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/* Target-dependent code for Linux running on i386's, for GDB.

   Copyright (C) 2000 Free Software Foundation, Inc.

   This file is part of GDB.

   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.

   This program is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License

   along with this program; if not, write to the Free Software

   Foundation, Inc., 59 Temple Place - Suite 330,

   Boston, MA 02111-1307, USA.  */

#include "defs.h"

#include "gdbcore.h"

#include "frame.h"

#include "value.h"

/* Recognizing signal handler frames.  */

/* Linux has two flavors of signals.  Normal signal handlers, and

   "realtime" (RT) signals.  The RT signals can provide additional

   information to the signal handler if the SA_SIGINFO flag is set

   when establishing a signal handler using `sigaction'.  It is not

   unlikely that future versions of Linux will support SA_SIGINFO for

   normal signals too.  */

/* When the i386 Linux kernel calls a signal handler and the

   SA_RESTORER flag isn't set, the return address points to a bit of

   code on the stack.  This function returns whether the PC appears to

   be within this bit of code.

   The instruction sequence for normal signals is

       pop    %eax

       mov    $0x77,%eax

       int    $0x80

   or 0x58 0xb8 0x77 0x00 0x00 0x00 0xcd 0x80.

   Checking for the code sequence should be somewhat reliable, because

   the effect is to call the system call sigreturn.  This is unlikely

   to occur anywhere other than a signal trampoline.

   It kind of sucks that we have to read memory from the process in

   order to identify a signal trampoline, but there doesn't seem to be

   any other way.  The IN_SIGTRAMP macro in tm-linux.h arranges to

   only call us if no function name could be identified, which should

   be the case since the code is on the stack.

   Detection of signal trampolines for handlers that set the

   SA_RESTORER flag is in general not possible.  Unfortunately this is

   what the GNU C Library has been doing for quite some time now.

   However, as of version 2.1.2, the GNU C Library uses signal

   trampolines (named __restore and __restore_rt) that are identical

   to the ones used by the kernel.  Therefore, these trampolines are

   supported too.  */

#define LINUX_SIGTRAMP_INSN0 (0x58)     /* pop %eax */

#define LINUX_SIGTRAMP_OFFSET0 (0)

#define LINUX_SIGTRAMP_INSN1 (0xb8)     /* mov $NNNN,%eax */

#define LINUX_SIGTRAMP_OFFSET1 (1)

#define LINUX_SIGTRAMP_INSN2 (0xcd)     /* int */

#define LINUX_SIGTRAMP_OFFSET2 (6)

static const unsigned char linux_sigtramp_code[] =

{

  LINUX_SIGTRAMP_INSN0,                                 /* pop %eax */

  LINUX_SIGTRAMP_INSN1, 0x77, 0x00, 0x00, 0x00,         /* mov $0x77,%eax */

  LINUX_SIGTRAMP_INSN2, 0x80                            /* int $0x80 */

};

#define LINUX_SIGTRAMP_LEN (sizeof linux_sigtramp_code)

/* If PC is in a sigtramp routine, return the address of the start of

   the routine.  Otherwise, return 0.  */

static CORE_ADDR

i386_linux_sigtramp_start (CORE_ADDR pc)

{

  unsigned char buf[LINUX_SIGTRAMP_LEN];

  /* We only recognize a signal trampoline if PC is at the start of

     one of the three instructions.  We optimize for finding the PC at

     the start, as will be the case when the trampoline is not the

     first frame on the stack.  We assume that in the case where the

     PC is not at the start of the instruction sequence, there will be

     a few trailing readable bytes on the stack.  */

  if (read_memory_nobpt (pc, (char *) buf, LINUX_SIGTRAMP_LEN) != 0)

    return 0;

  if (buf[0] != LINUX_SIGTRAMP_INSN0)

    {

      int adjust;

      switch (buf[0])

        {

        case LINUX_SIGTRAMP_INSN1:

          adjust = LINUX_SIGTRAMP_OFFSET1;

          break;

        case LINUX_SIGTRAMP_INSN2:

          adjust = LINUX_SIGTRAMP_OFFSET2;

          break;

        default:

          return 0;

        }

      pc -= adjust;

      if (read_memory_nobpt (pc, (char *) buf, LINUX_SIGTRAMP_LEN) != 0)

        return 0;

    }

  if (memcmp (buf, linux_sigtramp_code, LINUX_SIGTRAMP_LEN) != 0)

    return 0;

  return pc;

}

/* This function does the same for RT signals.  Here the instruction

   sequence is

       mov    $0xad,%eax

       int    $0x80

   or 0xb8 0xad 0x00 0x00 0x00 0xcd 0x80.

