libdwfl/linux-pid-attach.c - platform/external/elfutils.git - Git at Google

 /* Get Dwarf Frame state for target live PID process.
    Copyright (C) 2013, 2014, 2015, 2018 Red Hat, Inc.
    This file is part of elfutils.

    This file is free software; you can redistribute it and/or modify
    it under the terms of either

      * the GNU Lesser General Public License as published by the Free
        Software Foundation; either version 3 of the License, or (at
        your option) any later version

    or

      * 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

    or both in parallel, as here.

    elfutils 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 copies of the GNU General Public License and
    the GNU Lesser General Public License along with this program.  If
    not, see <http://www.gnu.org/licenses/>.  */

 #ifdef HAVE_CONFIG_H
 # include <config.h>
 #endif

 #include "libelfP.h"
 #include "libdwflP.h"
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <dirent.h>
 #include <unistd.h>

 #ifdef __linux__

 #include <sys/uio.h>
 #include <sys/ptrace.h>
 #include <sys/syscall.h>
 #include <sys/wait.h>

 static bool
 linux_proc_pid_is_stopped (pid_t pid)
 {
   char buffer[64];
   FILE *procfile;
   bool retval, have_state;

   snprintf (buffer, sizeof (buffer), "/proc/%ld/status", (long) pid);
   procfile = fopen (buffer, "r");
   if (procfile == NULL)
     return false;

   have_state = false;
   while (fgets (buffer, sizeof (buffer), procfile) != NULL)
     if (strncmp (buffer, "State:", 6) == 0)
       {
 	have_state = true;
 	break;
       }
   retval = (have_state && strstr (buffer, "T (stopped)") != NULL);
   fclose (procfile);
   return retval;
 }

 bool
 internal_function
 __libdwfl_ptrace_attach (pid_t tid, bool *tid_was_stoppedp)
 {
   if (ptrace (PTRACE_ATTACH, tid, NULL, NULL) != 0)
     {
       __libdwfl_seterrno (DWFL_E_ERRNO);
       return false;
     }
   *tid_was_stoppedp = linux_proc_pid_is_stopped (tid);
   if (*tid_was_stoppedp)
     {
       /* Make sure there is a SIGSTOP signal pending even when the process is
 	 already State: T (stopped).  Older kernels might fail to generate
 	 a SIGSTOP notification in that case in response to our PTRACE_ATTACH
 	 above.  Which would make the waitpid below wait forever.  So emulate
 	 it.  Since there can only be one SIGSTOP notification pending this is
 	 safe.  See also gdb/linux-nat.c linux_nat_post_attach_wait.  */
       syscall (__NR_tkill, tid, SIGSTOP);
       ptrace (PTRACE_CONT, tid, NULL, NULL);
     }
   for (;;)
     {
       int status;
       if (waitpid (tid, &status, __WALL) != tid || !WIFSTOPPED (status))
 	{
 	  int saved_errno = errno;
 	  ptrace (PTRACE_DETACH, tid, NULL, NULL);
 	  errno = saved_errno;
 	  __libdwfl_seterrno (DWFL_E_ERRNO);
 	  return false;
 	}
       if (WSTOPSIG (status) == SIGSTOP)
 	break;
       if (ptrace (PTRACE_CONT, tid, NULL,
 		  (void *) (uintptr_t) WSTOPSIG (status)) != 0)
 	{
 	  int saved_errno = errno;
 	  ptrace (PTRACE_DETACH, tid, NULL, NULL);
 	  errno = saved_errno;
 	  __libdwfl_seterrno (DWFL_E_ERRNO);
 	  return false;
 	}
     }
   return true;
 }

 #ifdef HAVE_PROCESS_VM_READV
 /* Note that the result word size depends on the architecture word size.
    That is sizeof long. */
 static bool
 read_cached_memory (struct __libdwfl_pid_arg *pid_arg,
 		    Dwarf_Addr addr, Dwarf_Word *result)
 {
   /* Let the ptrace fallback deal with the corner case of the address
      possibly crossing a page boundery.  */
   if ((addr & ((Dwarf_Addr)__LIBDWFL_REMOTE_MEM_CACHE_SIZE - 1))
       > (Dwarf_Addr)__LIBDWFL_REMOTE_MEM_CACHE_SIZE - sizeof (unsigned long))
     return false;

