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

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

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?

    return 0;

  return read_memory_typed_address (info->debug_base + lmo->r_ldsomap_offset,
                                    ptr_type);
}

-- 
H.J.