   The effect is to call the system call rt_sigreturn.  */

#define LINUX_RT_SIGTRAMP_INSN0 (0xb8)  /* mov $NNNN,%eax */

#define LINUX_RT_SIGTRAMP_OFFSET0 (0)

#define LINUX_RT_SIGTRAMP_INSN1 (0xcd)  /* int */

#define LINUX_RT_SIGTRAMP_OFFSET1 (5)

static const unsigned char linux_rt_sigtramp_code[] =

{

  LINUX_RT_SIGTRAMP_INSN0, 0xad, 0x00, 0x00, 0x00,      /* mov $0xad,%eax */

  LINUX_RT_SIGTRAMP_INSN1, 0x80                         /* int $0x80 */

};

#define LINUX_RT_SIGTRAMP_LEN (sizeof linux_rt_sigtramp_code)

/* If PC is in a RT sigtramp routine, return the address of the start

   of the routine.  Otherwise, return 0.  */

static CORE_ADDR

i386_linux_rt_sigtramp_start (CORE_ADDR pc)

{

  unsigned char buf[LINUX_RT_SIGTRAMP_LEN];

  /* We only recognize a signal trampoline if PC is at the start of

     one of the two instructions.  We optimize for finding the PC at

     the start, as will be the case when the trampoline is not the

     first frame on the stack.  We assume that in the case where the

     PC is not at the start of the instruction sequence, there will be

     a few trailing readable bytes on the stack.  */

  if (read_memory_nobpt (pc, (char *) buf, LINUX_RT_SIGTRAMP_LEN) != 0)

    return 0;

  if (buf[0] != LINUX_RT_SIGTRAMP_INSN0)

    {

      if (buf[0] != LINUX_RT_SIGTRAMP_INSN1)

        return 0;

      pc -= LINUX_RT_SIGTRAMP_OFFSET1;

      if (read_memory_nobpt (pc, (char *) buf, LINUX_RT_SIGTRAMP_LEN) != 0)

        return 0;

    }

  if (memcmp (buf, linux_rt_sigtramp_code, LINUX_RT_SIGTRAMP_LEN) != 0)

    return 0;

  return pc;

}

/* Return whether PC is in a Linux sigtramp routine.  */

int

i386_linux_in_sigtramp (CORE_ADDR pc, char *name)

{

  if (name)

    return STREQ ("__restore", name) || STREQ ("__restore_rt", name);

  return (i386_linux_sigtramp_start (pc) != 0

          || i386_linux_rt_sigtramp_start (pc) != 0);

}

/* Assuming FRAME is for a Linux sigtramp routine, return the address

   of the associated sigcontext structure.  */

CORE_ADDR

i386_linux_sigcontext_addr (struct frame_info *frame)

{

  CORE_ADDR pc;

  pc = i386_linux_sigtramp_start (frame->pc);

  if (pc)

    {

      CORE_ADDR sp;

      if (frame->next)

        /* If this isn't the top frame, the next frame must be for the

           signal handler itself.  The sigcontext structure lives on

           the stack, right after the signum argument.  */

        return frame->next->frame + 12;

      /* This is the top frame.  We'll have to find the address of the

         sigcontext structure by looking at the stack pointer.  Keep

         in mind that the first instruction of the sigtramp code is

         "pop %eax".  If the PC is at this instruction, adjust the

         returned value accordingly.  */

      sp = read_register (SP_REGNUM);

      if (pc == frame->pc)

        return sp + 4;

      return sp;

    }

  pc = i386_linux_rt_sigtramp_start (frame->pc);

  if (pc)

    {

      if (frame->next)

        /* If this isn't the top frame, the next frame must be for the

           signal handler itself.  The sigcontext structure is part of

           the user context.  A pointer to the user context is passed

           as the third argument to the signal handler.  */

        return read_memory_integer (frame->next->frame + 16, 4) + 20;

      /* This is the top frame.  Again, use the stack pointer to find

         the address of the sigcontext structure.  */

      return read_memory_integer (read_register (SP_REGNUM) + 8, 4) + 20;

    }

  error ("Couldn't recognize signal trampoline.");

  return 0;

}

/* Offset to saved PC in sigcontext, from <asm/sigcontext.h>.  */

#define LINUX_SIGCONTEXT_PC_OFFSET (56)

/* Assuming FRAME is for a Linux sigtramp routine, return the saved

   program counter.  */

CORE_ADDR

i386_linux_sigtramp_saved_pc (struct frame_info *frame)

{

  CORE_ADDR addr;

  addr = i386_linux_sigcontext_addr (frame);

  return read_memory_integer (addr + LINUX_SIGCONTEXT_PC_OFFSET, 4);

}

/* Offset to saved SP in sigcontext, from <asm/sigcontext.h>.  */

#define LINUX_SIGCONTEXT_SP_OFFSET (28)

/* Assuming FRAME is for a Linux sigtramp routine, return the saved

   stack pointer.  */

CORE_ADDR

i386_linux_sigtramp_saved_sp (struct frame_info *frame)

{

  CORE_ADDR addr;

  addr = i386_linux_sigcontext_addr (frame);

  return read_memory_integer (addr + LINUX_SIGCONTEXT_SP_OFFSET, 4);

}

/* Immediately after a function call, return the saved pc.  */

CORE_ADDR

i386_linux_saved_pc_after_call (struct frame_info *frame)

{

  if (frame->signal_handler_caller)

    return i386_linux_sigtramp_saved_pc (frame);

  return read_memory_integer (read_register (SP_REGNUM), 4);

}