   struct __libdwfl_remote_mem_cache *mem_cache = pid_arg->mem_cache;
   if (mem_cache == NULL)
     {
       size_t mem_cache_size = sizeof (struct __libdwfl_remote_mem_cache);
       mem_cache = (struct __libdwfl_remote_mem_cache *) malloc (mem_cache_size);
       if (mem_cache == NULL)
 	return false;

       mem_cache->addr = 0;
       mem_cache->len = 0;
       pid_arg->mem_cache = mem_cache;
     }

   unsigned char *d;
   if (addr >= mem_cache->addr && addr - mem_cache->addr < mem_cache->len)
     {
       d = &mem_cache->buf[addr - mem_cache->addr];
       if ((((uintptr_t) d) & (sizeof (unsigned long) - 1)) == 0)
 	*result = *(unsigned long *) d;
       else
 	memcpy (result, d, sizeof (unsigned long));
       return true;
     }

   struct iovec local, remote;
   mem_cache->addr = addr & ~((Dwarf_Addr)__LIBDWFL_REMOTE_MEM_CACHE_SIZE - 1);
   local.iov_base = mem_cache->buf;
   local.iov_len = __LIBDWFL_REMOTE_MEM_CACHE_SIZE;
   remote.iov_base = (void *) (uintptr_t) mem_cache->addr;
   remote.iov_len = __LIBDWFL_REMOTE_MEM_CACHE_SIZE;

   ssize_t res = process_vm_readv (pid_arg->tid_attached,
 				  &local, 1, &remote, 1, 0);
   if (res != __LIBDWFL_REMOTE_MEM_CACHE_SIZE)
     {
       mem_cache->len = 0;
       return false;
     }

   mem_cache->len = res;
   d = &mem_cache->buf[addr - mem_cache->addr];
   if ((((uintptr_t) d) & (sizeof (unsigned long) - 1)) == 0)
     *result = *(unsigned long *) d;
   else
     memcpy (result, d, sizeof (unsigned long));
   return true;
 }
 #endif /* HAVE_PROCESS_VM_READV */

 static void
 clear_cached_memory (struct __libdwfl_pid_arg *pid_arg)
 {
   struct __libdwfl_remote_mem_cache *mem_cache = pid_arg->mem_cache;
   if (mem_cache != NULL)
     mem_cache->len = 0;
 }

 /* Note that the result word size depends on the architecture word size.
    That is sizeof long. */
 static bool
 pid_memory_read (Dwfl *dwfl, Dwarf_Addr addr, Dwarf_Word *result, void *arg)
 {
   struct __libdwfl_pid_arg *pid_arg = arg;
   pid_t tid = pid_arg->tid_attached;
   assert (tid > 0);

 #ifdef HAVE_PROCESS_VM_READV
   if (read_cached_memory (pid_arg, addr, result))
     return true;
 #endif

   Dwfl_Process *process = dwfl->process;
   if (ebl_get_elfclass (process->ebl) == ELFCLASS64)
     {
 #if SIZEOF_LONG == 8
       errno = 0;
       *result = ptrace (PTRACE_PEEKDATA, tid, (void *) (uintptr_t) addr, NULL);
       return errno == 0;
 #else /* SIZEOF_LONG != 8 */
       /* This should not happen.  */
       return false;
 #endif /* SIZEOF_LONG != 8 */
     }
 #if SIZEOF_LONG == 8
   /* We do not care about reads unaliged to 4 bytes boundary.
      But 0x...ffc read of 8 bytes could overrun a page.  */
   bool lowered = (addr & 4) != 0;
   if (lowered)
     addr -= 4;
 #endif /* SIZEOF_LONG == 8 */
   errno = 0;
   *result = ptrace (PTRACE_PEEKDATA, tid, (void *) (uintptr_t) addr, NULL);
   if (errno != 0)
     return false;
 #if SIZEOF_LONG == 8
 # if BYTE_ORDER == BIG_ENDIAN
   if (! lowered)
     *result >>= 32;
 # else
   if (lowered)
     *result >>= 32;
 # endif
 #endif /* SIZEOF_LONG == 8 */
   *result &= 0xffffffff;
   return true;
 }

