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