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 #include <asm/desc.h>
   8 #include <asm/mmu_context.h>
   9
  10 unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs)
  11 {
  12         unsigned long addr, seg;
  13
  14         addr = regs->ip;
  15         seg = regs->cs & 0xffff;
  16         if (v8086_mode(regs)) {
  17                 addr = (addr & 0xffff) + (seg << 4);
  18                 return addr;
  19         }
  20
  21 #ifdef CONFIG_MODIFY_LDT_SYSCALL
  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                 struct desc_struct *desc;
  30                 unsigned long base;
  31
  32                 seg >>= 3;
  33
  34                 mutex_lock(&child->mm->context.lock);
  35                 if (unlikely(!child->mm->context.ldt ||
  36                              seg >= child->mm->context.ldt->size))
  37                         addr = -1L; /* bogus selector, access would fault */
  38                 else {
  39                         desc = &child->mm->context.ldt->entries[seg];
  40                         base = get_desc_base(desc);
  41
  42                         /* 16-bit code segment? */
  43                         if (!desc->d)
  44                                 addr &= 0xffff;
  45                         addr += base;
  46                 }
  47                 mutex_unlock(&child->mm->context.lock);
  48         }
  49 #endif
  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_ip_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 (!user_64bit_mode(regs))
  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         unsigned long oflags;
 112
 113         /*
 114          * If we stepped into a sysenter/syscall insn, it trapped in
 115          * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
 116          * If user-mode had set TF itself, then it's still clear from
 117          * do_debug() and we need to set it again to restore the user
 118          * state so we don't wrongly set TIF_FORCED_TF below.
 119          * If enable_single_step() was used last and that is what
 120          * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
 121          * already set and our bookkeeping is fine.
 122          */
 123         if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
 124                 regs->flags |= X86_EFLAGS_TF;
 125
 126         /*
 127          * Always set TIF_SINGLESTEP - this guarantees that
 128          * we single-step system calls etc..  This will also
 129          * cause us to set TF when returning to user mode.
 130          */
 131         set_tsk_thread_flag(child, TIF_SINGLESTEP);
 132
 133         oflags = regs->flags;
 134
 135         /* Set TF on the kernel stack.. */
 136         regs->flags |= X86_EFLAGS_TF;
 137
 138         /*
 139          * ..but if TF is changed by the instruction we will trace,
 140          * don't mark it as being "us" that set it, so that we
 141          * won't clear it by hand later.
 142          *
 143          * Note that if we don't actually execute the popf because
 144          * of a signal arriving right now or suchlike, we will lose
 145          * track of the fact that it really was "us" that set it.
 146          */
 147         if (is_setting_trap_flag(child, regs)) {
 148                 clear_tsk_thread_flag(child, TIF_FORCED_TF);
 149                 return 0;
 150         }
 151
 152         /*
 153          * If TF was already set, check whether it was us who set it.
 154          * If not, we should never attempt a block step.
 155          */
 156         if (oflags & X86_EFLAGS_TF)
 157                 return test_tsk_thread_flag(child, TIF_FORCED_TF);
 158
 159         set_tsk_thread_flag(child, TIF_FORCED_TF);
 160
 161         return 1;
 162 }
 163
 164 void set_task_blockstep(struct task_struct *task, bool on)
 165 {
 166         unsigned long debugctl;
 167
 168         /*
 169          * Ensure irq/preemption can't change debugctl in between.
 170          * Note also that both TIF_BLOCKSTEP and debugctl should
 171          * be changed atomically wrt preemption.
 172          *
 173          * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
 174          * task is current or it can't be running, otherwise we can race
 175          * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
 176          * PTRACE_KILL is not safe.
 177          */
 178         local_irq_disable();
 179         debugctl = get_debugctlmsr();
 180         if (on) {
 181                 debugctl |= DEBUGCTLMSR_BTF;
 182                 set_tsk_thread_flag(task, TIF_BLOCKSTEP);
 183         } else {
 184                 debugctl &= ~DEBUGCTLMSR_BTF;
 185                 clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
 186         }
 187         if (task == current)
 188                 update_debugctlmsr(debugctl);
 189         local_irq_enable();
 190 }
 191
 192 /*
 193  * Enable single or block step.
 194  */
 195 static void enable_step(struct task_struct *child, bool block)
 196 {
 197         /*
 198          * Make sure block stepping (BTF) is not enabled unless it should be.
 199          * Note that we don't try to worry about any is_setting_trap_flag()
 200          * instructions after the first when using block stepping.
 201          * So no one should try to use debugger block stepping in a program
 202          * that uses user-mode single stepping itself.
 203          */
 204         if (enable_single_step(child) && block)
 205                 set_task_blockstep(child, true);
 206         else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
 207                 set_task_blockstep(child, false);
 208 }
 209
 210 void user_enable_single_step(struct task_struct *child)
 211 {
 212         enable_step(child, 0);
 213 }
 214
 215 void user_enable_block_step(struct task_struct *child)
 216 {
 217         enable_step(child, 1);
 218 }
 219
 220 void user_disable_single_step(struct task_struct *child)
 221 {
 222         /*
 223          * Make sure block stepping (BTF) is disabled.
 224          */
 225         if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
 226                 set_task_blockstep(child, false);
 227
 228         /* Always clear TIF_SINGLESTEP... */
 229         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 230
 231         /* But touch TF only if it was set by us.. */
 232         if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
 233                 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
 234 }