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[l4.git] / l4 / pkg / valgrind / src / valgrind-3.6.0-svn / coregrind / m_gdbserver / remote-utils.c

/* Remote utility routines for the remote server for GDB.
   Copyright (C) 1986, 1989, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
   2001, 2002, 2003, 2004, 2005, 2006, 2011
   Free Software Foundation, Inc.

   This file is part of GDB.
   It has been modified to integrate it in valgrind

   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., 51 Franklin Street, Fifth Floor,
   Boston, MA 02110-1301, USA.  */

#include "pub_core_basics.h"
#include "pub_core_vki.h"
#include "pub_core_vkiscnums.h"
#include "pub_core_libcsignal.h"
#include "pub_core_options.h"

#include "server.h"

#  if defined(VGO_linux)
#include <sys/prctl.h>
#  endif

Bool noack_mode;

static int readchar (int single);

void remote_utils_output_status(void);

static int remote_desc;

static VgdbShared *shared;
static int  last_looked_cntr = -1;
static struct vki_pollfd remote_desc_pollfdread_activity;
#define INVALID_DESCRIPTOR -1

/* for a gdbserver embedded in valgrind, we read from a FIFO and write
   to another FIFO So, we need two descriptors */
static int write_remote_desc = INVALID_DESCRIPTOR;
static int pid_from_to_creator;
/* only this pid will remove the FIFOs: if an exec fails, we must avoid
   that the exiting child believes it has to remove the FIFOs of its parent */
static int mknod_done = 0;

static char *from_gdb = NULL;
static char *to_gdb = NULL;
static char *shared_mem = NULL;

static
int open_fifo (char *side, char *path, int flags)
{
  SysRes o;
  int fd;
  dlog(1, "Opening %s side %s\n", side, path);
  o = VG_(open) (path, flags, 0);
  if (sr_isError (o)) {
     sr_perror(o, "open fifo %s\n", path);
     fatal ("valgrind: fatal error: vgdb FIFO cannot be opened.\n");
  } else {
     fd = sr_Res(o);
     dlog(1, "result fd %d\n", fd);
  }
  fd = VG_(safe_fd)(fd);
  dlog(1, "result safe_fd %d\n", fd);
  if (fd == -1)
     fatal("safe_fd for vgdb FIFO failed\n");
  return fd;
}

void remote_utils_output_status(void)
{
   if (shared == NULL)
      VG_(umsg)("remote communication not initialized\n");
   else
      VG_(umsg)("shared->written_by_vgdb %d shared->seen_by_valgrind %d\n",
                shared->written_by_vgdb, shared->seen_by_valgrind);
}

/* Returns 0 if vgdb and connection state looks good,
   otherwise returns an int value telling which check failed. */
static
int vgdb_state_looks_bad(char* where)
{
   if (VG_(kill)(shared->vgdb_pid, 0) != 0)
      return 1; // vgdb process does not exist anymore.

   if (remote_desc_activity(where) == 2)
      return 2; // check for error on remote desc shows a problem

   if (remote_desc == INVALID_DESCRIPTOR)
      return 3; // after check, remote_desc not ok anymore
       
   return 0; // all is ok.
}

/* On systems that defines PR_SET_PTRACER, verify if ptrace_scope is
   is permissive enough for vgdb. Otherwise, call set_ptracer.
   This is especially aimed at Ubuntu >= 10.10 which has added
   the ptrace_scope context. */
static
void set_ptracer(void)
{
#ifdef PR_SET_PTRACER
   SysRes o;
   char *ptrace_scope_setting_file = "/proc/sys/kernel/yama/ptrace_scope";
   int fd;
   char ptrace_scope;
   int ret;

   o = VG_(open) (ptrace_scope_setting_file, VKI_O_RDONLY, 0);
   if (sr_isError(o)) {
      if (VG_(debugLog_getLevel)() >= 1) {
         sr_perror(o, "error VG_(open) %s\n", ptrace_scope_setting_file);
      }
      /* can't read setting. Assuming ptrace can be called by vgdb. */
      return;
   }
   fd = sr_Res(o);
   if (VG_(read) (fd, &ptrace_scope, 1) == 1) {
      dlog(1, "ptrace_scope %c\n", ptrace_scope);
      if (ptrace_scope != '0') {
         /* insufficient default ptrace_scope.
            Indicate to the kernel that we accept to be
            ptraced by our vgdb. */
         ret = VG_(prctl) (PR_SET_PTRACER, shared->vgdb_pid, 0, 0, 0);
         dlog(1, "set_ptracer to vgdb_pid %d result %d\n",
              shared->vgdb_pid, ret);
      }
   } else {
      dlog(0, "Could not read the ptrace_scope setting from %s\n",
           ptrace_scope_setting_file);
   }

