RE: [PATCH] gdb/remote: use current_inferior in read_ptid if multi-process not supported

["Aktemur, Tankut Baris" <tankut.baris.aktemur@intel.com>]

Kindly pinging. 

Thanks
-Baris

On Wednesday, February 9, 2022 8:58 AM, Aktemur, Tankut Baris wrote:
> When parsing the ptid out of a reply package, if the multi-process
> extensions are not supported, use current_inferior's pid as the pid of
> the reported thread, instead of inferior_ptid.  This is needed because
> the inferior_ptid may be null_ptid although a legit context exists,
> due to a prior context switch via switch_to_inferior_no_thread.
> 
> Below is a scenario that illustrates what could go wrong.  First,
> setup a multi-target scenario.  This is needed, because in a
> multi-target setting, the inferior_ptid is cleared out before waiting
> on targets.  The second inferior below sits on top of a remote target.
> Multi-process packets are disabled.
> 
>   $ # First, spawn a process with PID 26253 to attach to later.
>   $ gdb-up a.out
>   Reading symbols from a.out...
>   (gdb) maint set target-non-stop on
>   (gdb) set remote multiprocess-feature-packet off
>   (gdb) start
>   ...
>   (gdb) add-inferior -no-connection
>   [New inferior 2]
>   Added inferior 2
>   (gdb) inferior 2
>   [Switching to inferior 2 [<null>] (<noexec>)]
>   (gdb) target extended-remote | gdbserver --multi -
>   Remote debugging using | gdbserver --multi -
>   Remote debugging using stdio
>   (gdb) attach 26253
>   Attaching to Remote target
>   Attached; pid = 26253
>   [New Thread 26253]
>   [New inferior 3]
>   Reading /tmp/a.out from remote target...
>   ...
>   [New Thread 26253]
>   ...
>   Reading /usr/local/lib/debug/....debug from remote target...
>   >>> GDB seems to hang here.
> 
> After attaching to a process and reading some library files, GDB
> seems to hang.  One interesting thing to note is that
> 
>   [New Thread 26253]
> 
> appears twice.  We also see
> 
>   [New inferior 3]
> 
> Running the same scenario with "debug infrun on" reveals more details.
> 
>   ...
>   (gdb) attach 26253
>   [infrun] scoped_disable_commit_resumed: reason=attaching
>   Attaching to Remote target
>   Attached; pid = 26253
>   [New Thread 26253]
>   [infrun] infrun_async: enable=1
>   [infrun] attach_command: immediately after attach:
>   [infrun] attach_command:   thread 26253.26253.0, executing = 1, resumed = 0, state =
> RUNNING
>   [infrun] clear_proceed_status_thread: 26253.26253.0
>   [infrun] reset: reason=attaching
>   [infrun] maybe_set_commit_resumed_all_targets: not requesting commit-resumed for target
> native, no resumed threads
>   [infrun] maybe_set_commit_resumed_all_targets: enabling commit-resumed for target
> extended-remote
>   [infrun] fetch_inferior_event: enter
>     [infrun] scoped_disable_commit_resumed: reason=handling event
>     [infrun] do_target_wait: Found 2 inferiors, starting at #1
>     [infrun] random_pending_event_thread: None found.
>     [infrun] print_target_wait_results: target_wait (-1.0.0 [Thread 0], status) =
>     [infrun] print_target_wait_results:   26253.26253.0 [Thread 26253],
>     [infrun] print_target_wait_results:   status->kind = STOPPED, sig = GDB_SIGNAL_0
>     [infrun] handle_inferior_event: status->kind = STOPPED, sig = GDB_SIGNAL_0
>     [infrun] start_step_over: enter
>       [infrun] start_step_over: stealing global queue of threads to step, length = 0
>       [infrun] operator(): step-over queue now empty
>     [infrun] start_step_over: exit
>     [infrun] context_switch: Switching context from 0.0.0 to 26253.26253.0
>     [infrun] handle_signal_stop: stop_pc=0x7f849d8cf151
>     [infrun] stop_waiting: stop_waiting
>     [infrun] stop_all_threads: starting
>     [infrun] stop_all_threads: pass=0, iterations=0
>   [New inferior 3]
>   Reading /tmp/a.out from remote target...
>   warning: File transfers from remote targets can be slow. Use "set sysroot" to access files
> locally instead.
>   Reading /tmp/a.out from remote target...
>   Reading symbols from target:/tmp/a.out...
>   [New Thread 26253]
