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

Jan Kratochvil
2012-03-13T09:43:39Z

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