Re: [PATCH v2] gdbserver: Check r_version < 1 for Linux debugger interface

[Simon Marchi via Libc-alpha <libc-alpha@sourceware.org>]

On 2021-08-16 6:02 p.m., H.J. Lu wrote:
> On Mon, Aug 16, 2021 at 1:30 PM H.J. Lu <hjl.tools@gmail.com> 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.  Since so far, the gdbserver
>> only reads fields defined for r_version == 1, it is compatible with
>> r_version >= 1.
>>
>> All future glibc debugger interface changes will be backward compatible.
>> If there is ever the need for backward incompatible change to the glibc
>> debugger interface, a new DT_XXX element will be provided to access the
>> new incompatible interface.
>>
>>         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
>>
> 
> Set r_version == 2 breaks GDB due to
> 
> static CORE_ADDR
> solib_svr4_r_ldsomap (struct svr4_info *info)
> {
>   struct link_map_offsets *lmo = svr4_fetch_link_map_offsets ();
>   struct type *ptr_type = builtin_type (target_gdbarch ())->builtin_data_ptr;
>   enum bfd_endian byte_order = type_byte_order (ptr_type);
>   ULONGEST version = 0;
> 
>   try
>     {
>       /* Check version, and return zero if `struct r_debug' doesn't have
>          the r_ldsomap member.  */
>       version
>         = read_memory_unsigned_integer (info->debug_base +
> lmo->r_version_offset,
>                                         lmo->r_version_size, byte_order);
>     }
>   catch (const gdb_exception_error &ex)
>     {
>       exception_print (gdb_stderr, ex);
>     }
> 
>   if (version < 2 || lmo->r_ldsomap_offset == -1)
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> glibc doesn't have r_ldsomap.  But r_ldsomap_offset is set
> unconditionally.   Shouldn't it be set only if the target debugger
> interface has it?

Ideally, I think that Solaris support would implement its own
"solaris_{ilp32,lp64}_fetch_link_map_offsets", where it would set
r_ldsomap_offset.  And either there wouldn't be a generic
svr4_{ilp32,lp64}_fetch_link_map_offsets (each OS support would provide
its own, because things vary so much between OSes) or
svr4_{ilp32,lp64}_fetch_link_map_offsets would just specify what's
really the common denominator between all of them.

I think that would allow remove that "version < 2" check that is a bit
ugly.

Simon