[Bug dynamic-link/25938] New: ld.so.cache should store meaning of hwcap mask bits

["fw at deneb dot enyo.de" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=25938

            Bug ID: 25938
           Summary: ld.so.cache should store meaning of hwcap mask bits
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: minor
          Priority: P3
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: fw at deneb dot enyo.de
  Target Milestone: ---
             Flags: security-

The meaning of the bits depends on the glibc version: _DL_FIRST_EXTRA depends
on _DL_FIRST_PLATFORM and _DL_PLATFORMS_COUNT, and the latter increases if more
platform subdirectories are added (e.g., "power9").

This does not appear to be a significant issue today because the
_DL_FIRST_EXTRA mechanism was only ever used for the i686 nosegneg bit, and on
i686, _DL_FIRST_EXTRA has been unchanged for a long time.

The always-set bit for  the "tls" subdirectory is put by ldconfig at the MSB of
the 64-bit hwcap word, so its position is already constant.

However, if we ever want to reuse hwcap bits for different CPU features, the
cache needs to contain additional information about the meaning of the bits.
One way to express this mean is to list the subdirectory names the bits
correspond to.

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