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

[Alan Hayward <Alan dot Hayward at arm dot com>]

> 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;