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Re: [patch 2/3] attach-fail-reasons: Say more than ptrace: Operation not permitted.


On Thu, 08 Mar 2012 14:55:22 +0100, Pedro Alves wrote:
> > +extern pid_t linux_proc_get_tracerpid (int lwpid);
> ..
> 
> Would be neater with `pid_t lwpid'.

I chose 'int' specifically for you as this is TID, not PID and specifically
you typically use 'int' instead of 'pid_t' in your patches / in gdbserver,
where pid_t is available.  I do not address this in reviews as I find it as
a real nitpick.

OK, so changed it to pid_t, thanks.


> linux_ptrace_attach_warnings has a tiny race, in that by the time you open /proc/pid/... you
> may be looking at a different process.  There's no way to fix that, I think, and this
> will be right in the large majority of the cases, so we can just live with it, IMO.

I agree, this is affecting all the /proc operations.  As the code examines
/proc only in the case the attachment failed so any races have only
informational impact.


> You may have considered the following already, but I haven't seen it mentioned.  I think it
> may be better if the new warning strings are actually part of the thrown error string:

I fully agree, IIRC I considered it a bit but:

(1) Plain C code dealing with dynamic strings in gdbserver has to be ugly.

(2) I did not want to bring in more of the gdb/ and libiberty/ framework into
    gdbserver as:

  (2a) All the work would be getting a bit out of the scope of this patch.

  (2b) If introducing a new framework to gdbserver code we should bring in STL
       and not the libiberty/gdb code.  But we cannot yet use STL.

So I chose the (1) way in this patch.

No regressions on {x86_64,x86_64-m32,i686}-fedora17-linux-gnu and in
non-extended gdbserver mode.


Thanks,
Jan


gdb/
2012-03-13  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* common/linux-procfs.c (linux_proc_get_int): New, from
	linux_proc_get_tgid, change its LWPID type to pid_t, add parameter
	field.
	(linux_proc_get_tgid): Only call linux_proc_get_int.
	(linux_proc_get_tracerpid): New.
	(linux_proc_pid_has_state): New, from linux_proc_pid_is_zombie.
	(linux_proc_pid_is_stopped, linux_proc_pid_is_zombie): Only call
	linux_proc_pid_has_state.
	* common/linux-procfs.h (linux_proc_get_tracerpid): New declaration.
	* common/linux-ptrace.c: Include linux-procfs.h.
	(linux_ptrace_attach_warnings): New.
	* common/linux-ptrace.h (linux_ptrace_attach_warnings): New declaration.
	* linux-nat.c: Include exceptions.h and linux-ptrace.h.
	(linux_nat_attach): New variables ex and message.  Wrap to_attach by
	TRY_CATCH and call linux_ptrace_attach_warnings.

gdb/gdbserver/
2012-03-13  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* linux-low.c (linux_attach_lwp_1): New variable message.  Call
	linux_ptrace_attach_warnings.

gdb/testsuite/
2012-03-06  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.base/attach-twice.c: New files.
	* gdb.base/attach-twice.exp: New files.

--- a/gdb/common/linux-procfs.c
+++ b/gdb/common/linux-procfs.c
@@ -28,67 +28,54 @@
 /* Return the TGID of LWPID from /proc/pid/status.  Returns -1 if not
    found.  */
 
-int
-linux_proc_get_tgid (int lwpid)
+static int
+linux_proc_get_int (pid_t lwpid, const char *field)
 {
+  size_t field_len = strlen (field);
   FILE *status_file;
   char buf[100];
-  int tgid = -1;
+  int retval = -1;
 
   snprintf (buf, sizeof (buf), "/proc/%d/status", (int) lwpid);
   status_file = fopen (buf, "r");
-  if (status_file != NULL)
+  if (status_file == NULL)
     {
-      while (fgets (buf, sizeof (buf), status_file))
-	{
-	  if (strncmp (buf, "Tgid:", 5) == 0)
-	    {
-	      tgid = strtoul (buf + strlen ("Tgid:"), NULL, 10);
-	      break;
-	    }
-	}
-
-      fclose (status_file);
+      warning (_("unable to open /proc file '%s'"), buf);
+      return -1;
     }
 
-  return tgid;
+  while (fgets (buf, sizeof (buf), status_file))
+    if (strncmp (buf, field, field_len) == 0 && buf[field_len] == ':')
+      {
+	retval = strtol (&buf[field_len + 1], NULL, 10);
+	break;
+      }
+
+  fclose (status_file);
+  return retval;
 }
 