   VG_(close) (fd);
#endif
}

/* returns 1 if one or more poll "errors" is set.
   Errors are: VKI_POLLERR or VKI_POLLHUP or VKI_POLLNAL */
static
int poll_cond (short revents)
{
   return (revents & (VKI_POLLERR | VKI_POLLHUP | VKI_POLLNVAL));
}

/* Ensures we have a valid write file descriptor.
   Returns 1 if we have a valid write file descriptor,
   0 if the write fd could not be opened. */
static
int ensure_write_remote_desc(void)
{
   struct vki_pollfd write_remote_desc_ok;
   int ret;
   if (write_remote_desc != INVALID_DESCRIPTOR) {
      write_remote_desc_ok.fd = write_remote_desc;
      write_remote_desc_ok.events = VKI_POLLOUT;
      write_remote_desc_ok.revents = 0;
      ret = VG_(poll)(&write_remote_desc_ok, 1, 0);
      if (ret && poll_cond(write_remote_desc_ok.revents)) {
         dlog(1, "POLLcond %d closing write_remote_desc %d\n", 
              write_remote_desc_ok.revents, write_remote_desc);
         VG_(close) (write_remote_desc);
         write_remote_desc = INVALID_DESCRIPTOR;
      }
   }
   if (write_remote_desc == INVALID_DESCRIPTOR) {
      /* open_fifo write will block if the receiving vgdb
         process is dead.  So, let's check for vgdb state to
         be reasonably sure someone is reading on the other
         side of the fifo. */
      if (!vgdb_state_looks_bad("bad?@ensure_write_remote_desc")) {
         set_ptracer();
         write_remote_desc = open_fifo ("write", to_gdb, VKI_O_WRONLY);
      }
   }

   return (write_remote_desc != INVALID_DESCRIPTOR);
}

#if defined(VGO_darwin)
#define VKI_S_IFIFO 0010000
#endif
static
void safe_mknod (char *nod)
{
   SysRes m;
   m = VG_(mknod) (nod, VKI_S_IFIFO|0666, 0);
   if (sr_isError (m)) {
      if (sr_Err (m) == VKI_EEXIST) {
         if (VG_(clo_verbosity) > 1) {
            VG_(umsg)("%s already created\n", nod);
         }
      } else {
         sr_perror(m, "mknod %s\n", nod);
         VG_(umsg) ("valgrind: fatal error: vgdb FIFOs cannot be created.\n");
         VG_(exit)(1);
      }
   }
}

/* Open a connection to a remote debugger.
   NAME is the filename used for communication.  
   For Valgrind, name is the prefix for the two read and write FIFOs
   The two FIFOs names will be build by appending 
   -from-vgdb-to-pid and -to-vgdb-from-pid
   with pid being the pidnr of the valgrind process These two FIFOs
   will be created if not existing yet. They will be removed when
   the gdbserver connection is closed or the process exits */

void remote_open (char *name)
{
   int save_fcntl_flags;
   VgdbShared vgdbinit = 
      {0, 0, 0, (Addr) VG_(invoke_gdbserver),
       (Addr) VG_(threads), sizeof(ThreadState), 
       offsetof(ThreadState, status),
       offsetof(ThreadState, os_state) + offsetof(ThreadOSstate, lwpid)};
   const int pid = VG_(getpid)();
   const int name_default = strcmp(name, VG_CLO_VGDB_PREFIX_DEFAULT) == 0;
   Addr addr_shared;
   SysRes o;
   int shared_mem_fd = INVALID_DESCRIPTOR;
   
