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

[Simon Marchi <simon.marchi@polymtl.ca>]

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