 static pid_t
 pid_next_thread (Dwfl *dwfl __attribute__ ((unused)), void *dwfl_arg,
 		 void **thread_argp)
 {
   struct __libdwfl_pid_arg *pid_arg = dwfl_arg;
   struct dirent *dirent;
   /* Start fresh on first traversal. */
   if (*thread_argp == NULL)
     rewinddir (pid_arg->dir);
   do
     {
       errno = 0;
       dirent = readdir (pid_arg->dir);
       if (dirent == NULL)
 	{
 	  if (errno != 0)
 	    {
 	      __libdwfl_seterrno (DWFL_E_ERRNO);
 	      return -1;
 	    }
 	  return 0;
 	}
     }
   while (strcmp (dirent->d_name, ".") == 0
 	 || strcmp (dirent->d_name, "..") == 0);
   char *end;
   errno = 0;
   long tidl = strtol (dirent->d_name, &end, 10);
   if (errno != 0)
     {
       __libdwfl_seterrno (DWFL_E_ERRNO);
       return -1;
     }
   pid_t tid = tidl;
   if (tidl <= 0 || (end && *end) || tid != tidl)
     {
       __libdwfl_seterrno (DWFL_E_PARSE_PROC);
       return -1;
     }
   *thread_argp = dwfl_arg;
   return tid;
 }

 /* Just checks that the thread id exists.  */
 static bool
 pid_getthread (Dwfl *dwfl __attribute__ ((unused)), pid_t tid,
 	       void *dwfl_arg, void **thread_argp)
 {
   *thread_argp = dwfl_arg;
   if (kill (tid, 0) < 0)
     {
       __libdwfl_seterrno (DWFL_E_ERRNO);
       return false;
     }
   return true;
 }

 /* Implement the ebl_set_initial_registers_tid setfunc callback.  */

 static bool
 pid_thread_state_registers_cb (int firstreg, unsigned nregs,
 			       const Dwarf_Word *regs, void *arg)
 {
   Dwfl_Thread *thread = (Dwfl_Thread *) arg;
   if (firstreg < 0)
     {
       assert (firstreg == -1);
       assert (nregs == 1);
       INTUSE(dwfl_thread_state_register_pc) (thread, *regs);
       return true;
     }
   assert (nregs > 0);
   return INTUSE(dwfl_thread_state_registers) (thread, firstreg, nregs, regs);
 }

 static bool
 pid_set_initial_registers (Dwfl_Thread *thread, void *thread_arg)
 {
   struct __libdwfl_pid_arg *pid_arg = thread_arg;
   assert (pid_arg->tid_attached == 0);
   pid_t tid = INTUSE(dwfl_thread_tid) (thread);
   if (! pid_arg->assume_ptrace_stopped
       && ! __libdwfl_ptrace_attach (tid, &pid_arg->tid_was_stopped))
     return false;
   pid_arg->tid_attached = tid;
   Dwfl_Process *process = thread->process;
   Ebl *ebl = process->ebl;
   return ebl_set_initial_registers_tid (ebl, tid,
 					pid_thread_state_registers_cb, thread);
 }

 static void
 pid_detach (Dwfl *dwfl __attribute__ ((unused)), void *dwfl_arg)
 {
   struct __libdwfl_pid_arg *pid_arg = dwfl_arg;
   elf_end (pid_arg->elf);
   free (pid_arg->mem_cache);
   close (pid_arg->elf_fd);
   closedir (pid_arg->dir);
   free (pid_arg);
 }

 void
 internal_function
 __libdwfl_ptrace_detach (pid_t tid, bool tid_was_stopped)
 {
   /* This handling is needed only on older Linux kernels such as
      2.6.32-358.23.2.el6.ppc64.  Later kernels such as
      3.11.7-200.fc19.x86_64 remember the T (stopped) state
      themselves and no longer need to pass SIGSTOP during
      PTRACE_DETACH.  */
   ptrace (PTRACE_DETACH, tid, NULL,
 	  (void *) (intptr_t) (tid_was_stopped ? SIGSTOP : 0));
 }

 static void
 pid_thread_detach (Dwfl_Thread *thread, void *thread_arg)
 {
   struct __libdwfl_pid_arg *pid_arg = thread_arg;
   pid_t tid = INTUSE(dwfl_thread_tid) (thread);
   assert (pid_arg->tid_attached == tid);
   pid_arg->tid_attached = 0;
   clear_cached_memory (pid_arg);
   if (! pid_arg->assume_ptrace_stopped)
     __libdwfl_ptrace_detach (tid, pid_arg->tid_was_stopped);
 }

 static const Dwfl_Thread_Callbacks pid_thread_callbacks =
 {
   pid_next_thread,
   pid_getthread,
   pid_memory_read,
   pid_set_initial_registers,
   pid_detach,
   pid_thread_detach,
 };

 int
 dwfl_linux_proc_attach (Dwfl *dwfl, pid_t pid, bool assume_ptrace_stopped)
 {
   char buffer[36];
   FILE *procfile;
   int err = 0; /* The errno to return and set for dwfl->attcherr.  */

