Re: [PATCH] Return unconditionally ptid.pid () in get_ptrace_pid() for NetBSD

[Simon Marchi <simark@simark.ca>]

On 2020-03-17 1:45 p.m., Kamil Rytarowski wrote:
> On 17.03.2020 17:39, Simon Marchi wrote:
>> On 2020-03-17 12:30 p.m., Kamil Rytarowski wrote:
>>> NetBSD tracks the PID and LWP pair separately and both values are
>>> needed and meaningful.
>>> ---
>>>  gdb/inf-ptrace.c | 6 +++++-
>>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/gdb/inf-ptrace.c b/gdb/inf-ptrace.c
>>> index db17a76d946..6a6cb554ba7 100644
>>> --- a/gdb/inf-ptrace.c
>>> +++ b/gdb/inf-ptrace.c
>>> @@ -321,10 +321,14 @@ get_ptrace_pid (ptid_t ptid)
>>>  {
>>>    pid_t pid;
>>>
>>> +#if !defined(__NetBSD__)
>>>    /* If we have an LWPID to work with, use it.  Otherwise, we're
>>> -     dealing with a non-threaded program/target.  */
>>> +     dealing with a non-threaded program/target.
>>> +
>>> +     NetBSD tracks the PID and LWP pair separately. */
>>>    pid = ptid.lwp ();
>>>    if (pid == 0)
>>> +#endif
>>>      pid = ptid.pid ();
>>>    return pid;
>>>  }
>>> --
>>> 2.25.0
>>>
>>
>> I think you should just avoid using get_ptrace_pid on NetBSD altogether, since
>> it is meant for OSes that require passing a single thread identifier to ptrace
>> (whereas NetBSD requires the (pid, lwp) pair).
>>
>> Even with this modification in get_ptrace_pid, you need to change all the ptrace
>> call sites to pass the lwp on top of it.
>>
>> I would suggest to instead #ifdef out get_ptrace_pid entirely on NetBSD, to avoid
>> using it by mistake, and just replace all ptrace call sites possibly used on BSD
>> to be
>>
>>   ptrace (request, ptid.pid (), addr, ptid.lwp ());
>>
>> This matches what I suggested in:
>>
>>   https://sourceware.org/pipermail/gdb-patches/2020-March/166735.html
>>
>> Simon
>>
> 
> Avoiding is possibly nice.. however in the current code it is much more
> intrusive. We would need to patch now generic and OS/CPU specific code
> (some of that is also shared with other OSs due to legacy reasons).
> 
> I think it is much cleaner to return ptid. pid() for NetBSD and reflect
> the meaning of get_ptrace_pid().
> 
> If I follow your advice I end up with ifdefs like here:
> 
> diff --git a/gdb/inf-ptrace.c b/gdb/inf-ptrace.c
> index b63a1bf88ef..a5d9c1d10ea 100644
> --- a/gdb/inf-ptrace.c
> +++ b/gdb/inf-ptrace.c
> @@ -349,7 +349,11 @@ inf_ptrace_target::resume (ptid_t ptid, int step,
> enum gdb_signal signal)
>         single-threaded processes, so simply resume the inferior.  */
>      pid = inferior_ptid.pid ();
>    else
> +#ifdef __NetBSD__
> +    pid = ptid. pid();
> +#else
>      pid = get_ptrace_pid (ptid);
> +#endif
> 
>    if (catch_syscall_enabled () > 0)
>      request = PT_SYSCALL;
> @@ -533,7 +537,11 @@ inf_ptrace_target::xfer_partial (enum target_object
> object,
>  				 const gdb_byte *writebuf,
>  				 ULONGEST offset, ULONGEST len, ULONGEST *xfered_len)
>  {
> +#ifdef __NetBSD__
> +  pid_t pid = inferior_ptid. pid();
> +#else
>    pid_t pid = get_ptrace_pid (inferior_ptid);
> +#endif
> 
>    switch (object)
>      {

I was thinking more about using a "gdb_ptrace" function in this file, as
you have added in the other patch.  The only ifdef would be in that function.
get_ptrace_pid would only be used in the !__NetBSD__ branch of that function.

inf_ptrace_target::xfer_partial and inf_ptrace_target::resume would just call
gdb_ptrace, passing the right ptid.

> Maintaining that will be certainly harder and it will be prone to
> recurring regressions.
> 
> If we want to take the route of cleanups and refactoring I think it
> would be better to rethink the pid,lwp separation in Linux; but that is
> much beyond the scope of my patches.

I'm not sure what do mean by "rethink the pid,lwp separation in Linux".

> Last but not least, get_ptrace_pid() would work now for NetBSD literally
> as specified in the function name now... just extracting pid from ptid,
> not calculating it from lwp/pid. It's now questionable whether a wrapper
> function is still needed, but that would be optimized to .pid () in future.

Yeah, but since NetBSD doesn't need get_ptrace_pid, I'd like if we could avoid
making get_ptrace_pid more complex and if the ifdefs were concentrated around
the ptrace calls (ideally, in as few spots possible, thanks to the gdb_ptrace
functions).

Simon