   if (from_gdb != NULL) 
      free (from_gdb);
   from_gdb = malloc (strlen(name) + 30);
   if (to_gdb != NULL)
      free (to_gdb);
   to_gdb = malloc (strlen(name) + 30);
   if (shared_mem != NULL)
      free (shared_mem);
   shared_mem = malloc (strlen(name) + 30);
   /* below 3 lines must match the equivalent in vgdb.c */
   VG_(sprintf) (from_gdb,   "%s-from-vgdb-to-%d",    name, pid);
   VG_(sprintf) (to_gdb,     "%s-to-vgdb-from-%d",    name, pid);
   VG_(sprintf) (shared_mem, "%s-shared-mem-vgdb-%d", name, pid);
   if (VG_(clo_verbosity) > 1) {
      VG_(umsg)("embedded gdbserver: reading from %s\n", from_gdb);
      VG_(umsg)("embedded gdbserver: writing to   %s\n", to_gdb);
      VG_(umsg)("embedded gdbserver: shared mem   %s\n", shared_mem);
      VG_(umsg)("CONTROL ME using: vgdb --pid=%d%s%s ...command...\n",
                pid, (name_default ? "" : " --vgdb="),
                (name_default ? "" : name));
      VG_(umsg)("DEBUG ME using: (gdb) target remote | vgdb --pid=%d%s%s\n",
                pid, (name_default ? "" : " --vgdb="), 
                (name_default ? "" : name));
      VG_(umsg)("   --pid optional if only one valgrind process is running\n");
   }

   if (!mknod_done) {
      mknod_done++;
      safe_mknod(from_gdb);
      safe_mknod(to_gdb);

      pid_from_to_creator = pid;
      
      o = VG_(open) (shared_mem, VKI_O_CREAT|VKI_O_RDWR, 0666);
      if (sr_isError (o)) {
         sr_perror(o, "cannot create shared_mem file %s\n", shared_mem);
         fatal("");
      } else {
         shared_mem_fd = sr_Res(o);
      }
      
      if (VG_(write)(shared_mem_fd, &vgdbinit, sizeof(VgdbShared)) 
          != sizeof(VgdbShared)) {
         fatal("error writing %d bytes to shared mem %s\n",
               (int) sizeof(VgdbShared), shared_mem);
      }
      shared_mem_fd = VG_(safe_fd)(shared_mem_fd);
      if (shared_mem_fd == -1) {
         fatal("safe_fd for vgdb shared_mem %s failed\n", shared_mem);
      }
      {
         SysRes res = VG_(am_shared_mmap_file_float_valgrind)
            (sizeof(VgdbShared), VKI_PROT_READ|VKI_PROT_WRITE, 
             shared_mem_fd, (Off64T)0);
         if (sr_isError(res)) {
            sr_perror(res, "error VG_(am_shared_mmap_file_float_valgrind) %s\n",
                      shared_mem);
            fatal("");
         }  
         addr_shared = sr_Res (res);
      }
      shared = (VgdbShared*) addr_shared;
   }
   
   /* we open the read side FIFO in non blocking mode
      We then set the fd in blocking mode.
      Opening in non-blocking read mode always succeeds while opening
      in non-blocking write mode succeeds only if the fifo is already
      opened in read mode. So, we wait till we have read the first
      character from the read side before opening the write side. */
   remote_desc = open_fifo ("read", from_gdb, VKI_O_RDONLY|VKI_O_NONBLOCK);
   save_fcntl_flags = VG_(fcntl) (remote_desc, VKI_F_GETFL, 0);
   VG_(fcntl) (remote_desc, VKI_F_SETFL, save_fcntl_flags & ~VKI_O_NONBLOCK);
   remote_desc_pollfdread_activity.fd = remote_desc;
   remote_desc_pollfdread_activity.events = VKI_POLLIN;
   remote_desc_pollfdread_activity.revents = 0;
}

