[RFC][PATCH v12 0/8] Implementation of RTLD_SHARED for dlmopen

[Adhemerval Zanella]

On 08/07/2021 13:32, Vivek Das Mohapatra via Libc-alpha wrote:
> This is a revision of a previous patchset that I posted here
> regarding https://sourceware.org/bugzilla/show_bug.cgi?id=22745 
> 
> Introduction:
> 
> =======================================================================
>   As discussed in the URL above dlmopen requires a mechanism for
>   [optionally] sharing some objects between more than one namespace.
> 
>   The following patchset provides an implementation for this: If an
>   object is loaded with the new RTLD_SHARED flag we instead ensure
>   that a "master" copy exists (and is flagged as no-delete) in the
>   main namespace and a thin wrapper or clone is placed in the target
>   namespace.
> 
>   This patch series should address all the comments received on the
>   earlier (v1-v10) series.
> 
> =======================================================================
> 
> Changes from v11:
> 
>  - If a DSO is required in a non-base namespace because it is mentioned
>    in a DT_NEEDED entry and it is itself flagged DF_GNU_1_UNIQUE then
>    a proxy is generated for it.
> 
>  - Relocations via non-base namespace proxies work reliably (some code
>    paths did not do the address calculation relative to the DSO base
>    correctly when a proxy was involved).
> 
>  - Tests extended cover the above two scenarios.
> 
> Changes from v10:
> 
>  - A segfault in a dlmopen error pathway (which does not seem to have existed
>    when v10 was applied to the then-HEAD commit) has been fixed.
> 
>  - The fallback mechanism for adding DT_GNU_FLAGS_1 sections to the required
>    binaries has had some infrastructure moved to the elf/ directory
> 
>  - The runstatedir setting introduced by recent autoconf has been omitted from
>    the patchset as it is not relevant to this feature.
> 
>  - libpthread no longer tagged DT_GNU_FLAGS_1/DF_GNU_1_UNIQUE.
>    (with both -z unique supporting linkers and if the .os hack is
>    used to add the new flag - this was inconsistent before).
> 
> Not changed:
> 
>  - There is still some diagnostic info in the config.log when the linker
>    layout is acceptable but -z unique is not yet supported. I believe this
>    _is_ useful diagnostic information as a developer might otherwise wonder
>    why the vanilla linker was being rejected when its layout output seemed
>    fine.
> 
> I have not yet implemented, but plan to address once this series is
> accepted/acceptable:
> 
>  - dl_iterate_ns_phdr (cf dl_iterate_phdr but taking a namespace argument)
> 
>  - Check RTLD_GLOBAL interacts properly and unsurprisingly with RTLD_SHARED.

Hi Vivek, 

This patchset looks much better than before, I have added my comments on the
set. There are some spots that need rework, like some patches that should be
merged, some style fixes, and testcase; but in general I think we can move 
forward.

I have fixed all my remarks on a personal branch [1] and checked on both
x86_64-linux-gnu and i686-linux-gnu with binutils with and without DT_GNU_FLAGS_1 
support. If you are ok with my changes, please report the patches without the RFC 
so I can ack them and I will push them upstream.

Thanks for working on this.

[1] https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/azanella/rtld-shared