-/* Detect `T (stopped)' in `/proc/PID/status'.
-   Other states including `T (tracing stop)' are reported as false.  */
+/* Return the TGID of LWPID from /proc/pid/status.  Returns -1 if not
+   found.  */
 
 int
-linux_proc_pid_is_stopped (pid_t pid)
+linux_proc_get_tgid (pid_t lwpid)
 {
-  FILE *status_file;
-  char buf[100];
-  int retval = 0;
+  return linux_proc_get_int (lwpid, "Tgid");
+}
 
-  snprintf (buf, sizeof (buf), "/proc/%d/status", (int) pid);
-  status_file = fopen (buf, "r");
-  if (status_file != NULL)
-    {
-      int have_state = 0;
-
-      while (fgets (buf, sizeof (buf), status_file))
-	{
-	  if (strncmp (buf, "State:", 6) == 0)
-	    {
-	      have_state = 1;
-	      break;
-	    }
-	}
-      if (have_state && strstr (buf, "T (stopped)") != NULL)
-	retval = 1;
-      fclose (status_file);
-    }
-  return retval;
+/* See linux-procfs.h.  */
+
+pid_t
+linux_proc_get_tracerpid (pid_t lwpid)
+{
+  return linux_proc_get_int (lwpid, "TracerPid");
 }
 
-/* See linux-procfs.h declaration.  */
+/* Return non-zero if 'State' of /proc/PID/status contains STATE.  */
 
-int
-linux_proc_pid_is_zombie (pid_t pid)
+static int
+linux_proc_pid_has_state (pid_t pid, const char *state)
 {
   char buffer[100];
   FILE *procfile;
@@ -110,8 +97,24 @@ linux_proc_pid_is_zombie (pid_t pid)
 	have_state = 1;
 	break;
       }
-  retval = (have_state
-	    && strcmp (buffer, "State:\tZ (zombie)\n") == 0);
+  retval = (have_state && strstr (buffer, state) != NULL);
   fclose (procfile);
   return retval;
 }
+
+/* Detect `T (stopped)' in `/proc/PID/status'.
+   Other states including `T (tracing stop)' are reported as false.  */
+
+int
+linux_proc_pid_is_stopped (pid_t pid)
+{
+  return linux_proc_pid_has_state (pid, "T (stopped)");
+}
+
+/* See linux-procfs.h declaration.  */
+
+int
+linux_proc_pid_is_zombie (pid_t pid)
+{
+  return linux_proc_pid_has_state (pid, "Z (zombie)");
+}
--- a/gdb/common/linux-procfs.h
+++ b/gdb/common/linux-procfs.h
@@ -24,7 +24,12 @@
 /* Return the TGID of LWPID from /proc/pid/status.  Returns -1 if not
    found.  */
 
-extern int linux_proc_get_tgid (int lwpid);
+extern int linux_proc_get_tgid (pid_t lwpid);
+
+/* Return the TracerPid of LWPID from /proc/pid/status.  Returns -1 if not
+   found.  */
+
+extern pid_t linux_proc_get_tracerpid (pid_t lwpid);
 
 /* Detect `T (stopped)' in `/proc/PID/status'.
    Other states including `T (tracing stop)' are reported as false.  */
--- a/gdb/common/linux-ptrace.c
+++ b/gdb/common/linux-ptrace.c
@@ -24,3 +24,36 @@
 #endif
 
 #include "linux-ptrace.h"
+#include "linux-procfs.h"
+
+/* Find all possible reasons we could fail to attach PID.  Prepend strings for
+   those reasons to *MESSAGEP each one delimited by a newline.  *MESSAGEP
+   should be '\0'-terminated string allocated by xmalloc, its address may
+   change.  */
+
+void
+linux_ptrace_attach_warnings (pid_t pid, char **messagep)
+{
+  pid_t tracerpid;
+
+  tracerpid = linux_proc_get_tracerpid (pid);
+  if (tracerpid > 0)
+    {
+      char *s = xstrprintf (_("warning: "
+			      "process %d is already traced by process %d\n%s"),
+			    (int) pid, (int) tracerpid, *messagep);
+
+      xfree (*messagep);
+      *messagep = s;
+    }
+
+  if (linux_proc_pid_is_zombie (pid))
+    {
+      char *s = xstrprintf (_("warning: process %d is a zombie "
+			      "- the process has already terminated\n%s"),
+			    (int) pid, *messagep);
+
+      xfree (*messagep);
+      *messagep = s;
+    }
+}
--- a/gdb/common/linux-ptrace.h
+++ b/gdb/common/linux-ptrace.h
@@ -65,4 +65,6 @@
 #define __WALL          0x40000000 /* Wait for any child.  */
 #endif
 
