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         unsigned long oflags;
 109
 110         /*
 111          * If we stepped into a sysenter/syscall insn, it trapped in
 112          * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
 113          * If user-mode had set TF itself, then it's still clear from
 114          * do_debug() and we need to set it again to restore the user
 115          * state so we don't wrongly set TIF_FORCED_TF below.
 116          * If enable_single_step() was used last and that is what
 117          * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
 118          * already set and our bookkeeping is fine.
 119          */
 120         if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
 121                 regs->flags |= X86_EFLAGS_TF;
 122
 123         /*
 124          * Always set TIF_SINGLESTEP - this guarantees that
 125          * we single-step system calls etc..  This will also
 126          * cause us to set TF when returning to user mode.
 127          */
 128         set_tsk_thread_flag(child, TIF_SINGLESTEP);
 129
 130         oflags = regs->flags;
 131
 132         /* Set TF on the kernel stack.. */
 133         regs->flags |= X86_EFLAGS_TF;
 134
 135         /*
 136          * ..but if TF is changed by the instruction we will trace,
 137          * don't mark it as being "us" that set it, so that we
 138          * won't clear it by hand later.
 139          *
 140          * Note that if we don't actually execute the popf because
 141          * of a signal arriving right now or suchlike, we will lose
 142          * track of the fact that it really was "us" that set it.
 143          */
 144         if (is_setting_trap_flag(child, regs)) {
 145                 clear_tsk_thread_flag(child, TIF_FORCED_TF);
 146                 return 0;
 147         }
 148
 149         /*
 150          * If TF was already set, check whether it was us who set it.
 151          * If not, we should never attempt a block step.
 152          */
 153         if (oflags & X86_EFLAGS_TF)
 154                 return test_tsk_thread_flag(child, TIF_FORCED_TF);
 155
 156         set_tsk_thread_flag(child, TIF_FORCED_TF);
 157
 158         return 1;
 159 }
 160
 161 /*
 162  * Install this value in MSR_IA32_DEBUGCTLMSR whenever child is running.
 163  */
 164 static void write_debugctlmsr(struct task_struct *child, unsigned long val)
 165 {
 166         if (child->thread.debugctlmsr == val)
 167                 return;
 168
 169         child->thread.debugctlmsr = val;
 170
 171         if (child != current)
 172                 return;
 173
 174         update_debugctlmsr(val);
 175 }
 176
 177 /*
 178  * Enable single or block step.
 179  */
 180 static void enable_step(struct task_struct *child, bool block)
 181 {
 182         /*
 183          * Make sure block stepping (BTF) is not enabled unless it should be.
 184          * Note that we don't try to worry about any is_setting_trap_flag()
 185          * instructions after the first when using block stepping.
 186          * So noone should try to use debugger block stepping in a program
 187          * that uses user-mode single stepping itself.
 188          */
 189         if (enable_single_step(child) && block) {
 190                 set_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
 191                 write_debugctlmsr(child,
 192                                   child->thread.debugctlmsr | DEBUGCTLMSR_BTF);
 193         } else {
 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 }
 201
 202 void user_enable_single_step(struct task_struct *child)
 203 {
 204         enable_step(child, 0);
 205 }
 206
 207 void user_enable_block_step(struct task_struct *child)
 208 {
 209         enable_step(child, 1);
 210 }
 211
 212 void user_disable_single_step(struct task_struct *child)
 213 {
 214         /*
 215          * Make sure block stepping (BTF) is disabled.
 216          */
 217         write_debugctlmsr(child,
 218                           child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF);
 219
 220         if (!child->thread.debugctlmsr)
 221                 clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
 222
 223         /* Always clear TIF_SINGLESTEP... */
 224         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 225
 226         /* But touch TF only if it was set by us.. */
 227         if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
 228                 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
 229 }