[RFC PATCH 0/5] Proof-of-Concept implementation of RTLD_SHARED for dlmopen

Vivek Das Mohapatra vivek@collabora.com
Thu Apr 5 17:52:00 GMT 2018

As discussed in https://sourceware.org/bugzilla/show_bug.cgi?id=22745
dlmopen requires a mechanism for [optionally] sharing some objects
between more than one namespace.

The following patchset attempts 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.

I have attached the test program(s) I am using to the bug above.

It is not intended as a final implementation but I wanted to check
that the basic approach is acceptable/workable.

If it is, then I plan to extend the patchset as follows:

 - dlmopen will implicitly apply RTLD_SHARED to the libc/libpthread group
 - The user will be able to request that this sharing _not_ occur
   by passing a different flag to dlmopen (name TBD)
 - LD_AUDIT paths will not apply this implict sharing rule, so audit libraries
   will continue to be completely isolated.

If it isn't, then I guess it's back to the drawing board (but reasons why
it isn't acceptable/workable would be appreciated so I can figure out how
to do it right).

Vivek Das Mohapatra (5):
  bits/dlfcn.h: Declare and describe the dlmopen RTLD_SHARED flag
  include/link.h: Update the link_map struct to allow clones
  elf/dl-object.c: Implement a helper function to clone link_map entries
  elf/dl-load.c, elf-dl-open.c: Implement RTLD_SHARED dlmopen cloning
  elf/dl-fini.c: Handle cloned link_map entries in the shutdown path

 bits/dlfcn.h               |  7 +++++
 elf/dl-fini.c              | 51 ++++++++++++++++++++++++++++++
 elf/dl-load.c              | 34 ++++++++++++++++++++
 elf/dl-object.c            | 78 ++++++++++++++++++++++++++++++++++++++++++++++
 elf/dl-open.c              | 31 ++++++++++++++++--
 include/link.h             |  6 ++--
 sysdeps/generic/ldsodefs.h |  6 ++++
 7 files changed, 209 insertions(+), 4 deletions(-)


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