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[Bug remote/22597] Regression: backwards compatibility with old GDBservers broken
- From: "cvs-commit at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>
- To: gdb-prs at sourceware dot org
- Date: Thu, 11 Jan 2018 00:31:22 +0000
- Subject: [Bug remote/22597] Regression: backwards compatibility with old GDBservers broken
- Auto-submitted: auto-generated
- References: <bug-22597-4717@http.sourceware.org/bugzilla/>
https://sourceware.org/bugzilla/show_bug.cgi?id=22597
--- Comment #3 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The gdb-8.1-branch branch has been updated by Pedro Alves
<palves@sourceware.org>:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=936a312c04f9f1dd856571bf7573b5a8f51ed895
commit 936a312c04f9f1dd856571bf7573b5a8f51ed895
Author: Pedro Alves <palves@redhat.com>
Date: Thu Jan 11 00:24:59 2018 +0000
Fix backwards compatibility with old GDBservers (PR remote/22597)
At <https://sourceware.org/ml/gdb-patches/2017-12/msg00285.html>,
Maciej reported that commit:
commit 5cd63fda035d4ba949e6478406162c4673b3c9ef
Date: Wed Oct 4 18:21:10 2017 +0100
Subject: Fix "Remote 'g' packet reply is too long" problems with multiple
inferiors
made GDB stop working with older stubs. Any attempt to continue
execution after the initial connection fails with:
[...]
Process .../gdb/testsuite/outputs/gdb.base/advance/advance created; pid =
2670
Listening on port 2346
target remote [...]:2346
Remote debugging using [...]:2346
Reading symbols from .../lib64/ld.so.1...done.
[Switching to Thread <main>]
(gdb) continue
Cannot execute this command without a live selected thread.
(gdb)
The problem is:
(gdb) c
Cannot execute this command without a live selected thread.
(gdb) info threads
Id Target Id Frame
1 Thread 14917 0x00007f341cd98ed0 in _start () from
/lib64/ld-linux-x86-64.so.2
The current thread <Thread ID 2> has terminated. See `help thread'.
^^^^^^^^^^^
(gdb)
Note, thread _2_. There's really only one thread in the inferior
(it's still at the entry point), but still GDB added a bogus second
thread.
The reason GDB started adding a second thread after 5cd63fda035d is
this hunk:
+ if (event->ptid == null_ptid)
+ {
+ const char *thr = strstr (p1 + 1, ";thread:");
+ if (thr != NULL)
+ event->ptid = read_ptid (thr + strlen (";thread:"),
+ NULL);
+ else
+ event->ptid = magic_null_ptid;
+ }
Note the else branch that falls back to magic_null_ptid. We reach
that when we process the initial stop reply sent back in response to
the the "?" (status) packet early in the connection setup:
Sending packet: $?#3f...Ack
Packet received:
T0506:0000000000000000;07:40a510f4fd7f0000;10:d0fe1201577f0000;
And note that that response does not include a ";thread:XXX" part.
This stop reply is processed after listing threads with qfThreadInfo /
qsThreadInfo :
Sending packet: $qfThreadInfo#bb...Ack
Packet received: m3915
Sending packet: $qsThreadInfo#c8...Ack
Packet received: l
meaning, when we process that stop reply, we treat the event as coming
from a thread with ptid == magic_null_ptid, which is not yet in the
thread list, so we add it then:
(top-gdb) p ptid
$1 = {m_pid = 42000, m_lwp = -1, m_tid = 1}
(top-gdb) bt
#0 0x0000000000840a8c in add_thread_silent(ptid_t) (ptid=...) at
src/gdb/thread.c:269
#1 0x00000000007ad61d in remote_add_thread(ptid_t, int, int) (ptid=...,
running=0, executing=0)
at src/gdb/remote.c:1838
#2 0x00000000007ad8de in remote_notice_new_inferior(ptid_t, int)
(currthread=..., executing=0)
at src/gdb/remote.c:1921
#3 0x00000000007b758b in process_stop_reply(stop_reply*,
target_waitstatus*) (stop_reply=0x1158860, status=0x7fffffffcc00)
at src/gdb/remote.c:7217
#4 0x00000000007b7a38 in remote_wait_as(ptid_t, target_waitstatus*, int)
(ptid=..., status=0x7fffffffcc00, options=0)
at src/gdb/remote.c:7380
#5 0x00000000007b7cd1 in remote_wait(target_ops*, ptid_t,
target_waitstatus*, int) (ops=0x102fac0 <remote_ops>, ptid=...,
status=0x7fffffffcc00, options=0) at src/gdb/remote.c:7446
#6 0x000000000081587b in delegate_wait(target_ops*, ptid_t,
target_waitstatus*, int) (self=0x102fac0 <remote_ops>, arg1=...,
arg2=0x7fffffffcc00, arg3=0) at src/gdb/target-delegates.c:138
#7 0x0000000000827d77 in target_wait(ptid_t, target_waitstatus*, int)
(ptid=..., status=0x7fffffffcc00, options=0)
at src/gdb/target.c:2179
#8 0x0000000000715fda in do_target_wait(ptid_t, target_waitstatus*, int)
(ptid=..., status=0x7fffffffcc00, options=0)
at src/gdb/infrun.c:3589
#9 0x0000000000716351 in wait_for_inferior() () at src/gdb/infrun.c:3707
#10 0x0000000000715435 in start_remote(int) (from_tty=1) at
src/gdb/infrun.c:3212
things go downhill from this.
We don't see the problem with current master gdbserver, because that
version always sends the ";thread:" part in the initial stop reply:
Sending packet: $?#3f...Packet received:
T0506:0000000000000000;07:a0d4ffffff7f0000;10:d05eddf7ff7f0000;thread:p3cea.3cea;core:3;
Years ago I had added a "--disable-packet=" command line option to
gdbserver which comes in handy for testing this, since the existing
"--disable-packet=Tthread" precisely makes gdbserver not send that
";thread:" part in stop replies. The testcase added by this commit
emulates old gdbserver making use of that.
I've compared a testrun at 5cd63fda035d^ (before regression) with
'current master+patch', against old gdbserver at f8b73d13b7ca^. I
hacked out --once, and "monitor exit" to be able to test. The results
are a bit too unstable to tell accurately, but it looked like there
were no regressions. Maciej confirmed this worked for him as well.
No regressions on master (against master gdbserver).
gdb/ChangeLog:
2018-01-11 Pedro Alves <palves@redhat.com>
PR remote/22597
* remote.c (remote_parse_stop_reply): Default to the last-set
general thread instead of to 'magic_null_ptid'.
gdb/testsuite/ChangeLog:
2018-01-11 Pedro Alves <palves@redhat.com>
PR remote/22597
* gdb.server/stop-reply-no-thread.c: New file.
* gdb.server/stop-reply-no-thread.exp: New file.
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