/* sync_gdb_connection wait a time long enough to let the connection
   be properly closed if needed when closing the connection (in case
   of detach or error), if we reopen it too quickly, it seems there
   are some events queued in the kernel concerning the "old"
   connection/remote_desc which are discovered with poll or select on
   the "new" connection/remote_desc.  We bypass this by waiting some
   time to let a proper cleanup to be donex */
void sync_gdb_connection(void)
{
   VG_(poll)(0, 0, 100);
}

static
char * ppFinishReason (FinishReason reason)
{
   switch (reason) {
   case orderly_finish:    return "orderly_finish";
   case reset_after_error: return "reset_after_error";
   case reset_after_fork:  return "reset_after_fork";
   default: vg_assert (0);
   }
}

void remote_finish (FinishReason reason)
{
   dlog(1, "remote_finish (reason %s) %d %d\n", 
        ppFinishReason(reason), remote_desc, write_remote_desc);
   reset_valgrind_sink(ppFinishReason(reason));
   if (write_remote_desc != INVALID_DESCRIPTOR)
      VG_(close) (write_remote_desc);
   write_remote_desc = INVALID_DESCRIPTOR;
   if (remote_desc != INVALID_DESCRIPTOR) {
      remote_desc_pollfdread_activity.fd = INVALID_DESCRIPTOR;
      remote_desc_pollfdread_activity.events = 0;
      remote_desc_pollfdread_activity.revents = 0;
      VG_(close) (remote_desc);
   }
   remote_desc = INVALID_DESCRIPTOR;
   noack_mode = False;
   
   /* ensure the child will create its own FIFOs */
   if (reason == reset_after_fork)
      mknod_done = 0;
   
   if (reason == reset_after_error)
      sync_gdb_connection();
}

/* orderly close, cleans up everything */
void remote_close (void)
{
   const int pid = VG_(getpid)();
   remote_finish(orderly_finish);
   if (pid == pid_from_to_creator) {
      dlog(1, "unlinking\n    %s\n    %s\n    %s\n", 
           from_gdb, to_gdb, shared_mem);
      if (VG_(unlink) (from_gdb) == -1)
         warning ("could not unlink %s\n", from_gdb);
      if (VG_(unlink) (to_gdb) == -1)
         warning ("could not unlink %s\n", to_gdb);
      if (VG_(unlink) (shared_mem) == -1)
         warning ("could not unlink %s\n", shared_mem);
   }
   else {
      dlog(1, "not creator => not unlinking %s and %s\n", from_gdb, to_gdb);
   }
   free (from_gdb);
   free (to_gdb);
}

Bool remote_connected(void)
{
   return write_remote_desc != INVALID_DESCRIPTOR;
}

/* cleanup after an error detected by poll_cond */
static
void error_poll_cond(void)
{
   /* if we will close the connection, we assume either that
      all characters have been seen or that they will be dropped. */
   shared->seen_by_valgrind = shared->written_by_vgdb;
   remote_finish(reset_after_error);
}

/* remote_desc_activity might be used at high frequency if the user
   gives a small value to --vgdb-poll. So, the function avoids
   doing repetitively system calls by rather looking at the
   counter values maintained in shared memory by vgdb. */
int remote_desc_activity(char *msg)
{
   int ret;
   const int looking_at = shared->written_by_vgdb;
   if (shared->seen_by_valgrind == looking_at)
   //   if (last_looked_cntr == looking_at)
      return 0;
   if (remote_desc == INVALID_DESCRIPTOR)
      return 0;

   /* poll the remote desc */
   remote_desc_pollfdread_activity.revents = 0;
   ret = VG_(poll) (&remote_desc_pollfdread_activity, 1, 0);
   if (ret && poll_cond(remote_desc_pollfdread_activity.revents)) {
      dlog(1, "POLLcond %d remote_desc_pollfdread %d\n", 
           remote_desc_pollfdread_activity.revents, remote_desc);
      error_poll_cond();
      ret = 2;
   }
   dlog(1,
        "remote_desc_activity %s %d last_looked_cntr %d looking_at %d"
        " shared->written_by_vgdb %d shared->seen_by_valgrind %d"
        " ret %d\n", 
        msg, remote_desc, last_looked_cntr, looking_at, 
        shared->written_by_vgdb, shared->seen_by_valgrind,
        ret);
   /* if no error from poll, indicate we have "seen" up to looking_at */
   if (ret != 2)
      last_looked_cntr = looking_at;
   return ret;
}

