Re: [PATCH] gdbserver: When attaching, add process before lwps

Alan Hayward
2019-02-01T11:20:42Z

> On 25 Jan 2019, at 18:23, Pedro Alves <palves@redhat.com> wrote:
>
> On 01/25/2019 10:48 AM, Alan Hayward wrote:
>> The recent BP/WP changes for AArch64 swapping the order in add_lwp()
>> so that the process was added before the lwp. This was due to the lwp
>> creation requiring the process data.
>>
>> This also needs changing in linux_attach().
>>
>> Fixes gdb.server/ext-attach.exp on Aarch64.
>>
>> (This regression was hidden due to the racy nature of the gdb.server
>> tests - now they are no longer racy it'll be easier to spot. Also
>> checked X86).
>>
>> gdb/gdbserver/ChangeLog:
>>
>> 2019-01-25 Alan Hayward <alan.hayward@arm.com>
>>
>> * linux-low.c (linux_attach): Add process before lwp.
>
>> ---
>> gdb/gdbserver/linux-low.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
>> index 17cce24d76..1ab2cfb1eb 100644
>> --- a/gdb/gdbserver/linux-low.c
>> +++ b/gdb/gdbserver/linux-low.c
>> @@ -1188,6 +1188,8 @@ linux_attach (unsigned long pid)
>> ptid_t ptid = ptid_t (pid, pid, 0);
>> int err;
>>
>> + proc = linux_add_process (pid, 1);
>> +
>> /* Attach to PID. We will check for other threads
>> soon. */
>> err = linux_attach_lwp (ptid);
>> @@ -1198,8 +1200,6 @@ linux_attach (unsigned long pid)
>> error ("Cannot attach to process %ld: %s", pid, reason.c_str ());
>> }
>>
>> - proc = linux_add_process (pid, 1);
>> -
>
> This fails to consider the error conditions. That error call
> visible above throws an exception.
>
> - If GDBserver is already attached to the process, this will
> create another process_info object for the already-attached
> process, and leave it behind.
>
> For this case, it would seem better to check whether we're
> already attached to a given PID in common code, around
> server.c:attach_inferior.

Assuming you are suggesting adding a check if the process object
exists in gdbserver, rather than adding a new target func which
then calls out to /proc.

>
> - If GDBserver isn't already attached, but the attach fails,
> this will likewise leave a stale process_info object behind.
>

I think I’m probably missing something here, but if the attach
fails for any reason then gdbserver will error and exit - so it
won’t matter if the object is not deleted?

New patch with the above changes. Ok?

gdb/gdbserver/ChangeLog:

2019-02-01 Alan Hayward <alan.hayward@arm.com>

* linux-low.c (linux_attach): Add process before lwp.
* server.c (attach_inferior): Check if already attached.

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 44016d2310..8c5a51f23c 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -1188,18 +1188,19 @@ linux_attach (unsigned long pid)
ptid_t ptid = ptid_t (pid, pid, 0);
int err;

+ proc = linux_add_process (pid, 1);
+
/* Attach to PID. We will check for other threads
soon. */
err = linux_attach_lwp (ptid);
if (err != 0)
{
- std::string reason = linux_ptrace_attach_fail_reason_string (ptid, err);
+ remove_process (proc);

+ std::string reason = linux_ptrace_attach_fail_reason_string (ptid, err);
error ("Cannot attach to process %ld: %s", pid, reason.c_str ());
}

- proc = linux_add_process (pid, 1);
-
/* Don't ignore the initial SIGSTOP if we just attached to this
process. It will be collected by wait shortly. */
initial_thread = find_thread_ptid (ptid_t (pid, pid, 0));
diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index f9bfdd7307..e960c10d40 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -295,6 +295,9 @@ attach_inferior (int pid)
/* myattach should return -1 if attaching is unsupported,
0 if it succeeded, and call error() otherwise. */

+ if (find_process_pid (pid) != nullptr)
+ error ("Already attached to process %d\n", pid);
+
if (myattach (pid) != 0)
return -1;