   /* Make sure to report the actual PID (thread group leader) to
      dwfl_attach_state.  */
   snprintf (buffer, sizeof (buffer), "/proc/%ld/status", (long) pid);
   procfile = fopen (buffer, "r");
   if (procfile == NULL)
     {
       err = errno;
     fail:
       if (dwfl->process == NULL && dwfl->attacherr == DWFL_E_NOERROR)
 	{
 	  errno = err;
 	  dwfl->attacherr = __libdwfl_canon_error (DWFL_E_ERRNO);
 	}
       return err;
     }

   char *line = NULL;
   size_t linelen = 0;
   while (getline (&line, &linelen, procfile) >= 0)
     if (strncmp (line, "Tgid:", 5) == 0)
       {
 	errno = 0;
 	char *endptr;
 	long val = strtol (&line[5], &endptr, 10);
 	if ((errno == ERANGE && val == LONG_MAX)
 	    || *endptr != '\n' || val < 0 || val != (pid_t) val)
 	  pid = 0;
 	else
 	  pid = (pid_t) val;
 	break;
       }
   free (line);
   fclose (procfile);

   if (pid == 0)
     {
       err = ESRCH;
       goto fail;
     }

   char name[64];
   int i = snprintf (name, sizeof (name), "/proc/%ld/task", (long) pid);
   assert (i > 0 && i < (ssize_t) sizeof (name) - 1);
   DIR *dir = opendir (name);
   if (dir == NULL)
     {
       err = errno;
       goto fail;
     }

   Elf *elf;
   i = snprintf (name, sizeof (name), "/proc/%ld/exe", (long) pid);
   assert (i > 0 && i < (ssize_t) sizeof (name) - 1);
   int elf_fd = open (name, O_RDONLY);
   if (elf_fd >= 0)
     {
       elf = elf_begin (elf_fd, ELF_C_READ_MMAP, NULL);
       if (elf == NULL)
 	{
 	  /* Just ignore, dwfl_attach_state will fall back to trying
 	     to associate the Dwfl with one of the existing DWfl_Module
 	     ELF images (to know the machine/class backend to use).  */
 	  close (elf_fd);
 	  elf_fd = -1;
 	}
     }
   else
     elf = NULL;
   struct __libdwfl_pid_arg *pid_arg = malloc (sizeof *pid_arg);
   if (pid_arg == NULL)
     {
       elf_end (elf);
       close (elf_fd);
       closedir (dir);
       err = ENOMEM;
       goto fail;
     }
   pid_arg->dir = dir;
   pid_arg->elf = elf;
   pid_arg->elf_fd = elf_fd;
   pid_arg->mem_cache = NULL;
   pid_arg->tid_attached = 0;
   pid_arg->assume_ptrace_stopped = assume_ptrace_stopped;
   if (! INTUSE(dwfl_attach_state) (dwfl, elf, pid, &pid_thread_callbacks,
 				   pid_arg))
     {
       elf_end (elf);
       close (elf_fd);
       closedir (dir);
       free (pid_arg);
       return -1;
     }
   return 0;
 }
 INTDEF (dwfl_linux_proc_attach)

 struct __libdwfl_pid_arg *
 internal_function
 __libdwfl_get_pid_arg (Dwfl *dwfl)
 {
   if (dwfl != NULL && dwfl->process != NULL
       && dwfl->process->callbacks == &pid_thread_callbacks)
     return (struct __libdwfl_pid_arg *) dwfl->process->callbacks_arg;

   return NULL;
 }

 #else	/* __linux__ */

 bool
 internal_function
 __libdwfl_ptrace_attach (pid_t tid __attribute__ ((unused)),
 			 bool *tid_was_stoppedp __attribute__ ((unused)))
 {
   errno = ENOSYS;
   __libdwfl_seterrno (DWFL_E_ERRNO);
   return false;
 }

 void
 internal_function
 __libdwfl_ptrace_detach (pid_t tid __attribute__ ((unused)),
 			 bool tid_was_stopped __attribute__ ((unused)))
 {
 }

 int
 dwfl_linux_proc_attach (Dwfl *dwfl __attribute__ ((unused)),
 			pid_t pid __attribute__ ((unused)),
 			bool assume_ptrace_stopped __attribute__ ((unused)))
 {
   return ENOSYS;
 }
 INTDEF (dwfl_linux_proc_attach)

 struct __libdwfl_pid_arg *
 internal_function
 __libdwfl_get_pid_arg (Dwfl *dwfl __attribute__ ((unused)))
 {
   return NULL;
 }

 #endif /* ! __linux __ */
	/* Get Dwarf Frame state for target live PID process.
	Copyright (C) 2013, 2014, 2015, 2018 Red Hat, Inc.
	This file is part of elfutils.