>     [infrun] stop_all_threads:   4723.4723.0 not executing
>     [infrun] stop_all_threads:   26253.26253.0 not executing
>     [infrun] stop_all_threads:   42000.26253.0 executing, need stop
>     [infrun] print_target_wait_results: target_wait (-1.0.0 [Thread 0], status) =
>     [infrun] print_target_wait_results:   -1.0.0 [Thread 0],
>     [infrun] print_target_wait_results:   status->kind = IGNORE
>     [infrun] print_target_wait_results: target_wait (-1.0.0 [Thread 0], status) =
>     [infrun] print_target_wait_results:   -1.0.0 [Thread 0],
>     [infrun] print_target_wait_results:   status->kind = IGNORE
> 
> GDB tried to stop Thread 42000.26253.0, which does not exist, and we
> are waiting for a stop event that will never happen.  The PID in
> '42000.26253.0', namely 42000, is the PID of magic_null_ptid.
> It comes from gdb/remote.c:read_ptid:
> 
>   /* Since the stub is not sending a process id, then default to
>      what's in inferior_ptid, unless it's null at this point.  If so,
>      then since there's no way to know the pid of the reported
>      threads, use the magic number.  */
>   if (inferior_ptid == null_ptid)
>     pid = magic_null_ptid.pid ();
>   else
>     pid = inferior_ptid.pid ();
> 
>   if (obuf)
>     *obuf = pp;
>   return ptid_t (pid, tid);
> 
> Because multi-process was turned off, GDB did not parse an explicitly
> specified PID.  Furthermore, inferior_ptid == null_ptid, and
> eventually GDB picked the PID from magic_null_ptid.
> 
> If target-non-stop is not turned on at the beginning, the same bug
> reveals itself as a duplicated thread as shown below.
> 
>   # Same setup as above, without 'maint set target-non-stop on'.
>   ...
>   (gdb) attach 26253
>   Attaching to Remote target
>   Attached; pid = 26253
>   [New inferior 3]
>   ...
>   [New Thread 26253]
>   ...
>   (gdb) info threads
>     Id   Target Id             Frame
>     1.1  process 13517 "a.out" main () at test.c:3
>   * 2.1  Thread 26253 "a.out"  0x00007f12750c5151 in read () from target:/lib/x86_64-linux-
> gnu/libc.so.6
>     3.1  Thread 26253 "a.out"  Remote 'g' packet reply is too long (expected 560 bytes, got
> 2496 bytes):
> 00feffffffffffff000a3a75127f000051510c75127f0000000400000000000060d24ef6af550000000000000000
> 0000680d000000000000b85b31e3fc7f0000c0283a75127f000000e55b75127f000010d04ef6af55000046020000
> 0000000060c73975127f0000a0d23975127f0000000a3a75127f0000000000000000000051510c75127f00004602
> 0000330000002b000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000007f03000000000000ffff00000000000000000000000000000000000000000000
> 80143a75127f000080143a75127f000040143a75127f000040143a75127f00007d0000007e0000007f0000008000
> 0000300c3a75127f0000300c3a75127f00000e000000000000000e00000000000000000000000000000000000000
> 00000000ffffffffffffffffffffffffffffffff0400000004000000040000000400000020143a75127f00002014
> 3a75127f000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 0000000000000000000000000000000000000000000000000000801f0000000000000000000000e55b75127f0000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 000000000000000000000000
>   (gdb)
> 
> Fix the problem by preferring current_inferior()'s pid instead of
> magic_null_ptid.
> 
> Regression-tested on X86-64 Linux.
> 
> Co-authored-by: Aleksandar Paunovic <aleksandar.paunovic@intel.com>
> ---
>  gdb/remote.c                                  |  9 +-
>  .../gdb.multi/attach-no-multi-process.c       | 34 +++++++
>  .../gdb.multi/attach-no-multi-process.exp     | 90 +++++++++++++++++++
>  3 files changed, 129 insertions(+), 4 deletions(-)
>  create mode 100644 gdb/testsuite/gdb.multi/attach-no-multi-process.c
>  create mode 100644 gdb/testsuite/gdb.multi/attach-no-multi-process.exp
> 
> diff --git a/gdb/remote.c b/gdb/remote.c
> index 49eeb63445d..281260af971 100644
> --- a/gdb/remote.c
> +++ b/gdb/remote.c
> @@ -3145,14 +3145,15 @@ read_ptid (const char *buf, const char **obuf)
>        return null_ptid;
>      }
> 
> -  /* Since the stub is not sending a process id, then default to
