[PATCH] gdbserver: Check r_version < 1 for Linux debugger interface

[Simon Marchi]

On 2021-08-16 2:02 p.m., H.J. Lu wrote:
> On Mon, Aug 16, 2021 at 9:26 AM Simon Marchi <simon.marchi@polymtl.ca> wrote:
>>
>> On 2021-08-16 11:27 a.m., H.J. Lu via Gdb-patches wrote:
>>> Update gdbserver to check r_version < 1 instead of r_version != 1 so
>>> that r_version can be bumped for a new field in the glibc debugger
>>> interface to support multiple namespaces.
>>>
>>>       PR gdb/11839
>>>       * linux-low.cc (linux_process_target::qxfer_libraries_svr4):
>>>       Check r_version < 1 instead of r_version != 1.
>>> ---
>>>  gdbserver/linux-low.cc | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/gdbserver/linux-low.cc b/gdbserver/linux-low.cc
>>> index 5c6191d941c..fc7a995351d 100644
>>> --- a/gdbserver/linux-low.cc
>>> +++ b/gdbserver/linux-low.cc
>>> @@ -6845,7 +6845,7 @@ linux_process_target::qxfer_libraries_svr4 (const char *annex,
>>>         if (linux_read_memory (priv->r_debug + lmo->r_version_offset,
>>>                                (unsigned char *) &r_version,
>>>                                sizeof (r_version)) != 0
>>> -           || r_version != 1)
>>> +           || r_version < 1)
>>>           {
>>>             warning ("unexpected r_debug version %d", r_version);
>>>           }
>>>
>>
>> I don't understand how this change on its own is useful.  If r_version
>> gets bumped from 1 to 2, it's presumably because there are backwards
> 
> No.   The all glibc debugger interface changes will be backward compatible.

Ok, this is an important piece of information, please add it to the
commit message.

>> incompatible changes done to the interface.  Without the corresponding
>> changes to adjust to that new interface, then we just risk having a
>> gdbserver trying to read a library list with r_version == 2 without
>> actually knowing how to read a library list with r_version == 2.  So
> 
> Since all future interface changes will be backward compatible, gdbserver
> just needs to check r_version for incompatible implementation.  Since the
> current gdbserver only reads fields defined for r_version == 1, it is compatible
> with r_version >= 1.

I'm just curious, if there is ever the need to do a backwards
incompatible change, how would that be handled?

Simon