	This file is free software; you can redistribute it and/or modify
	it under the terms of either

	* the GNU Lesser General Public License as published by the Free
	Software Foundation; either version 3 of the License, or (at
	your option) any later version

	or

	* 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

	or both in parallel, as here.

	elfutils 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 copies of the GNU General Public License and
	the GNU Lesser General Public License along with this program. If
	not, see <http://www.gnu.org/licenses/>. */

	#ifdef HAVE_CONFIG_H
	# include <config.h>
	#endif

	#include "libelfP.h"
	#include "libdwflP.h"
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <fcntl.h>
	#include <dirent.h>
	#include <unistd.h>

	#ifdef __linux__

	#include <sys/uio.h>
	#include <sys/ptrace.h>
	#include <sys/syscall.h>
	#include <sys/wait.h>

	static bool
	linux_proc_pid_is_stopped (pid_t pid)
	{
	char buffer[64];
	FILE *procfile;
	bool retval, have_state;

	snprintf (buffer, sizeof (buffer), "/proc/%ld/status", (long) pid);
	procfile = fopen (buffer, "r");
	if (procfile == NULL)
	return false;

	have_state = false;
	while (fgets (buffer, sizeof (buffer), procfile) != NULL)
	if (strncmp (buffer, "State:", 6) == 0)
	{
	have_state = true;
	break;
	}
	retval = (have_state && strstr (buffer, "T (stopped)") != NULL);
	fclose (procfile);
	return retval;
	}

	bool
	internal_function
	__libdwfl_ptrace_attach (pid_t tid, bool *tid_was_stoppedp)
	{
	if (ptrace (PTRACE_ATTACH, tid, NULL, NULL) != 0)
	{
	__libdwfl_seterrno (DWFL_E_ERRNO);
	return false;
	}
	*tid_was_stoppedp = linux_proc_pid_is_stopped (tid);
	if (*tid_was_stoppedp)
	{
	/* Make sure there is a SIGSTOP signal pending even when the process is
	already State: T (stopped). Older kernels might fail to generate
	a SIGSTOP notification in that case in response to our PTRACE_ATTACH
	above. Which would make the waitpid below wait forever. So emulate
	it. Since there can only be one SIGSTOP notification pending this is
	safe. See also gdb/linux-nat.c linux_nat_post_attach_wait. */
	syscall (__NR_tkill, tid, SIGSTOP);
	ptrace (PTRACE_CONT, tid, NULL, NULL);
	}
	for (;;)
	{
	int status;
	if (waitpid (tid, &status, __WALL) != tid \|\| !WIFSTOPPED (status))
	{
	int saved_errno = errno;
	ptrace (PTRACE_DETACH, tid, NULL, NULL);
	errno = saved_errno;
	__libdwfl_seterrno (DWFL_E_ERRNO);
	return false;
	}
	if (WSTOPSIG (status) == SIGSTOP)
	break;
	if (ptrace (PTRACE_CONT, tid, NULL,
	(void *) (uintptr_t) WSTOPSIG (status)) != 0)
	{
	int saved_errno = errno;
	ptrace (PTRACE_DETACH, tid, NULL, NULL);
	errno = saved_errno;
	__libdwfl_seterrno (DWFL_E_ERRNO);
	return false;
	}
	}
	return true;
	}

	#ifdef HAVE_PROCESS_VM_READV
	/* Note that the result word size depends on the architecture word size.
	That is sizeof long. */
	static bool
	read_cached_memory (struct __libdwfl_pid_arg *pid_arg,
	Dwarf_Addr addr, Dwarf_Word *result)
	{
	/* Let the ptrace fallback deal with the corner case of the address
	possibly crossing a page boundery. */
	if ((addr & ((Dwarf_Addr)__LIBDWFL_REMOTE_MEM_CACHE_SIZE - 1))
	> (Dwarf_Addr)__LIBDWFL_REMOTE_MEM_CACHE_SIZE - sizeof (unsigned long))
	return false;

