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
   9 unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs)
  10 {
  11         unsigned long addr, seg;
  12
  13         addr = regs->ip;
  14         seg = regs->cs & 0xffff;
  15         if (v8086_mode(regs)) {
  16                 addr = (addr & 0xffff) + (seg << 4);
  17                 return addr;
  18         }
  19
  20         /*
  21          * We'll assume that the code segments in the GDT
  22          * are all zero-based. That is largely true: the
  23          * TLS segments are used for data, and the PNPBIOS
  24          * and APM bios ones we just ignore here.
  25          */
  26         if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
  27                 struct desc_struct *desc;
  28                 unsigned long base;
  29
  30                 seg &= ~7UL;
  31
  32                 mutex_lock(&child->mm->context.lock);
  33                 if (unlikely((seg >> 3) >= child->mm->context.size))
  34                         addr = -1L; /* bogus selector, access would fault */
  35                 else {
  36                         desc = child->mm->context.ldt + seg;
  37                         base = get_desc_base(desc);
  38
  39                         /* 16-bit code segment? */
  40                         if (!desc->d)
  41                                 addr &= 0xffff;
  42                         addr += base;
  43                 }
  44                 mutex_unlock(&child->mm->context.lock);
  45         }
  46
  47         return addr;
  48 }
  49
  50 static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
  51 {
  52         int i, copied;
  53         unsigned char opcode[15];
  54         unsigned long addr = convert_ip_to_linear(child, regs);
  55
  56         copied = access_process_vm(child, addr, opcode, sizeof(opcode), 0);
  57         for (i = 0; i < copied; i++) {
  58                 switch (opcode[i]) {
  59                 /* popf and iret */
  60                 case 0x9d: case 0xcf:
  61                         return 1;
  62
  63                         /* CHECKME: 64 65 */
  64
  65                 /* opcode and address size prefixes */
  66                 case 0x66: case 0x67:
  67                         continue;
  68                 /* irrelevant prefixes (segment overrides and repeats) */
  69                 case 0x26: case 0x2e:
  70                 case 0x36: case 0x3e:
  71                 case 0x64: case 0x65:
  72                 case 0xf0: case 0xf2: case 0xf3:
  73                         continue;
  74
  75 #ifdef CONFIG_X86_64
  76                 case 0x40 ... 0x4f:
  77                         if (!user_64bit_mode(regs))
  78                                 /* 32-bit mode: register increment */
  79                                 return 0;
  80                         /* 64-bit mode: REX prefix */
  81                         continue;
  82 #endif
  83
  84                         /* CHECKME: f2, f3 */
  85
  86                 /*
  87                  * pushf: NOTE! We should probably not let
  88                  * the user see the TF bit being set. But
  89                  * it's more pain than it's worth to avoid
  90                  * it, and a debugger could emulate this
  91                  * all in user space if it _really_ cares.
  92                  */
  93                 case 0x9c:
  94                 default:
  95                         return 0;
  96                 }
  97         }
  98         return 0;
  99 }
 100
 101 /*
 102  * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
 103  */
 104 static int enable_single_step(struct task_struct *child)
 105 {
 106         struct pt_regs *regs = task_pt_regs(child);
 107         unsigned long oflags;
 108
 109         /*
 110          * If we stepped into a sysenter/syscall insn, it trapped in
 111          * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
 112          * If user-mode had set TF itself, then it's still clear from
 113          * do_debug() and we need to set it again to restore the user
 114          * state so we don't wrongly set TIF_FORCED_TF below.
 115          * If enable_single_step() was used last and that is what
 116          * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
 117          * already set and our bookkeeping is fine.
 118          */
 119         if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
 120                 regs->flags |= X86_EFLAGS_TF;
 121
 122         /*
 123          * Always set TIF_SINGLESTEP - this guarantees that
 124          * we single-step system calls etc..  This will also
 125          * cause us to set TF when returning to user mode.
 126          */
 127         set_tsk_thread_flag(child, TIF_SINGLESTEP);
 128
 129         oflags = regs->flags;
 130
 131         /* Set TF on the kernel stack.. */
 132         regs->flags |= X86_EFLAGS_TF;
 133
 134         /*
 135          * ..but if TF is changed by the instruction we will trace,
 136          * don't mark it as being "us" that set it, so that we
 137          * won't clear it by hand later.
 138          *
 139          * Note that if we don't actually execute the popf because
 140          * of a signal arriving right now or suchlike, we will lose
 141          * track of the fact that it really was "us" that set it.
 142          */
 143         if (is_setting_trap_flag(child, regs)) {
 144                 clear_tsk_thread_flag(child, TIF_FORCED_TF);
 145                 return 0;
 146         }
 147
 148         /*
 149          * If TF was already set, check whether it was us who set it.
 150          * If not, we should never attempt a block step.
 151          */
 152         if (oflags & X86_EFLAGS_TF)
 153                 return test_tsk_thread_flag(child, TIF_FORCED_TF);
 154
 155         set_tsk_thread_flag(child, TIF_FORCED_TF);
 156
 157         return 1;
 158 }
 159
 160 void set_task_blockstep(struct task_struct *task, bool on)
 161 {
 162         unsigned long debugctl;
 163
 164         /*
 165          * Ensure irq/preemption can't change debugctl in between.
 166          * Note also that both TIF_BLOCKSTEP and debugctl should
 167          * be changed atomically wrt preemption.
 168          *
 169          * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
 170          * task is current or it can't be running, otherwise we can race
 171          * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
 172          * PTRACE_KILL is not safe.
 173          */
 174         local_irq_disable();
 175         debugctl = get_debugctlmsr();
 176         if (on) {
 177                 debugctl |= DEBUGCTLMSR_BTF;
 178                 set_tsk_thread_flag(task, TIF_BLOCKSTEP);
 179         } else {
 180                 debugctl &= ~DEBUGCTLMSR_BTF;
 181                 clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
 182         }
 183         if (task == current)
 184                 update_debugctlmsr(debugctl);
 185         local_irq_enable();
 186 }
 187
 188 /*
 189  * Enable single or block step.
 190  */
 191 static void enable_step(struct task_struct *child, bool block)
 192 {
 193         /*
 194          * Make sure block stepping (BTF) is not enabled unless it should be.
 195          * Note that we don't try to worry about any is_setting_trap_flag()
 196          * instructions after the first when using block stepping.
 197          * So no one should try to use debugger block stepping in a program
 198          * that uses user-mode single stepping itself.
 199          */
 200         if (enable_single_step(child) && block)
 201                 set_task_blockstep(child, true);
 202         else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
 203                 set_task_blockstep(child, false);
 204 }
 205
 206 void user_enable_single_step(struct task_struct *child)
 207 {
 208         enable_step(child, 0);
 209 }
 210
 211 void user_enable_block_step(struct task_struct *child)
 212 {
 213         enable_step(child, 1);
 214 }
 215
 216 void user_disable_single_step(struct task_struct *child)
 217 {
 218         /*
 219          * Make sure block stepping (BTF) is disabled.
 220          */
 221         if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
 222                 set_task_blockstep(child, false);
 223
 224         /* Always clear TIF_SINGLESTEP... */
 225         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 226
 227         /* But touch TF only if it was set by us.. */
 228         if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
 229                 task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
 230 }