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

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

[H.J. Lu]

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);
 	    }
-- 
2.31.1

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

[Simon Marchi]

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
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
it will potentially interpret the data in a wrong way and behave badly.

Simon

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

[H.J. Lu]

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.

> 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.

> it will potentially interpret the data in a wrong way and behave badly.
>
> Simon



-- 
H.J.

Re: [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

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

[H.J. Lu]

On Mon, Aug 16, 2021 at 11:08 AM Simon Marchi <simon.marchi@polymtl.ca> wrote:
>
> 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.

Will do.

> >> 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?

If it does happen, we will provide its access via a new DT_XXX element.

-- 
H.J.