	struct __libdwfl_remote_mem_cache *mem_cache = pid_arg->mem_cache;
	if (mem_cache == NULL)
	{
	size_t mem_cache_size = sizeof (struct __libdwfl_remote_mem_cache);
	mem_cache = (struct __libdwfl_remote_mem_cache *) malloc (mem_cache_size);
	if (mem_cache == NULL)
	return false;

	mem_cache->addr = 0;
	mem_cache->len = 0;
	pid_arg->mem_cache = mem_cache;
	}

	unsigned char *d;
	if (addr >= mem_cache->addr && addr - mem_cache->addr < mem_cache->len)
	{
	d = &mem_cache->buf[addr - mem_cache->addr];
	if ((((uintptr_t) d) & (sizeof (unsigned long) - 1)) == 0)
	result = (unsigned long *) d;
	else
	memcpy (result, d, sizeof (unsigned long));
	return true;
	}

	struct iovec local, remote;
	mem_cache->addr = addr & ~((Dwarf_Addr)__LIBDWFL_REMOTE_MEM_CACHE_SIZE - 1);
	local.iov_base = mem_cache->buf;
	local.iov_len = __LIBDWFL_REMOTE_MEM_CACHE_SIZE;
	remote.iov_base = (void *) (uintptr_t) mem_cache->addr;
	remote.iov_len = __LIBDWFL_REMOTE_MEM_CACHE_SIZE;

	ssize_t res = process_vm_readv (pid_arg->tid_attached,
	&local, 1, &remote, 1, 0);
	if (res != __LIBDWFL_REMOTE_MEM_CACHE_SIZE)
	{
	mem_cache->len = 0;
	return false;
	}

	mem_cache->len = res;
	d = &mem_cache->buf[addr - mem_cache->addr];
	if ((((uintptr_t) d) & (sizeof (unsigned long) - 1)) == 0)
	result = (unsigned long *) d;
	else
	memcpy (result, d, sizeof (unsigned long));
	return true;
	}
	#endif /* HAVE_PROCESS_VM_READV */

	static void
	clear_cached_memory (struct __libdwfl_pid_arg *pid_arg)
	{
	struct __libdwfl_remote_mem_cache *mem_cache = pid_arg->mem_cache;
	if (mem_cache != NULL)
	mem_cache->len = 0;
	}

	/* Note that the result word size depends on the architecture word size.
	That is sizeof long. */
	static bool
	pid_memory_read (Dwfl dwfl, Dwarf_Addr addr, Dwarf_Word result, void *arg)
	{
	struct __libdwfl_pid_arg *pid_arg = arg;
	pid_t tid = pid_arg->tid_attached;
	assert (tid > 0);

	#ifdef HAVE_PROCESS_VM_READV
	if (read_cached_memory (pid_arg, addr, result))
	return true;
	#endif

	Dwfl_Process *process = dwfl->process;
	if (ebl_get_elfclass (process->ebl) == ELFCLASS64)
	{
	#if SIZEOF_LONG == 8
	errno = 0;
	result = ptrace (PTRACE_PEEKDATA, tid, (void ) (uintptr_t) addr, NULL);
	return errno == 0;
	#else /* SIZEOF_LONG != 8 */
	/* This should not happen. */
	return false;
	#endif /* SIZEOF_LONG != 8 */
	}
	#if SIZEOF_LONG == 8
	/* We do not care about reads unaliged to 4 bytes boundary.
	But 0x...ffc read of 8 bytes could overrun a page. */
	bool lowered = (addr & 4) != 0;
	if (lowered)
	addr -= 4;
	#endif /* SIZEOF_LONG == 8 */
	errno = 0;
	result = ptrace (PTRACE_PEEKDATA, tid, (void ) (uintptr_t) addr, NULL);
	if (errno != 0)
	return false;
	#if SIZEOF_LONG == 8
	# if BYTE_ORDER == BIG_ENDIAN
	if (! lowered)
	*result >>= 32;
	# else
	if (lowered)
	*result >>= 32;
	# endif
	#endif /* SIZEOF_LONG == 8 */
	*result &= 0xffffffff;
	return true;
	}

