[patch] --filter option in ld: srcfix

[PODOPLELOV Andrei <Andrei.PODOPLELOV@3ds.com>]

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Hello Michael,

First of all I should admit I am a big fan of your "The Linux Programming Interface" book. I've been Windows guy since mid-90th and your book helped me to become a local Linux expert.

Working recently on a bug in our product, happening on Ubuntu, but not on our official RHEL, I found that it caused by -filter option in ld. Unfortunately the description of this option in https://man7.org/linux/man-pages/man1/ld.1.html is not very clear and that delayed my resolution.
I was lucky to find https://docs.oracle.com/cd/E23824_01/html/819-0690/chapter4-4.html, containing a step-by-step sample, I played with, and figured how it all works. My final sample is attached.
In this change I want to emphasize the relationship between the two shared objects.

The current doc says:

       --filter=name
           When creating an ELF shared object, set the internal
           DT_FILTER field to the specified name.  This tells the
           dynamic linker that the symbol table of the shared object
           which is being created should be used as a filter on the
           symbol table of the shared object name.

           If you later link a program against this filter object, then,
           when you run the program, the dynamic linker will see the
           DT_FILTER field.  The dynamic linker will resolve symbols
           according to the symbol table of the filter object as usual,
           but it will actually link to the definitions found in the
           shared object name.  Thus the filter object can be used to
           select a subset of the symbols provided by the object name.

I believe it would be beneficial to change it to something like:

       --filter=name
           When creating an ELF shared object (a "filter"), set the
           internal DT_FILTER field to the specified name - another
           ELF shared object (a "filtee"). This tells the dynamic linker
           that the symbol table of the "filter" should be used to
           select a subset of the symbols provided by the "filtee".

           When you link a program against this "filter" and run it,
           the dynamic linker will see the DT_FILTER field and resolve
           symbols according to the symbol table of the "filter" object
           as usual. However, when a certain symbol of the "filter" is
           also present in "filtee", it will actually link to the
           definition in the "filtee".

Best Regards,

Andrei PODOPLELOV
Senior Software Architect - Customer Success, Spatial
Office: +1 508 305 5247
mailto:Andrei.PODOPLELOV@3ds.com
http://www.3ds.com/
Spatial Corp. | 310 Interlocken Parkway, Suite 200 | Broomfield, CO 80021 | United States

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