aboutsummaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorPedro Alves <pedro@palves.net>2020-07-22 12:32:53 +0100
committerPedro Alves <pedro@palves.net>2020-07-22 12:32:53 +0100
commit028a46039a22842e41030a94848d086d5db05617 (patch)
tree7acf840acddcdf949b21a1ef47f5d44faa8b2471
parent0e6a3f07f50723d1831291492b96fdf74bcbdc11 (diff)
downloadgdb-028a46039a22842e41030a94848d086d5db05617.zip
gdb-028a46039a22842e41030a94848d086d5db05617.tar.gz
gdb-028a46039a22842e41030a94848d086d5db05617.tar.bz2
gdbserver: handle running threads in qXfer:threads:read
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.
-rw-r--r--gdbserver/ChangeLog11
-rw-r--r--gdbserver/inferiors.cc10
-rw-r--r--gdbserver/inferiors.h3
-rw-r--r--gdbserver/server.cc49
-rw-r--r--gdbserver/thread-db.cc8
5 files changed, 68 insertions, 13 deletions
diff --git a/gdbserver/ChangeLog b/gdbserver/ChangeLog
index d924e72..f42b5c7 100644
--- a/gdbserver/ChangeLog
+++ b/gdbserver/ChangeLog
@@ -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.
diff --git a/gdbserver/inferiors.cc b/gdbserver/inferiors.cc
index 88adb16..861c9f3 100644
--- a/gdbserver/inferiors.cc
+++ b/gdbserver/inferiors.cc
@@ -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 *
diff --git a/gdbserver/inferiors.h b/gdbserver/inferiors.h
index 4e24b2c..bf2a16f 100644
--- a/gdbserver/inferiors.h
+++ b/gdbserver/inferiors.h
@@ -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 *);
diff --git a/gdbserver/server.cc b/gdbserver/server.cc
index ab5363e..aadcb9b 100644
--- a/gdbserver/server.cc
+++ b/gdbserver/server.cc
@@ -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)
diff --git a/gdbserver/thread-db.cc b/gdbserver/thread-db.cc
index 89c45cd..32dc376 100644
--- a/gdbserver/thread-db.cc
+++ b/gdbserver/thread-db.cc
@@ -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