[PATCH 1/2][RFC] #17645, fix slow DSO sorting behavior in dynamic loader

[Chung-Lin Tang <chunglin_tang at mentor dot com>]

Hi, this patch is our attempt at resolving the slow shared object sorting
situation in #17645, #15310, and some effort at #15311.  I realize this is
pretty unsuitable timing to be submitting a patch of such nature now (probably
way too late to be included into 2.30), but still sending now anyways as this
will probably need quite some discussion before being approved.

Prior attempts at solving this slow sorting behavior appeared to have failed
due to inadequate proposed testing, therefore cannot convince reviewers to
touch what seems to be perceived as a sensitive and easy to break part of ld.so.

Therefore the first part of this patch is not a change to the dynamic loader
code proper, but a testing framework for constructing DSO sorting tests.
It consists of a new Python script 'dso-ordering-test.py' that serves to
generate both testcase source files and the needed Makefile fragments from
a short description string, for example:

    a->b->c->d          // four objects linked one after another

    a->[bc]->d;b->c     // a depends on b and c, which both depend on d,
                        // b depends on c (b,c linked to object a in fixed order)

    a->b->c;{+a;%a;-a}  // a, b, c serially dependent, main program uses
                        // dlopen/dlsym/dlclose on object a

    a->b->c;{}!->[abc]  // a, b, c serially dependent; multiple tests generated
                        // to test all permutations of a, b, c ordering linked
                        // to main program

   (Above is just a short description of what the script can do, more
    documentation is in the script comments.)

and, a patch to glibc/elf/Makefile which uses this script to add a few
DSO sorting testcases.  The description string notation and output form of the
generated testcases is short enough that both the test descriptions
and expected outcomes can all directly be specified in the Makefile.

In terms of the tests I added using this script, I am not completely sure they are
(together with existing tests) adequate to prove algorithmic integrity in face
of any ld.so code changes, but the script should provide a solid tool to further
improve on coverage.  Also welcome suggestions if the current features are still
lacking in expressing some case of shared object relations, or if the documentation
still feels unclear.

Thanks,
Chung-Lin

2019-07-20  Chung-Lin Tang  <cltang@codesourcery.com>

        [BZ #17645]
        [BZ #15311]
        [BZ #15310]
        * elf/Makefile (test_dso_ordering): New make function.
        (tst-dso-ordering[123456789]): Define new DSO sorting tests.
        (tst-bz15311): Testcase from #15311.
        * scripts/dso-ordering-test.py: New script.

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