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 unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs)
   9 {
  10         unsigned long addr, seg;
  11
  12         addr = regs->ip;
  13         seg = regs->cs & 0xffff;
  14         if (v8086_mode(regs)) {
  15                 addr = (addr & 0xffff) + (seg << 4);
  16                 return addr;
  17         }
  18
  19         /*
  20          * We'll assume that the code segments in the GDT
  21          * are all zero-based. That is largely true: the
  22          * TLS segments are used for data, and the PNPBIOS
  23          * and APM bios ones we just ignore here.
  24          */
  25         if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
  26                 u32 *desc;
  27                 unsigned long base;
  28
  29                 seg &= ~7UL;
  30
  31                 mutex_lock(&child->mm->context.lock);
  32                 if (unlikely((seg >> 3) >= child->mm->context.size))
  33                         addr = -1L; /* bogus selector, access would fault */
  34                 else {
  35                         desc = child->mm->context.ldt + seg;
  36                         base = ((desc[0] >> 16) |
  37                                 ((desc[1] & 0xff) << 16) |
  38                                 (desc[1] & 0xff000000));
  39
  40                         /* 16-bit code segment? */
  41                         if (!((desc[1] >> 22) & 1))
  42                                 addr &= 0xffff;
  43                         addr += base;
  44                 }
  45                 mutex_unlock(&child->mm->context.lock);
  46         }
  47
  48         return addr;
  49 }
  50
  51 static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
  52 {
  53         int i, copied;
  54         unsigned char opcode[15];
  55         unsigned long addr = convert_ip_to_linear(child, regs);
  56
  57         copied = access_process_vm(child, addr, opcode, sizeof(opcode), 0);
  58         for (i = 0; i < copied; i++) {
  59                 switch (opcode[i]) {
  60                 /* popf and iret */
  61                 case 0x9d: case 0xcf:
  62                         return 1;
  63
  64                         /* CHECKME: 64 65 */
  65
  66                 /* opcode and address size prefixes */
  67                 case 0x66: case 0x67:
  68                         continue;
  69                 /* irrelevant prefixes (segment overrides and repeats) */
  70                 case 0x26: case 0x2e:
  71                 case 0x36: case 0x3e:
  72                 case 0x64: case 0x65:
  73                 case 0xf0: case 0xf2: case 0xf3:
  74                         continue;
  75
  76 #ifdef CONFIG_X86_64
  77                 case 0x40 ... 0x4f:
  78                         if (regs->cs != __USER_CS)
  79                                 /* 32-bit mode: register increment */
  80                                 return 0;
  81                         /* 64-bit mode: REX prefix */
  82                         continue;
  83 #endif
  84
  85                         /* CHECKME: f2, f3 */
  86
  87                 /*
  88                  * pushf: NOTE! We should probably not let
  89                  * the user see the TF bit being set. But
  90                  * it's more pain than it's worth to avoid
  91                  * it, and a debugger could emulate this
  92                  * all in user space if it _really_ cares.
  93                  */
  94                 case 0x9c:
  95                 default:
  96                         return 0;
  97                 }
  98         }
  99         return 0;
 100 }
 101
 102 /*
 103  * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
 104  */
 105 static int enable_single_step(struct task_struct *child)
 106 {
 107         struct pt_regs *regs = task_pt_regs(child);
 108
 109         /*
 110          * Always set TIF_SINGLESTEP - this guarantees that
 111          * we single-step system calls etc..  This will also
 112          * cause us to set TF when returning to user mode.
 113          */
 114         set_tsk_thread_flag(child, TIF_SINGLESTEP);
 115
 116         /*
 117          * If TF was already set, don't do anything else
 118          */
 119         if (regs->flags & X86_EFLAGS_TF)
 120                 return 0;
 121
 122         /* Set TF on the kernel stack.. */
 123         regs->flags |= X86_EFLAGS_TF;
 124
 125         /*
 126          * ..but if TF is changed by the instruction we will trace,
 127          * don't mark it as being "us" that set it, so that we
 128          * won't clear it by hand later.
 129          */
 130         if (is_setting_trap_flag(child, regs))
 131                 return 0;
 132
 133         set_tsk_thread_flag(child, TIF_FORCED_TF);
 134
 135         return 1;
 136 }
 137
 138 /*
 139  * Install this value in MSR_IA32_DEBUGCTLMSR whenever child is running.
 140  */
 141 static void write_debugctlmsr(struct task_struct *child, unsigned long val)
 142 {
 143         if (child->thread.debugctlmsr == val)
 144                 return;
 145
 146         child->thread.debugctlmsr = val;
 147
 148         if (child != current)
 149                 return;
 150
 151         wrmsrl(MSR_IA32_DEBUGCTLMSR, val);
 152 }
 153
 154 /*
 155  * Enable single or block step.
 156  */
 157 static void enable_step(struct task_struct *child, bool block)
 158 {
 159         /*
 160          * Make sure block stepping (BTF) is not enabled unless it should be.
 161          * Note that we don't try to worry about any is_setting_trap_flag()
 162          * instructions after the first when using block stepping.
 163          * So noone should try to use debugger block stepping in a program
 164          * that uses user-mode single stepping itself.
 165          */
 166         if (enable_single_step(child) && block) {
 167                 set_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
 168                 write_debugctlmsr(child,
 169                                   child->thread.debugctlmsr | DEBUGCTLMSR_BTF);
 170         } else {
 171                 write_debugctlmsr(child,
 172                                   child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF);
 173
 174                 if (!child->thread.debugctlmsr)
 175                         clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
 176         }
 177 }
 178
 179 void user_enable_single_step(struct task_struct *child)
 180 {
 181         enable_step(child, 0);
 182 }
 183
 184 void user_enable_block_step(struct task_struct *child)
 185 {
 186         enable_step(child, 1);
 187 }
 188
 189 void user_disable_single_step(struct task_struct *child)
 190 {
 191         /*
 192          * Make sure block stepping (BTF) is disabled.
 193          */
 194         write_debugctlmsr(child,
 195                           child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF);
 196
 197         if (!child->thread.debugctlmsr)
 198                 clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
 199
 200         /* Always clear TIF_SINGLESTEP... */
 201         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 202
 203         /* But touch TF only if it was set by us.. */
 204         if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
 205                 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
 206 }