/* Convert hex digit A to a number.  */

static
int fromhex (int a)
{
   if (a >= '0' && a <= '9')
      return a - '0';
   else if (a >= 'a' && a <= 'f')
      return a - 'a' + 10;
   else
      error ("Reply contains invalid hex digit 0x%x\n", a);
   return 0;
}

int unhexify (char *bin, const char *hex, int count)
{
   int i;
   
   for (i = 0; i < count; i++) {
      if (hex[0] == 0 || hex[1] == 0) {
         /* Hex string is short, or of uneven length.
            Return the count that has been converted so far. */
         return i;
      }
      *bin++ = fromhex (hex[0]) * 16 + fromhex (hex[1]);
      hex += 2;
   }
   return i;
}

void decode_address (CORE_ADDR *addrp, const char *start, int len)
{
   CORE_ADDR addr;
   char ch;
   int i;
   
   addr = 0;
   for (i = 0; i < len; i++) {
      ch = start[i];
      addr = addr << 4;
      addr = addr | (fromhex (ch) & 0x0f);
   }
   *addrp = addr;
}

/* Convert number NIB to a hex digit.  */

static
int tohex (int nib)
{
   if (nib < 10)
      return '0' + nib;
   else
      return 'a' + nib - 10;
}

int hexify (char *hex, const char *bin, int count)
{
   int i;

   /* May use a length, or a nul-terminated string as input. */
   if (count == 0)
      count = strlen (bin);

  for (i = 0; i < count; i++) {
     *hex++ = tohex ((*bin >> 4) & 0xf);
     *hex++ = tohex (*bin++ & 0xf);
  }
  *hex = 0;
  return i;
}

/* Convert BUFFER, binary data at least LEN bytes long, into escaped
   binary data in OUT_BUF.  Set *OUT_LEN to the length of the data
   encoded in OUT_BUF, and return the number of bytes in OUT_BUF
   (which may be more than *OUT_LEN due to escape characters).  The
   total number of bytes in the output buffer will be at most
   OUT_MAXLEN.  */

int
remote_escape_output (const gdb_byte *buffer, int len,
                      gdb_byte *out_buf, int *out_len,
                      int out_maxlen)
{
   int input_index, output_index;
   
   output_index = 0;
   for (input_index = 0; input_index < len; input_index++) {
      gdb_byte b = buffer[input_index];

      if (b == '$' || b == '#' || b == '}' || b == '*') {
         /* These must be escaped.  */
         if (output_index + 2 > out_maxlen)
            break;
         out_buf[output_index++] = '}';
         out_buf[output_index++] = b ^ 0x20;
      } else {
         if (output_index + 1 > out_maxlen)
            break;
         out_buf[output_index++] = b;
      }
   }

   *out_len = input_index;
   return output_index;
}

/* Convert BUFFER, escaped data LEN bytes long, into binary data
   in OUT_BUF.  Return the number of bytes written to OUT_BUF.
   Raise an error if the total number of bytes exceeds OUT_MAXLEN.

   This function reverses remote_escape_output.  It allows more
   escaped characters than that function does, in particular because
   '*' must be escaped to avoid the run-length encoding processing
   in reading packets.  */

static
int remote_unescape_input (const gdb_byte *buffer, int len,
                       gdb_byte *out_buf, int out_maxlen)
{
   int input_index, output_index;
   int escaped;
   
   output_index = 0;
   escaped = 0;
   for (input_index = 0; input_index < len; input_index++) {
      gdb_byte b = buffer[input_index];

      if (output_index + 1 > out_maxlen)
         error ("Received too much data (len %d) from the target.\n", len);

      if (escaped) {
         out_buf[output_index++] = b ^ 0x20;
         escaped = 0;
      } else if (b == '}') {
         escaped = 1;
      } else {
         out_buf[output_index++] = b;
      }
   }

   if (escaped)
      error ("Unmatched escape character in target response.\n");

   return output_index;
}

/* Look for a sequence of characters which can be run-length encoded.
   If there are any, update *CSUM and *P.  Otherwise, output the
   single character.  Return the number of characters consumed.  */

static
int try_rle (char *buf, int remaining, unsigned char *csum, char **p)
{
   int n;

   /* Always output the character.  */
   *csum += buf[0];
   *(*p)++ = buf[0];

   /* Don't go past '~'.  */
   if (remaining > 97)
      remaining = 97;
   
   for (n = 1; n < remaining; n++)
      if (buf[n] != buf[0])
         break;
   
