Re: Alternative solution to stat storm problem

[Tom Scogland]

Hi Ludovic, thanks for your thoughts.

You’re right, the LD_LIBRARY_PATH will not change the loading order, but using 
LD_PRELOAD will by the same mechanism we’re using, pre-filling the cache with a 
library at the same soname.  As part of other explorations we’re doing around 
tweaking or wrapping the loader, it may be possible to get semantics like 
LD_LIBRARY_PATH other ways, but at the moment the goal is to make a program 
that will correctly load all the dependencies it would have loaded were it run 
in the same environment it was built in, despite LD_LIBRARY_PATH or RUNPATH in 
dependencies or similar.  Making a little tool that would override the same way 
LD_LIBRARY_PATH would have would be relatively straightforward though, would 
that be worth exploring do you think?

-Tom

On 8 Jan 2022, at 19:05, Farid Zakaria wrote:

> I did forget to mention the point of LD_LIBRARY_PATH, that you can
> still make use of LD_PRELOAD and I am also thinking about maybe using
> something like dlopen-resolver[1] to further expand the NEEDED
> section.
>
> [1] 
> https://urldefense.us/v3/__https://github.com/Mic92/dlopen-resolver__;!!G2kpM7uM-TzIFchu!gFf3bOVCBsw_Ld35XMHl8Y0nYb0k7ikmOrOuo5SGPLCLqrCqx5qaP3giJkQTBeRD1A$
>
> On Sat, Jan 8, 2022 at 7:00 PM Farid Zakaria <fmzakari@ucsc.edu> wrote:
>>
>> Hi Ludovic,
>>
>> On Sat, Jan 8, 2022 at 1:22 PM Ludovic Courtès <ludo@gnu.org> wrote:
>>>
>>> Hi Farid,
>>>
>>> Farid Zakaria <fmzakari@ucsc.edu> skribis:
>>>
>>>> I have written a tool _shrinkwrap_ [2] that takes all transitive
>>>> dynamic shared object dependencies (only those listed in DT_NEEDED)
>>>> and turns them into an absolute path.
>>>>
>>>> This has the same result as caching the entries and avoids the
>>>> unnecessary failed attempts at trying each RUNPATH entry.
>>>>
>>>> Using the same demo application _emacs_ shows as much as well:
>>>
>>> Nice!  I think that’s another interesting way to address the problem.
>>>
>>> I guess the advantage is that you don’t need the ld.so patch.  The
>>> downside is that PatchELF needs to be able to write longer NEEDED
>>> strings in the dynamic section, which it may not always be successful at
>>> (I think?).
>>
>> I can't claim to be a ELF specification guru but I have not
>> encountered that longer NEEDED strings to be a cause for failure.
>> The emacs example is a pretty good test case because the transitive
>> closure of all NEEDED libraries is quite large, which all seem to be
>> added successfully to the ELF header.
>>
>> The benefit to me seems:
>> 1 - does not need a glibc patch for functionality (although for other
>> libc such as musl it might in this case
>> https://urldefense.us/v3/__https://www.openwall.com/lists/musl/2021/12/21/1__;!!G2kpM7uM-TzIFchu!gFf3bOVCBsw_Ld35XMHl8Y0nYb0k7ikmOrOuo5SGPLCLqrCqx5qaP3giJkSfjFvBcw$
>>  )
>> 2 - understanding the dependencies of an application become simpler
>> 3 - there are esoteric cases where in fact libraries might link to the
>> wrong libraries (although they are correct at build time) given a
>> RUNPATH/RPATH since there are subtleties with the inheritance model.
>>
>> I'm actually researching ways to improve (3) as well through
>> mentorship with Tom Scogland by researching alternative ways to do
>> linking:
>> - RUNPATH per NEEDED
>> - the ability to specify whether a RUNPATH should be inherited or not
>> to downstream dependencies
>>
>>> Also, I wonder if the absolute file names in NEEDED interfere with uses
>>> of $LD_LIBRARY_PATH (making it impossible to force use of another
>>> libxyz.so than the one that would be found in RUNPATH.)
>>
>> Correct. For a system with reproducibility in mind this can perhaps be
>> a desired feature.
>> It is the current limitation of the proposal.
>>
>> In fact, Carlos brought up a great philosophical question:
>> "Is linking to libraries through a content-addressable value allowed
>> for LGPL software?"
>> What if the linked address also forced the content-address by having
>> it resolve to something on IPFS ?
>>
>>> Thoughts?
>>>
>>> Thanks for sharing!
>>>
>>> Ludo’.