[RFC,v1,1/5] bits/dlfcn.h: Declare and describe the dlmopen RTLD_SHARED flag

  From: Vivek Das Mohapatra <vivek@collabora.co.uk>

This flag will instruct dlmopen to create a shared object present
in both the main namespace and the selected namespace when set.
---
 bits/dlfcn.h | 7 +++++++
 1 file changed, 7 insertions(+)
  

On 05/16/2018 01:11 PM, Vivek Das Mohapatra wrote:
> From: Vivek Das Mohapatra <vivek@collabora.co.uk>
> 
> This flag will instruct dlmopen to create a shared object present
> in both the main namespace and the selected namespace when set.
> ---
>  bits/dlfcn.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/bits/dlfcn.h b/bits/dlfcn.h
> index b0b129b66b..a96270d499 100644
> --- a/bits/dlfcn.h
> +++ b/bits/dlfcn.h
> @@ -32,6 +32,13 @@
>     visible as if the object were linked directly into the program.  */
>  #define RTLD_GLOBAL	0x00100
>  
> +/* If the following bit is set in the MODE argument to dlmopen
> +   then the target object is loaded into the main namespace (if
> +   it is not already there) and a shallow copy (clone) is placed
> +   in the target namespace: This allows multiple namespaces to
> +   share a single instance of a DSO.  */
> +#define RTLD_SHARED 0x00080

This is an RFC, but also note there is a MIPS-specific dlfcn.h which needs
patching also to define RTLD_SHARED. We should probably try to look at how
the constants overlap, and find a common value to start with for everyone.

I agree that we need a constant with which to choose between:

* Old dlmopen behaviour.

* New dlmopen behaviour.

> +
>  /* Unix98 demands the following flag which is the inverse to RTLD_GLOBAL.
>     The implementation does this by default and so we can define the
>     value to zero.  */
>
  

>> +#define RTLD_SHARED 0x00080
>
> This is an RFC, but also note there is a MIPS-specific dlfcn.h which needs
> patching also to define RTLD_SHARED. We should probably try to look at how
> the constants overlap, and find a common value to start with for everyone.

Noted.

> I agree that we need a constant with which to choose between:
>
> * Old dlmopen behaviour.
> * New dlmopen behaviour.

My plan is to put that in the "policy" patchset once this
"mechanism" patchset is acceptable:

Something like:

   RTLD_SHARED   - this library and its dependencies will be proxied
   RTLD_ISOLATED - nothing (except ld.so) will be proxied (we hope you
                   know what you're doing) aka the old behaviour
   -nothing-     - libc/libpthread/ld.so will be proxied from the main ns.
                   everything else will be isolated.

Details to be thrashed out later.