          */
 122         if (regs->flags & X86_EFLAGS_TF)
 123                 return 0;
 124
 125         /* Set TF on the kernel stack.. */
 126         regs->flags |= X86_EFLAGS_TF;
 127
 128         /*
 129          * ..but if TF is changed by the instruction we will trace,
 130          * don't mark it as being "us" that set it, so that we
 131          * won't clear it by hand later.
 132          */
 133         if (is_setting_trap_flag(child, regs))
 134                 return 0;
 135
 136         set_tsk_thread_flag(child, TIF_FORCED_TF);
 137
 138         return 1;
 139 }
 140
 141 /*
 142  * Install this value in MSR_IA32_DEBUGCTLMSR whenever child is running.
 143  */
 144 static void write_debugctlmsr(struct task_struct *child, unsigned long val)
 145 {
 146         child->thread.debugctlmsr = val;
 147
 148         if (child != current)
 149                 return;
 150
 151 #ifdef CONFIG_X86_64
 152         wrmsrl(MSR_IA32_DEBUGCTLMSR, val);
 153 #else
 154         wrmsr(MSR_IA32_DEBUGCTLMSR, val, 0);
 155 #endif
 156 }
 157
 158 /*
 159  * Enable single or block step.
 160  */
 161 static void enable_step(struct task_struct *child, bool block)
 162 {
 163         /*
 164          * Make sure block stepping (BTF) is not enabled unless it should be.
 165          * Note that we don't try to worry about any is_setting_trap_flag()
 166          * instructions after the first when using block stepping.
 167          * So noone should try to use debugger block stepping in a program
 168          * that uses user-mode single stepping itself.
 169          */
 170         if (enable_single_step(child) && block) {
 171                 set_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
 172                 write_debugctlmsr(child, DEBUGCTLMSR_BTF);
 173         } else if (test_and_clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR)) {
 174                 write_debugctlmsr(child, 0);
 175         }
 176 }
 177
 178 void user_enable_single_step(struct task_struct *child)
 179 {
 180         enable_step(child, 0);
 181 }
 182
 183 void user_enable_block_step(struct task_struct *child)
 184 {
 185         enable_step(child, 1);
 186 }
 187
 188 void user_disable_single_step(struct task_struct *child)
 189 {
 190         /*
 191          * Make sure block stepping (BTF) is disabled.
 192          */
 193         if (test_and_clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR))
 194                 write_debugctlmsr(child, 0);
 195
 196         /* Always clear TIF_SINGLESTEP... */
 197         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 198
 199         /* But touch TF only if it was set by us.. */
 200         if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
 201                 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
 202 }