+extern void linux_ptrace_attach_warnings (pid_t pid, char **messagep);
+
 #endif /* COMMON_LINUX_PTRACE_H */
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -652,6 +652,8 @@ linux_attach_lwp_1 (unsigned long lwpid, int initial)
 
   if (ptrace (PTRACE_ATTACH, lwpid, 0, 0) != 0)
     {
+      char *message;
+
       if (!initial)
 	{
 	  /* If we fail to attach to an LWP, just warn.  */
@@ -662,8 +664,10 @@ linux_attach_lwp_1 (unsigned long lwpid, int initial)
 	}
 
       /* If we fail to attach to a process, report an error.  */
-      error ("Cannot attach to lwp %ld: %s (%d)\n", lwpid,
-	     strerror (errno), errno);
+      message = xstrprintf ("Cannot attach to lwp %ld: %s (%d)", lwpid,
+			    strerror (errno), errno);
+      linux_ptrace_attach_warnings (lwpid, &message);
+      error ("%s\n", message);
     }
 
   if (initial)
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -62,6 +62,8 @@
 #include "symfile.h"
 #include "agent.h"
 #include "tracepoint.h"
+#include "exceptions.h"
+#include "linux-ptrace.h"
 
 #ifndef SPUFS_MAGIC
 #define SPUFS_MAGIC 0x23c9b64e
@@ -1612,11 +1614,26 @@ linux_nat_attach (struct target_ops *ops, char *args, int from_tty)
   struct lwp_info *lp;
   int status;
   ptid_t ptid;
+  volatile struct gdb_exception ex;
 
   /* Make sure we report all signals during attach.  */
   linux_nat_pass_signals (0, NULL);
 
-  linux_ops->to_attach (ops, args, from_tty);
+  TRY_CATCH (ex, RETURN_MASK_ERROR)
+    {
+      linux_ops->to_attach (ops, args, from_tty);
+    }
+  if (ex.reason < 0)
+    {
+      pid_t pid = parse_pid_to_attach (args);
+      char *message;
+
+      message = xstrdup (ex.message);
+      make_cleanup (free_current_contents, &message);
+
+      linux_ptrace_attach_warnings (pid, &message);
+      throw_error (ex.error, "%s", message);
+    }
 
   /* The ptrace base target adds the main thread with (pid,0,0)
      format.  Decorate it with lwp info.  */
--- /dev/null
+++ b/gdb/testsuite/gdb.base/attach-twice.c
@@ -0,0 +1,42 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2011-2012 Free Software Foundation, Inc.
+
+   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 3 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, see <http://www.gnu.org/licenses/>.  */
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/ptrace.h>
+#include <errno.h>
+
+int
+main (void)
+{
+  long l;
+
+  switch (fork ())
+  {
+    case -1:
+      perror ("fork");
+      exit (1);
+    case 0:
+      errno = 0;
+      ptrace (PTRACE_ATTACH, getppid (), NULL, NULL);
+      if (errno != 0)
+	perror ("PTRACE_ATTACH");
+      break;
+  }
+  sleep (600);
+  return 0;
+}
--- /dev/null
+++ b/gdb/testsuite/gdb.base/attach-twice.exp
@@ -0,0 +1,52 @@
+# Copyright (C) 2012 Free Software Foundation, Inc.
+#
+# 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 3 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, see <http://www.gnu.org/licenses/>.
+
+# Manipulation with PID on target is not supported.
+if [is_remote target] then {
+    return 0
+}
+
+set testfile attach-twice
+set executable ${testfile}
+set binfile ${objdir}/${subdir}/${executable}
+
+if { [prepare_for_testing ${testfile}.exp $executable] } {
+    return -1
+}
+
+set testpid [eval exec $binfile &]
+exec sleep 2
+
+set parentpid 0
+
+set test "attach"
+gdb_test_multiple "attach $testpid" $test {
+    -re "Attaching to program: \[^\r\n\]*, process $testpid\r\n.*warning: process $testpid is already traced by process (\[0-9\]+)\r\n.*ptrace: Operation not permitted\\.\r\n$gdb_prompt $" {
+	set parentpid $expect_out(1,string)
+	pass $test
+    }
+    -re "Attaching to program: \[^\r\n\]*, process $testpid\r\n.*ptrace: Operation not permitted\\.\r\n$gdb_prompt $" {
+	fail $test
+    }
+    -re "\r\n$gdb_prompt $" {
+	xfail $test
+    }
+}
+
+eval exec ps xfw
+if {$parentpid != 0} {
+  eval exec kill -9 $parentpid
+}
+eval exec kill -9 $testpid


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