elf: Add glibc-hwcaps support for LD_LIBRARY_PATH

This hacks non-power-set processing into _dl_important_hwcaps.
Once the legacy hwcaps handling goes away, the subdirectory
handling needs to be reworked, but it is premature to do this
while both approaches are still supported.

ld.so supports two new arguments, --glibc-hwcaps-prepend and
--glibc-hwcaps-mask.  Each accepts a colon-separated list of
glibc-hwcaps subdirectory names.  The prepend option adds additional
subdirectories that are searched first, in the specified order.  The
mask option restricts the automatically selected subdirectories to
those listed in the option argument.  For example, on systems where
/usr/lib64 is on the library search path,
--glibc-hwcaps-prepend=valgrind:debug causes the dynamic loader to
search the directories /usr/lib64/glibc-hwcaps/valgrind and
/usr/lib64/glibc-hwcaps/debug just before /usr/lib64 is searched.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/elf/markermodMARKER-VALUE.c b/elf/markermodMARKER-VALUE.c
new file mode 100644
index 0000000000..99bdcf71a4
--- /dev/null
+++ b/elf/markermodMARKER-VALUE.c
@@ -0,0 +1,29 @@
+/* Source file template for building shared objects with marker functions.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* MARKER and VALUE must be set on the compiler command line.  */
+
+#ifndef MARKER
+# error MARKER not defined
+#endif
+
+int
+MARKER (void)
+{
+  return VALUE;
+}