powerpc64le: enforce non-specific long double in .gnu.attributes section

We turn off this feature to avoid polluting our shared libary with
a specific value.  However, static libgcc is not under our control,
and has enabled this for ibm128 routines.  This pollutes the
resulting shared libraries with it.

Attach a post-linking hook to replace this section with one crafted
as hard-float + indeterminate ldbl.  This allows IEEE ldbl users to
avoid having to disable the gnu attributes feature which should
protect them from linking ibm ldbl libraries using the gnu attributes
feature.

Currently, this only replaces libc and libm which support both ldbl
formats and rely on application code to explicitly determine which
is to be used.

Strictly speaking, the section could be deleted with minimal lost value.
However correctly set attributes could prove useful for some future change,
and similarly missing attributes.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>

2 files changed, 56 insertions, 1 deletions

diff --git a/sysdeps/powerpc/powerpc64/le/Makefile b/sysdeps/powerpc/powerpc64/le/Makefile
index 96834d3e2b..6afa1228dd 100644
--- a/sysdeps/powerpc/powerpc64/le/Makefile
+++ b/sysdeps/powerpc/powerpc64/le/Makefile
@@ -10,7 +10,32 @@ ifeq ($(ibm128-fcts),yes)
 ibm128-abi-CFLAGS := -mabi=ibmlongdouble
 type-ldouble-CFLAGS += $(ibm128-abi-CFLAGS)
 no-gnu-attribute-CFLAGS = -mno-gnu-attribute
-endif
+
+# This is a very gnarly workaround to static libgcc providing a .gnu.attribute
+# in ibm128 functions.  This is not a bug in static libgcc as normal users
+# should only ever link ibm128 or ieee128 routines, never both.  We are the
+# exception which allows the decision to be deferred to user libraries or
+# applications.
+#
+# We built an object file and extract the desired .gnu.attributes section
+# to be inserted into lib{c,m}.so.
+ifeq ($(build-shared),yes)
+before-compile += $(common-objpfx)no_ldbl_gnu_attribute.bin
+
+$(common-objpfx)no_ldbl_gnu_attribute.bin: $(..)sysdeps/powerpc/powerpc64/le/no_ldbl_gnu_attribute.c
+	$(filter-out $(no-gnu-attribute-CFLAGS),$(compile.c)) -o $(basename $@).o
+	$(OBJCOPY) --dump-section=.gnu.attributes=$@.tmp $(basename $@).o
+	mv $@.tmp $@
+
+define after-link
+if [ ! -z "$(filter libm.so libc.so,$(notdir $1))" ]; then \
+  echo "Replacing .gnu.attributes in $1"; \
+  $(OBJCOPY) --update-section=.gnu.attributes=$(common-objpfx)no_ldbl_gnu_attribute.bin $1; \
+fi
+endef
+
+endif # ifeq ($(build-shared),yes)
+endif # ifeq ($(ibm128-fcts),yes)
 
 # All _Float128 files should be built assuming an ibm128 long double.
 # Likewise, this avoids some old GCC 7 compiler bugs whereby calls
diff --git a/sysdeps/powerpc/powerpc64/le/no_ldbl_gnu_attribute.c b/sysdeps/powerpc/powerpc64/le/no_ldbl_gnu_attribute.c
new file mode 100644
index 0000000000..8ffee30560
--- /dev/null
+++ b/sysdeps/powerpc/powerpc64/le/no_ldbl_gnu_attribute.c
@@ -0,0 +1,30 @@
+/* File to generate desired .gnu.attribute section
+   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/>.  */
+
+/* This generates a .gnu.attributes sections with
+   the default toolchain settings and an indeterminate
+   long double format.  */
+long double _not_used;
+
+/* Some older GCC versions need a little more convincing to
+   generate .gnu.attributes.  */
+double
+foo (double d)
+{
+  return d;
+}