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gdbserver: handle running threads in qXfer:threads:read

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On some systems, the gdb.multi/multi-target.exp testcase occasionally
fails like so:

 Running src/gdb/testsuite/gdb.multi/multi-target.exp ...
 FAIL: gdb.multi/multi-target.exp: info-inferiors: multi_process=on: inferior 1: info connections
 FAIL: gdb.multi/multi-target.exp: info-inferiors: multi_process=on: inferior 1: info inferiors
 FAIL: gdb.multi/multi-target.exp: info-inferiors: multi_process=on: inferior 2: info connections
 FAIL: gdb.multi/multi-target.exp: info-inferiors: multi_process=on: inferior 2: info inferiors
 FAIL: gdb.multi/multi-target.exp: info-inferiors: multi_process=on: inferior 3: inferior 3
 ... many more cascading fails.

The problem starts when the testcase runs an inferior against GDBserver:

 (gdb) run
 Starting program: build/gdb/testsuite/outputs/gdb.multi/multi-target/multi-target
 Reading /lib64/ld-linux-x86-64.so.2 from remote target...
 warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
 Reading /lib64/ld-linux-x86-64.so.2 from remote target...
 Reading /lib64/ld-2.31.so from remote target...
 Reading /lib64/.debug/ld-2.31.so from remote target...
 Reading /usr/lib/debug//lib64/ld-2.31.so from remote target...
 Reading /usr/lib/debug/lib64//ld-2.31.so from remote target...
 Reading target:/usr/lib/debug/lib64//ld-2.31.so from remote target...
 Reading /lib/x86_64-linux-gnu/libpthread.so.0 from remote target...
 Reading /lib/x86_64-linux-gnu/libc.so.6 from remote target...
 Reading /lib/x86_64-linux-gnu/libc-2.31.so from remote target...
 Reading /lib/x86_64-linux-gnu/.debug/libc-2.31.so from remote target...
 Reading /usr/lib/debug//lib/x86_64-linux-gnu/libc-2.31.so from remote target...
 Reading /usr/lib/debug//lib/x86_64-linux-gnu/libc-2.31.so from remote target...
 Remote connection closed
 ...

Note the "Remote connection closed" message.  That means GDBserver
exited abruptly.

I traced it down to the fact that GDB fetches the thread list from
GDBserver while the main thread of the process is still running.  On
my main system where I wrote the testcase, I have not observed the
failure because it is slow enough that the thread stops before
GDBserver fetches the thread list in the problem scenario which I'll
describe below.

With some --remote-debug logging from GDBserver side, we see the last
packets before the connection closes:

 ...
 getpkt ("vCont;c");  [no ack sent]
 putpkt ("$OK#9a"); [noack mode]
 getpkt ("Tp10f9a.10f9a");  [no ack sent]
 putpkt ("$OK#9a"); [noack mode]
 getpkt ("Hgp0.0");  [no ack sent]
 putpkt ("$OK#9a"); [noack mode]
 getpkt ("qXfer:threads:read::0,1000");  [no ack sent]

Note the vCont;c , which sets the program running, and then a
qXfer:threads:read packet at the end.

The problem happens when the thread list refresh (qXfer:threads:read)
is sent just while the main thread is running and it still hasn't
initialized its libpthread id internally.  In that state, the main
thread's lwp will remain with the thread_known flag clear.  See in
find_one_thread:

  /* If the new thread ID is zero, a final thread ID will be available
     later.  Do not enable thread debugging yet.  */
  if (ti.ti_tid == 0)
    return 0;

Now, back in server.cc, to handle the qXfer:threads:read, we reach
handle_qxfer_threads -> handle_qxfer_threads_proper, and the latter
then calls handle_qxfer_threads_worker for each known thread.  In
handle_qxfer_threads_worker, we call target_thread_handle.  This ends
up in thread_db_thread_handle, here:

  if (!lwp->thread_known && !find_one_thread (thread->id))
    return false;

Since the thread ID isn't known yet, we call find_one_thread.  This
calls into libthread_db.so, which accesses memory.  Because the
current thread is running, that fails and we throw an error, here:

  /* Get information about this thread.  */
  err = thread_db->td_ta_map_lwp2thr_p (thread_db->thread_agent, lwpid, &th);
  if (err != TD_OK)
    error ("Cannot get thread handle for LWP %d: %s",
	   lwpid, thread_db_err_str (err));

The current design is that whenever GDB-facing packets/requests need
to accesses memory, server.cc is supposed to prepare the target for
the access.  See gdb_read_memory / gdb_write_memory.  This preparation
means pausing threads if in non-stop mode (someday we could lift this
requirement, but we will still need to pause to access registers or do
other related ptrace accesses like PTRACE_GET_THREAD_AREA).  Note that
the multi-target.exp testcase forces "maint set target-non-stop on".

