[Bug dynamic-link/21942] _dl_dst_substitute incorrectly handles $ORIGIN: with AT_SECURE=1

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https://sourceware.org/bugzilla/show_bug.cgi?id=21942

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=5aad5f617892e75d91d4c8fb7594ff35b610c042

commit 5aad5f617892e75d91d4c8fb7594ff35b610c042
Author: Carlos O'Donell <carlos@redhat.com>
Date:   Tue Jun 5 23:55:17 2018 -0400

    Improve DST handling (Bug 23102, Bug 21942, Bug 18018, Bug 23259).

    This commit improves DST handling significantly in the following
    ways: firstly is_dst () is overhauled to correctly process DST
    sequences that would be accepted given the ELF gABI.  This means that
    we actually now accept slightly more sequences than before.  Now we
    accept $ORIGIN$ORIGIN, but in the past we accepted only $ORIGIN\0 or
    $ORIGIN/..., but this kind of behaviour results in unexpected
    and uninterpreted DST sequences being used as literal search paths
    leading to security defects.  Therefore the first step in correcting
    this defect is making is_dst () properly account for all DSTs
    and making the function context free in the sense that it counts
    DSTs without knowledge of path, or AT_SECURE.  Next, _dl_dst_count ()
    is also simplified to count all DSTs regardless of context.
    Then in _dl_dst_substitute () we reintroduce context-dependent
    processing for such things as AT_SECURE handling.  At the level of
    _dl_dst_substitute we can have access to things like the true start
    of the string sequence to validate $ORIGIN-based paths rooted in
    trusted directories.  Lastly, we tighten up the accepted sequences
    in AT_SECURE, and avoid leaving known unexpanded DSTs, this is
    noted in the NEWS entry.

    Verified with a sequence of 68 tests on x86_64 that cover
    non-AT_SECURE and AT_SECURE testing using a sysroot (requires root
    to run).  The tests cover cases for bug 23102, bug 21942, bug 18018,
    and bug 23259.  These tests are not yet appropriate for the glibc
    regression testsuite, but with the upcoming test-in-container testing
    framework it should be possible to include these tests upstream soon.

    See the mailing list for the tests:
    https://www.sourceware.org/ml/libc-alpha/2018-06/msg00251.html

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog     |   19 +++++
 NEWS          |   11 +++
 elf/dl-deps.c |    2 +-
 elf/dl-dst.h  |   13 ----
 elf/dl-load.c |  213 ++++++++++++++++++++++++++++++++++++---------------------
 5 files changed, 165 insertions(+), 93 deletions(-)

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