   /* N is the index of the first character not the same as buf[0].
      buf[0] is counted twice, so by decrementing N, we get the number
      of characters the RLE sequence will replace.  */
   n--;
   
   if (n < 3)
      return 1;
   
   /* Skip the frame characters.  The manual says to skip '+' and '-'
      also, but there's no reason to.  Unfortunately these two unusable
      characters double the encoded length of a four byte zero
      value.  */
   while (n + 29 == '$' || n + 29 == '#')
      n--;
   
   *csum += '*';
   *(*p)++ = '*';
   *csum += n + 29;
   *(*p)++ = n + 29;
  
   return n + 1;
}

/* Send a packet to the remote machine, with error checking.
   The data of the packet is in BUF, and the length of the
   packet is in CNT.  Returns >= 0 on success, -1 otherwise.  */

int putpkt_binary (char *buf, int cnt)
{
   int i;
   unsigned char csum = 0;
   char *buf2;
   char *p;
   int cc;

   buf2 = malloc (PBUFSIZ);

   /* Copy the packet into buffer BUF2, encapsulating it
      and giving it a checksum.  */

   p = buf2;
   *p++ = '$';

   for (i = 0; i < cnt;)
      i += try_rle (buf + i, cnt - i, &csum, &p);

   *p++ = '#';
   *p++ = tohex ((csum >> 4) & 0xf);
   *p++ = tohex (csum & 0xf);

   *p = '\0';

   /* we might have to write a pkt when out FIFO not yet/anymore opened */
   if (!ensure_write_remote_desc()) {
      warning ("putpkt(write) error: no write_remote_desc\n");
      return -1;
   }

   /* Send it once (noack_mode)
      or send it over and over until we get a positive ack.  */

   do {
      if (VG_(write) (write_remote_desc, buf2, p - buf2) != p - buf2) {
         warning ("putpkt(write) error\n");
         return -1;
      }

      if (noack_mode)
         dlog(1, "putpkt (\"%s\"); [no ack]\n", buf2);
      else
         dlog(1,"putpkt (\"%s\"); [looking for ack]\n", buf2);

      if (noack_mode)
         break;

      cc = readchar (1);
      if (cc > 0)
         dlog(1, "[received '%c' (0x%x)]\n", cc, cc);

      if (cc <= 0) {
         if (cc == 0)
            dlog(1, "putpkt(read): Got EOF\n");
         else
            warning ("putpkt(read) error\n");

         free (buf2);
         return -1;
      }

      /* Check for an input interrupt while we're here.  */
      if (cc == '\003')
         (*the_target->send_signal) (VKI_SIGINT);
   }
   while (cc != '+');

   free (buf2);
   return 1;                    /* Success! */
}

/* Send a packet to the remote machine, with error checking.  The data
   of the packet is in BUF, and the packet should be a NUL-terminated
   string.  Returns >= 0 on success, -1 otherwise.  */

int putpkt (char *buf)
{
   return putpkt_binary (buf, strlen (buf));
}

void monitor_output (char *s)
{
   const int len = strlen(s);
   char *buf = malloc(1 + 2*len + 1);

   buf[0] = 'O';
   hexify(buf+1, s, len);
   if (putpkt (buf) < 0) {
      /* We probably have lost the connection with vgdb. */
      reset_valgrind_sink("Error writing monitor output");
      /* write again after reset */
      VG_(printf) ("%s", s);
   }
   
   free (buf);
}

/* Returns next char from remote GDB.  -1 if error.  */
/* if single, only one character maximum can be read with
   read system call. Otherwise, when reading an ack character
   we might pile up the next gdb command in the static buf.
   The read loop is then blocked in poll till gdb times out. */
static
int readchar (int single)
{
   static unsigned char buf[PBUFSIZ];
   static int bufcnt = 0;
   static unsigned char *bufp;
   int ret;
  
   if (bufcnt-- > 0)
      return *bufp++;

   if (remote_desc == INVALID_DESCRIPTOR)
      return -1;

