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sstep.c

/*
 * Single-step support.
 *
 * Copyright (C) 2004 Paul Mackerras <paulus@au.ibm.com>, IBM
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version
 * 2 of the License, or (at your option) any later version.
 */
#include <linux/kernel.h>
#include <linux/kprobes.h>
#include <linux/ptrace.h>
#include <asm/sstep.h>
#include <asm/processor.h>

extern char system_call_common[];

#ifdef CONFIG_PPC64
/* Bits in SRR1 that are copied from MSR */
#define MSR_MASK  0xffffffff87c0ffffUL
#else
#define MSR_MASK  0x87c0ffff
#endif

/*
 * Determine whether a conditional branch instruction would branch.
 */
static int __kprobes branch_taken(unsigned int instr, struct pt_regs *regs)
{
      unsigned int bo = (instr >> 21) & 0x1f;
      unsigned int bi;

      if ((bo & 4) == 0) {
            /* decrement counter */
            --regs->ctr;
            if (((bo >> 1) & 1) ^ (regs->ctr == 0))
                  return 0;
      }
      if ((bo & 0x10) == 0) {
            /* check bit from CR */
            bi = (instr >> 16) & 0x1f;
            if (((regs->ccr >> (31 - bi)) & 1) != ((bo >> 3) & 1))
                  return 0;
      }
      return 1;
}

/*
 * Emulate instructions that cause a transfer of control.
 * Returns 1 if the step was emulated, 0 if not,
 * or -1 if the instruction is one that should not be stepped,
 * such as an rfid, or a mtmsrd that would clear MSR_RI.
 */
int __kprobes emulate_step(struct pt_regs *regs, unsigned int instr)
{
      unsigned int opcode, rs, rb, rd, spr;
      unsigned long int imm;

      opcode = instr >> 26;
      switch (opcode) {
      case 16:    /* bc */
            imm = (signed short)(instr & 0xfffc);
            if ((instr & 2) == 0)
                  imm += regs->nip;
            regs->nip += 4;
            if ((regs->msr & MSR_SF) == 0)
                  regs->nip &= 0xffffffffUL;
            if (instr & 1)
                  regs->link = regs->nip;
            if (branch_taken(instr, regs))
                  regs->nip = imm;
            return 1;
#ifdef CONFIG_PPC64
      case 17:    /* sc */
            /*
             * N.B. this uses knowledge about how the syscall
             * entry code works.  If that is changed, this will
             * need to be changed also.
             */
            regs->gpr[9] = regs->gpr[13];
            regs->gpr[11] = regs->nip + 4;
            regs->gpr[12] = regs->msr & MSR_MASK;
            regs->gpr[13] = (unsigned long) get_paca();
            regs->nip = (unsigned long) &system_call_common;
            regs->msr = MSR_KERNEL;
            return 1;
#endif
      case 18:    /* b */
            imm = instr & 0x03fffffc;
            if (imm & 0x02000000)
                  imm -= 0x04000000;
            if ((instr & 2) == 0)
                  imm += regs->nip;
            if (instr & 1) {
                  regs->link = regs->nip + 4;
                  if ((regs->msr & MSR_SF) == 0)
                        regs->link &= 0xffffffffUL;
            }
            if ((regs->msr & MSR_SF) == 0)
                  imm &= 0xffffffffUL;
            regs->nip = imm;
            return 1;
      case 19:
            switch (instr & 0x7fe) {
            case 0x20:  /* bclr */
            case 0x420: /* bcctr */
                  imm = (instr & 0x400)? regs->ctr: regs->link;
                  regs->nip += 4;
                  if ((regs->msr & MSR_SF) == 0) {
                        regs->nip &= 0xffffffffUL;
                        imm &= 0xffffffffUL;
                  }
                  if (instr & 1)
                        regs->link = regs->nip;
                  if (branch_taken(instr, regs))
                        regs->nip = imm;
                  return 1;
            case 0x24:  /* rfid, scary */
                  return -1;
            }
      case 31:
            rd = (instr >> 21) & 0x1f;
            switch (instr & 0x7fe) {
            case 0xa6:  /* mfmsr */
                  regs->gpr[rd] = regs->msr & MSR_MASK;
                  regs->nip += 4;
                  if ((regs->msr & MSR_SF) == 0)
                        regs->nip &= 0xffffffffUL;
                  return 1;
            case 0x124: /* mtmsr */
                  imm = regs->gpr[rd];
                  if ((imm & MSR_RI) == 0)
                        /* can't step mtmsr that would clear MSR_RI */
                        return -1;
                  regs->msr = imm;
                  regs->nip += 4;
                  return 1;
#ifdef CONFIG_PPC64
            case 0x164: /* mtmsrd */
                  /* only MSR_EE and MSR_RI get changed if bit 15 set */
                  /* mtmsrd doesn't change MSR_HV and MSR_ME */
                  imm = (instr & 0x10000)? 0x8002: 0xefffffffffffefffUL;
                  imm = (regs->msr & MSR_MASK & ~imm)
                        | (regs->gpr[rd] & imm);
                  if ((imm & MSR_RI) == 0)
                        /* can't step mtmsrd that would clear MSR_RI */
                        return -1;
                  regs->msr = imm;
                  regs->nip += 4;
                  if ((imm & MSR_SF) == 0)
                        regs->nip &= 0xffffffffUL;
                  return 1;
#endif
            case 0x26:  /* mfcr */
                  regs->gpr[rd] = regs->ccr;
                  regs->gpr[rd] &= 0xffffffffUL;
                  goto mtspr_out;
            case 0x2a6: /* mfspr */
                  spr = (instr >> 11) & 0x3ff;
                  switch (spr) {
                  case 0x20:  /* mfxer */
                        regs->gpr[rd] = regs->xer;
                        regs->gpr[rd] &= 0xffffffffUL;
                        goto mtspr_out;
                  case 0x100: /* mflr */
                        regs->gpr[rd] = regs->link;
                        goto mtspr_out;
                  case 0x120: /* mfctr */
                        regs->gpr[rd] = regs->ctr;
                        goto mtspr_out;
                  }
                  break;
            case 0x378: /* orx */
                  rs = (instr >> 21) & 0x1f;
                  rb = (instr >> 11) & 0x1f;
                  if (rs == rb) {         /* mr */
                        rd = (instr >> 16) & 0x1f;
                        regs->gpr[rd] = regs->gpr[rs];
                        goto mtspr_out;
                  }
                  break;
            case 0x3a6: /* mtspr */
                  spr = (instr >> 11) & 0x3ff;
                  switch (spr) {
                  case 0x20:  /* mtxer */
                        regs->xer = (regs->gpr[rd] & 0xffffffffUL);
                        goto mtspr_out;
                  case 0x100: /* mtlr */
                        regs->link = regs->gpr[rd];
                        goto mtspr_out;
                  case 0x120: /* mtctr */
                        regs->ctr = regs->gpr[rd];
mtspr_out:
                        regs->nip += 4;
                        return 1;
                  }
            }
      }
      return 0;
}

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