> -     what's in inferior_ptid, unless it's null at this point.  If so,
> +  /* Since the stub is not sending a process id, default to
> +     what's current_inferior, unless it's null at this point.  If so,
>       then since there's no way to know the pid of the reported
>       threads, use the magic number.  */
> -  if (inferior_ptid == null_ptid)
> +  inferior *inf = current_inferior ();
> +  if (inf == nullptr || inf->pid == 0)
>      pid = magic_null_ptid.pid ();
>    else
> -    pid = inferior_ptid.pid ();
> +    pid = inf->pid;
> 
>    if (obuf)
>      *obuf = pp;
> diff --git a/gdb/testsuite/gdb.multi/attach-no-multi-process.c
> b/gdb/testsuite/gdb.multi/attach-no-multi-process.c
> new file mode 100644
> index 00000000000..df09dd69ccc
> --- /dev/null
> +++ b/gdb/testsuite/gdb.multi/attach-no-multi-process.c
> @@ -0,0 +1,34 @@
> +/* This testcase is part of GDB, the GNU debugger.
> +
> +   Copyright 2020-2022 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/>.  */
> +
> +/* This program is intended to be started outside of gdb, and then
> +   attached to by GDB.  */
> +
> +#include <unistd.h>
> +
> +int
> +main ()
> +{
> +  /* Don't run forever in case GDB crashes and DejaGNU fails to kill
> +     this program.  */
> +  alarm (10);
> +
> +  while (1)
> +    ;
> +
> +  return 0;
> +}
> diff --git a/gdb/testsuite/gdb.multi/attach-no-multi-process.exp
> b/gdb/testsuite/gdb.multi/attach-no-multi-process.exp
> new file mode 100644
> index 00000000000..0ba99351ec5
> --- /dev/null
> +++ b/gdb/testsuite/gdb.multi/attach-no-multi-process.exp
> @@ -0,0 +1,90 @@
> +# This testcase is part of GDB, the GNU debugger.
> +
> +# Copyright 2020-2022 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/>.
> +
> +# Test attaching to a process, as a second inferior, through a
> +# gdbserver that does not support multi-process extensions.
> +
> +load_lib gdbserver-support.exp
> +
> +standard_testfile
> +
> +# The plain remote target can't do multiple inferiors.
> +if {[target_info gdb_protocol] != ""} {
> +    return
> +}
> +
> +if {![can_spawn_for_attach]} {
> +    return
> +}
> +
> +if {[build_executable "build" $testfile $srcfile {debug}] == -1} {
> +    return -1
> +}
> +
> +proc test {target_non_stop} {
> +    global binfile
> +
> +    save_vars { ::GDBFLAGS } {
> +	# Make GDB read files from the local file system, not through the
> +	# remote targets, to speed things up.
> +	set ::GDBFLAGS "${::GDBFLAGS} -ex \"set sysroot\""
> +	clean_restart ${binfile}
> +    }
> +
> +    gdb_test_no_output "set remote multiprocess-feature-packet off"
> +    gdb_test_no_output "maint set target-non-stop ${target_non_stop}"
> +
> +    # Start the first inferior.
> +    if {![runto_main]} {
> +	return
> +    }
> +
> +    # The second inferior is an extended remote.
> +    gdb_test "add-inferior -no-connection" "Added inferior 2.*" \
> +	"add the second inferior"
> +    gdb_test "inferior 2" ".*Switching to inferior 2.*" \
> +	"switch to inferior 2"
> +    set res [gdbserver_start "--multi" ""]
> +    set gdbserver_gdbport [lindex $res 1]
> +    gdb_target_cmd "extended-remote" $gdbserver_gdbport
> +
> +    # Start a program, then attach to it.
> +    set spawn_id_list [spawn_wait_for_attach [list $binfile]]
> +    set test_spawn_id [lindex $spawn_id_list 0]
> +    set testpid [spawn_id_get_pid $test_spawn_id]
> +    gdb_test "attach $testpid" \
> +	"Attaching to Remote target.*" \
> +	"attach to the program via remote"
> +
> +    # Check that we have two threads.  Bad GDB duplicated the
> +    # thread coming from the remote when target-non-stop is off;
> +    # or hanged during attach when target-non-stop is on.
> +    gdb_test "info threads" \
> +	[multi_line \
> +	     "  Id\[^\r\n\]+" \
> +	     "  1\.1\[^\r\n\]+" \
> +	     ". 2\.1\[^\r\n\]+"
> +	]
> +
> +    # Clean the spawned process and gdbserver.
> +    gdbserver_exit 0
> +    kill_wait_spawned_process $test_spawn_id
> +}
> +
> +foreach_with_prefix target_non_stop {off on} {
> +    test $target_non_stop
> +}
> --
> 2.33.1

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