   /* No characters available in buf =>
      wait for some characters to arrive */
   remote_desc_pollfdread_activity.revents = 0;
   ret = VG_(poll)(&remote_desc_pollfdread_activity, 1, -1);
   if (ret != 1) {
      dlog(0, "readchar: poll got %d\n", ret);
      return -1;
   }
   if (single)
      bufcnt = VG_(read) (remote_desc, buf, 1);
   else
      bufcnt = VG_(read) (remote_desc, buf, sizeof (buf));

   if (bufcnt <= 0) {
      if (bufcnt == 0)
         dlog (1, "readchar: Got EOF\n");
      else
         warning ("readchar read error\n");
      
      return -1;
   }

   shared->seen_by_valgrind += bufcnt;
   
   /* If we have received a character and we do not yet have a
      connection, we better open our "write" fifo to let vgdb open its
      read fifo side */
   if (write_remote_desc == INVALID_DESCRIPTOR 
       && !ensure_write_remote_desc()) {
      dlog(1, "reachar: write_remote_desc could not be created");
   }

   bufp = buf;
   bufcnt--;

   if (poll_cond(remote_desc_pollfdread_activity.revents)) {
      dlog(1, "readchar: POLLcond got %d\n",
           remote_desc_pollfdread_activity.revents);
      error_poll_cond();
   }

   return *bufp++;
}


/* Read a packet from the remote machine, with error checking,
   and store it in BUF.  Returns length of packet, or negative if error. */

int getpkt (char *buf)
{
   char *bp;
   unsigned char csum, c1, c2;
   int c;
  
   while (1) {
      csum = 0;
      
      while (1) {
         c = readchar (0);
         if (c == '$')
            break;
         dlog(1, "[getpkt: discarding char '%c']\n", c);
         if (c < 0)
            return -1;
      }

      bp = buf;
      while (1) {
         c = readchar (0);
         if (c < 0)
            return -1;
         if (c == '#')
            break;
         *bp++ = c;
         csum += c;
      }
      *bp = 0;

      c1 = fromhex (readchar (0));
      c2 = fromhex (readchar (0));

      if (csum == (c1 << 4) + c2)
         break;

      dlog (0, "Bad checksum, sentsum=0x%x, csum=0x%x, buf=%s\n",
            (c1 << 4) + c2, csum, buf);
      if (!ensure_write_remote_desc()) {
         dlog(1, "getpkt(write nack) no write_remote_desc");
      }
      VG_(write) (write_remote_desc, "-", 1);
   }

   if (noack_mode)
      dlog(1, "getpkt (\"%s\");  [no ack] \n", buf);
   else
      dlog(1, "getpkt (\"%s\");  [sending ack] \n", buf);

   if (!noack_mode) {
      if (!ensure_write_remote_desc()) {
         dlog(1, "getpkt(write ack) no write_remote_desc");
      }
      VG_(write) (write_remote_desc, "+", 1);
      dlog(1, "[sent ack]\n");
   }

   return bp - buf;
}

void write_ok (char *buf)
{
   buf[0] = 'O';
   buf[1] = 'K';
   buf[2] = '\0';
}

void write_enn (char *buf)
{
   /* Some day, we should define the meanings of the error codes... */
   buf[0] = 'E';
   buf[1] = '0';
   buf[2] = '1';
   buf[3] = '\0';
}

void convert_int_to_ascii (unsigned char *from, char *to, int n)
{
   int nib;
   int ch;
   while (n--) {
      ch = *from++;
      nib = ((ch & 0xf0) >> 4) & 0x0f;
      *to++ = tohex (nib);
      nib = ch & 0x0f;
      *to++ = tohex (nib);
   }
   *to++ = 0;
}


void convert_ascii_to_int (char *from, unsigned char *to, int n)
{
   int nib1, nib2;
   while (n--) {
      nib1 = fromhex (*from++);
      nib2 = fromhex (*from++);
      *to++ = (((nib1 & 0x0f) << 4) & 0xf0) | (nib2 & 0x0f);
   }
}

static
char * outreg (int regno, char *buf)
{
   if ((regno >> 12) != 0)
      *buf++ = tohex ((regno >> 12) & 0xf);
   if ((regno >> 8) != 0)
      *buf++ = tohex ((regno >> 8) & 0xf);
   *buf++ = tohex ((regno >> 4) & 0xf);
   *buf++ = tohex (regno & 0xf);
   *buf++ = ':';
   collect_register_as_string (regno, buf);
   buf += 2 * register_size (regno);
   *buf++ = ';';

   return buf;
}

void prepare_resume_reply (char *buf, char status, unsigned char sig)
{
   int nib;
   