So the fix here is to prepare the target to access memory when
handling qXfer:threads:read too.

gdbserver/ChangeLog:

	* inferiors.cc (switch_to_process): New, moved here from
	thread-db.cc, and made extern.
	* inferiors.h (switch_to_process): Declare.
	* server.cc: Include "gdbsupport/scoped_restore.h".
	(handle_qxfer_threads_proper): Now returns bool.  Prepare to
	access memory around target calls.
	(handle_qxfer_threads): Handle errors.
	* thread-db.cc (switch_to_process): Moved to inferiors.cc.
This commit is contained in:
Pedro Alves 2020-07-22 12:32:53 +01:00
parent 0e6a3f07f5
commit 028a46039a
5 changed files with 68 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
gdbserver/ChangeLog
View File

@ -1,3 +1,14 @@
2020-07-22 Pedro Alves <pedro@palves.net>
* inferiors.cc (switch_to_process): New, moved here from
thread-db.cc, and made extern.
* inferiors.h (switch_to_process): Declare.
* server.cc: Include "gdbsupport/scoped_restore.h".
(handle_qxfer_threads_proper): Now returns bool. Prepare to
access memory around target calls.
(handle_qxfer_threads): Handle errors.
* thread-db.cc (switch_to_process): Moved to inferiors.cc.
2020-07-21 Simon Marchi <simon.marchi@efficios.com>
* linux-low.cc (stopped_pids): Make static.

10
gdbserver/inferiors.cc
View File

@ -223,6 +223,16 @@ switch_to_thread (process_stratum_target *ops, ptid_t ptid)
current_thread = find_thread_ptid (ptid);
}
/* See inferiors.h. */
void
switch_to_process (process_info *proc)
{
int pid = pid_of (proc);
current_thread = find_any_thread_of_pid (pid);
}
/* See gdbsupport/common-inferior.h. */
const char *

3
gdbserver/inferiors.h
View File

@ -138,6 +138,9 @@ struct process_info *find_process_pid (int pid);
int have_started_inferiors_p (void);
int have_attached_inferiors_p (void);
/* Switch to a thread of PROC. */
void switch_to_process (process_info *proc);
void clear_inferiors (void);
void *thread_target_data (struct thread_info *);

49
gdbserver/server.cc
View File

@ -48,6 +48,7 @@
#include "gdbsupport/selftest.h"
#include "gdbsupport/scope-exit.h"
#include "gdbsupport/gdb_select.h"
#include "gdbsupport/scoped_restore.h"
#define require_running_or_return(BUF) \
if (!target_running ()) \
@ -1678,19 +1679,54 @@ handle_qxfer_threads_worker (thread_info *thread, struct buffer *buffer)
buffer_xml_printf (buffer, "/>\n");
}
/* Helper for handle_qxfer_threads. */
/* Helper for handle_qxfer_threads. Return true on success, false
otherwise. */
static void
static bool
handle_qxfer_threads_proper (struct buffer *buffer)
{
client_state &cs = get_client_state ();
scoped_restore save_current_thread
= make_scoped_restore (&current_thread);
scoped_restore save_current_general_thread
= make_scoped_restore (&cs.general_thread);
buffer_grow_str (buffer, "<threads>\n");
for_each_thread ([&] (thread_info *thread)
process_info *error_proc = find_process ([&] (process_info *process)
{
handle_qxfer_threads_worker (thread, buffer);
/* The target may need to access memory and registers (e.g. via
libthread_db) to fetch thread properties. Prepare for memory
access here, so that we potentially pause threads just once
for all accesses. Note that even if someday we stop needing
to pause threads to access memory, we will need to be able to
access registers, or other ptrace accesses like
PTRACE_GET_THREAD_AREA. */
/* Need to switch to each process in turn, because
prepare_to_access_memory prepares for an access in the
current process pointed to by general_thread. */
switch_to_process (process);
cs.general_thread = current_thread->id;
int res = prepare_to_access_memory ();
if (res == 0)
{
for_each_thread (process->pid, [&] (thread_info *thread)
{
handle_qxfer_threads_worker (thread, buffer);
});
done_accessing_memory ();
return false;
}
else
return true;
});
buffer_grow_str0 (buffer, "</threads>\n");
return error_proc == nullptr;
}
/* Handle qXfer:threads:read. */
@ -1719,11 +1755,14 @@ handle_qxfer_threads (const char *annex,
buffer_init (&buffer);
handle_qxfer_threads_proper (&buffer);
bool res = handle_qxfer_threads_proper (&buffer);
result = buffer_finish (&buffer);
result_length = strlen (result);
buffer_free (&buffer);
if (!res)
return -1;
}
if (offset >= result_length)

8
gdbserver/thread-db.cc
View File

@ -767,14 +767,6 @@ thread_db_init (void)
return 0;
}
static void
switch_to_process (struct process_info *proc)
{
int pid = pid_of (proc);
current_thread = find_any_thread_of_pid (pid);
}
/* Disconnect from libthread_db and free resources. */
static void