	static pid_t
	pid_next_thread (Dwfl dwfl __attribute__ ((unused)), void dwfl_arg,
	void **thread_argp)
	{
	struct __libdwfl_pid_arg *pid_arg = dwfl_arg;
	struct dirent *dirent;
	/* Start fresh on first traversal. */
	if (*thread_argp == NULL)
	rewinddir (pid_arg->dir);
	do
	{
	errno = 0;
	dirent = readdir (pid_arg->dir);
	if (dirent == NULL)
	{
	if (errno != 0)
	{
	__libdwfl_seterrno (DWFL_E_ERRNO);
	return -1;
	}
	return 0;
	}
	}
	while (strcmp (dirent->d_name, ".") == 0
	\|\| strcmp (dirent->d_name, "..") == 0);
	char *end;
	errno = 0;
	long tidl = strtol (dirent->d_name, &end, 10);
	if (errno != 0)
	{
	__libdwfl_seterrno (DWFL_E_ERRNO);
	return -1;
	}
	pid_t tid = tidl;
	if (tidl <= 0 \|\| (end && *end) \|\| tid != tidl)
	{
	__libdwfl_seterrno (DWFL_E_PARSE_PROC);
	return -1;
	}
	*thread_argp = dwfl_arg;
	return tid;
	}

	/* Just checks that the thread id exists. */
	static bool
	pid_getthread (Dwfl *dwfl __attribute__ ((unused)), pid_t tid,
	void dwfl_arg, void *thread_argp)
	{
	*thread_argp = dwfl_arg;
	if (kill (tid, 0) < 0)
	{
	__libdwfl_seterrno (DWFL_E_ERRNO);
	return false;
	}
	return true;
	}

	/* Implement the ebl_set_initial_registers_tid setfunc callback. */

	static bool
	pid_thread_state_registers_cb (int firstreg, unsigned nregs,
	const Dwarf_Word regs, void arg)
	{
	Dwfl_Thread thread = (Dwfl_Thread ) arg;
	if (firstreg < 0)
	{
	assert (firstreg == -1);
	assert (nregs == 1);
	INTUSE(dwfl_thread_state_register_pc) (thread, *regs);
	return true;
	}
	assert (nregs > 0);
	return INTUSE(dwfl_thread_state_registers) (thread, firstreg, nregs, regs);
	}

	static bool
	pid_set_initial_registers (Dwfl_Thread thread, void thread_arg)
	{
	struct __libdwfl_pid_arg *pid_arg = thread_arg;
	assert (pid_arg->tid_attached == 0);
	pid_t tid = INTUSE(dwfl_thread_tid) (thread);
	if (! pid_arg->assume_ptrace_stopped
	&& ! __libdwfl_ptrace_attach (tid, &pid_arg->tid_was_stopped))
	return false;
	pid_arg->tid_attached = tid;
	Dwfl_Process *process = thread->process;
	Ebl *ebl = process->ebl;
	return ebl_set_initial_registers_tid (ebl, tid,
	pid_thread_state_registers_cb, thread);
	}

	static void
	pid_detach (Dwfl dwfl __attribute__ ((unused)), void dwfl_arg)
	{
	struct __libdwfl_pid_arg *pid_arg = dwfl_arg;
	elf_end (pid_arg->elf);
	free (pid_arg->mem_cache);
	close (pid_arg->elf_fd);
	closedir (pid_arg->dir);
	free (pid_arg);
	}

	void
	internal_function
	__libdwfl_ptrace_detach (pid_t tid, bool tid_was_stopped)
	{
	/* This handling is needed only on older Linux kernels such as
	2.6.32-358.23.2.el6.ppc64. Later kernels such as
	3.11.7-200.fc19.x86_64 remember the T (stopped) state
	themselves and no longer need to pass SIGSTOP during
	PTRACE_DETACH. */
	ptrace (PTRACE_DETACH, tid, NULL,
	(void *) (intptr_t) (tid_was_stopped ? SIGSTOP : 0));
	}

	static void
	pid_thread_detach (Dwfl_Thread thread, void thread_arg)
	{
	struct __libdwfl_pid_arg *pid_arg = thread_arg;
	pid_t tid = INTUSE(dwfl_thread_tid) (thread);
	assert (pid_arg->tid_attached == tid);
	pid_arg->tid_attached = 0;
	clear_cached_memory (pid_arg);
	if (! pid_arg->assume_ptrace_stopped)
	__libdwfl_ptrace_detach (tid, pid_arg->tid_was_stopped);
	}

	static const Dwfl_Thread_Callbacks pid_thread_callbacks =
	{
	pid_next_thread,
	pid_getthread,
	pid_memory_read,
	pid_set_initial_registers,
	pid_detach,
	pid_thread_detach,
	};

	int
	dwfl_linux_proc_attach (Dwfl *dwfl, pid_t pid, bool assume_ptrace_stopped)
	{
	char buffer[36];
	FILE *procfile;
	int err = 0; /* The errno to return and set for dwfl->attcherr. */