   *buf++ = status;

   nib = ((sig & 0xf0) >> 4);
   *buf++ = tohex (nib);
   nib = sig & 0x0f;
   *buf++ = tohex (nib);

   if (status == 'T') {
      const char **regp = gdbserver_expedite_regs;
      
      if (the_target->stopped_by_watchpoint != NULL
          && (*the_target->stopped_by_watchpoint) ()) {
         CORE_ADDR addr;
         int i;

         strncpy (buf, "watch:", 6);
         buf += 6;

         addr = (*the_target->stopped_data_address) ();

         /* Convert each byte of the address into two hexadecimal chars.
            Note that we take sizeof (void *) instead of sizeof (addr);
            this is to avoid sending a 64-bit address to a 32-bit GDB.  */
         for (i = sizeof (void *) * 2; i > 0; i--) {
            *buf++ = tohex ((addr >> (i - 1) * 4) & 0xf);
         }
         *buf++ = ';';
      }

      while (*regp) {
         buf = outreg (find_regno (*regp), buf);
         regp ++;
      }

      {
         unsigned int gdb_id_from_wait;
         
         /* FIXME right place to set this? */
         thread_from_wait = 
            ((struct inferior_list_entry *)current_inferior)->id;
         gdb_id_from_wait = thread_to_gdb_id (current_inferior);
         
         dlog(1, "Writing resume reply for %ld\n", thread_from_wait);
         /* This if (1) ought to be unnecessary.  But remote_wait in GDB
            will claim this event belongs to inferior_ptid if we do not
            specify a thread, and there's no way for gdbserver to know
            what inferior_ptid is.  */
         if (1 || old_thread_from_wait != thread_from_wait) {
            general_thread = thread_from_wait;
            VG_(sprintf) (buf, "thread:%x;", gdb_id_from_wait);
            buf += strlen (buf);
            old_thread_from_wait = thread_from_wait;
         }
      }
   }
   /* For W and X, we're done.  */
   *buf++ = 0;
}

void decode_m_packet (char *from, CORE_ADDR *mem_addr_ptr, unsigned int *len_ptr)
{
   int i = 0, j = 0;
   char ch;
   *mem_addr_ptr = *len_ptr = 0;

   while ((ch = from[i++]) != ',') {
      *mem_addr_ptr = *mem_addr_ptr << 4;
      *mem_addr_ptr |= fromhex (ch) & 0x0f;
   }

   for (j = 0; j < 4; j++) {
      if ((ch = from[i++]) == 0)
         break;
      *len_ptr = *len_ptr << 4;
      *len_ptr |= fromhex (ch) & 0x0f;
   }
}

void decode_M_packet (char *from, CORE_ADDR *mem_addr_ptr, unsigned int *len_ptr,
                 unsigned char *to)
{
   int i = 0;
   char ch;
   *mem_addr_ptr = *len_ptr = 0;

   while ((ch = from[i++]) != ',') {
      *mem_addr_ptr = *mem_addr_ptr << 4;
      *mem_addr_ptr |= fromhex (ch) & 0x0f;
   }

   while ((ch = from[i++]) != ':') {
      *len_ptr = *len_ptr << 4;
      *len_ptr |= fromhex (ch) & 0x0f;
   }

   convert_ascii_to_int (&from[i++], to, *len_ptr);
}

int decode_X_packet (char *from, int packet_len, CORE_ADDR *mem_addr_ptr,
                 unsigned int *len_ptr, unsigned char *to)
{
   int i = 0;
   char ch;
   *mem_addr_ptr = *len_ptr = 0;
   
   while ((ch = from[i++]) != ',') {
      *mem_addr_ptr = *mem_addr_ptr << 4;
      *mem_addr_ptr |= fromhex (ch) & 0x0f;
   }

   while ((ch = from[i++]) != ':') {
      *len_ptr = *len_ptr << 4;
      *len_ptr |= fromhex (ch) & 0x0f;
   }

   if (remote_unescape_input ((const gdb_byte *) &from[i], packet_len - i,
                              to, *len_ptr) != *len_ptr)
      return -1;

   return 0;
}