	/* Make sure to report the actual PID (thread group leader) to
	dwfl_attach_state. */
	snprintf (buffer, sizeof (buffer), "/proc/%ld/status", (long) pid);
	procfile = fopen (buffer, "r");
	if (procfile == NULL)
	{
	err = errno;
	fail:
	if (dwfl->process == NULL && dwfl->attacherr == DWFL_E_NOERROR)
	{
	errno = err;
	dwfl->attacherr = __libdwfl_canon_error (DWFL_E_ERRNO);
	}
	return err;
	}

	char *line = NULL;
	size_t linelen = 0;
	while (getline (&line, &linelen, procfile) >= 0)
	if (strncmp (line, "Tgid:", 5) == 0)
	{
	errno = 0;
	char *endptr;
	long val = strtol (&line[5], &endptr, 10);
	if ((errno == ERANGE && val == LONG_MAX)
	\|\| *endptr != '\n' \|\| val < 0 \|\| val != (pid_t) val)
	pid = 0;
	else
	pid = (pid_t) val;
	break;
	}
	free (line);
	fclose (procfile);

	if (pid == 0)
	{
	err = ESRCH;
	goto fail;
	}

	char name[64];
	int i = snprintf (name, sizeof (name), "/proc/%ld/task", (long) pid);
	assert (i > 0 && i < (ssize_t) sizeof (name) - 1);
	DIR *dir = opendir (name);
	if (dir == NULL)
	{
	err = errno;
	goto fail;
	}

	Elf *elf;
	i = snprintf (name, sizeof (name), "/proc/%ld/exe", (long) pid);
	assert (i > 0 && i < (ssize_t) sizeof (name) - 1);
	int elf_fd = open (name, O_RDONLY);
	if (elf_fd >= 0)
	{
	elf = elf_begin (elf_fd, ELF_C_READ_MMAP, NULL);
	if (elf == NULL)
	{
	/* Just ignore, dwfl_attach_state will fall back to trying
	to associate the Dwfl with one of the existing DWfl_Module
	ELF images (to know the machine/class backend to use). */
	close (elf_fd);
	elf_fd = -1;
	}
	}
	else
	elf = NULL;
	struct __libdwfl_pid_arg pid_arg = malloc (sizeof pid_arg);
	if (pid_arg == NULL)
	{
	elf_end (elf);
	close (elf_fd);
	closedir (dir);
	err = ENOMEM;
	goto fail;
	}
	pid_arg->dir = dir;
	pid_arg->elf = elf;
	pid_arg->elf_fd = elf_fd;
	pid_arg->mem_cache = NULL;
	pid_arg->tid_attached = 0;
	pid_arg->assume_ptrace_stopped = assume_ptrace_stopped;
	if (! INTUSE(dwfl_attach_state) (dwfl, elf, pid, &pid_thread_callbacks,
	pid_arg))
	{
	elf_end (elf);
	close (elf_fd);
	closedir (dir);
	free (pid_arg);
	return -1;
	}
	return 0;
	}
	INTDEF (dwfl_linux_proc_attach)

	struct __libdwfl_pid_arg *
	internal_function
	__libdwfl_get_pid_arg (Dwfl *dwfl)
	{
	if (dwfl != NULL && dwfl->process != NULL
	&& dwfl->process->callbacks == &pid_thread_callbacks)
	return (struct __libdwfl_pid_arg *) dwfl->process->callbacks_arg;

	return NULL;
	}

	#else /* __linux__ */

	bool
	internal_function
	__libdwfl_ptrace_attach (pid_t tid __attribute__ ((unused)),
	bool *tid_was_stoppedp __attribute__ ((unused)))
	{
	errno = ENOSYS;
	__libdwfl_seterrno (DWFL_E_ERRNO);
	return false;
	}

	void
	internal_function
	__libdwfl_ptrace_detach (pid_t tid __attribute__ ((unused)),
	bool tid_was_stopped __attribute__ ((unused)))
	{
	}

	int
	dwfl_linux_proc_attach (Dwfl *dwfl __attribute__ ((unused)),
	pid_t pid __attribute__ ((unused)),
	bool assume_ptrace_stopped __attribute__ ((unused)))
	{
	return ENOSYS;
	}
	INTDEF (dwfl_linux_proc_attach)

	struct __libdwfl_pid_arg *
	internal_function
	__libdwfl_get_pid_arg (Dwfl *dwfl __attribute__ ((unused)))
	{
	return NULL;
	}

	#endif /* ! __linux __ */