Fix treatment of symbol versions with unused as-needed libraries.

When we have a weak reference to a symbol defined in an
as-needed library, and that library ends up not-needed, gold
simply clears the version information in the symbol table, even
if the symbol could have been resolved by a needed library later
in the link order. This results in a loss of version information,
which can cause the program to bind to the wrong version at run
time.

This patch lets a dynamic definition override an earlier one if
the earlier one is from a not-needed library, so that we can
retain the version information from the binding to the needed
library. In order to do that, the tracking of needed/not-needed
had to be moved up to symbol resolution time, instead of during
Symbol_table::set_dynsym_indexes().

In cases where we still end up discarding version information,
I've added a warning.

For the original problem report and discussion, see:

https://stackoverflow.com/questions/50751421/undefined-behavior-in-shared-lib-using-libpthread-but-not-having-it-in-elf-as-d

gold/
	* resolve.cc (Symbol_table::resolve): Rename tobinding to
	orig_tobinding.  Call set_is_needed() for objects that resolve
	non-weak references.
	(Symbol_table::should_override): Allow a dynamic definition to
	override an earlier one in a not-needed library.
	* symtab.cc (Symbol_table::set_dynsym_indexes): Remove separate
	processing for as-needed symbols.  Add warning when discarding
	version informatin.
	* testsuite/Makefile.am (weak_as_needed): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/weak_as_needed.sh: New test script.
	* testsuite/weak_as_needed_a.c: New source file.
	* testsuite/weak_as_needed_b.c: New source file.
	* testsuite/weak_as_needed_b.script: New version script.
	* testsuite/weak_as_needed_c.c: New source file.
	* testsuite/weak_as_needed_c.script: New version script.

8 files changed, 190 insertions, 2 deletions

diff --git a/gold/testsuite/Makefile.am b/gold/testsuite/Makefile.am
index b88b02f..ab3b258 100644
--- a/gold/testsuite/Makefile.am
+++ b/gold/testsuite/Makefile.am
@@ -1880,6 +1880,23 @@ ver_test_14.syms: ver_test_14
 ver_test_14: gcctestdir/ld ver_test_main.o ver_test_1.so ver_test_2.so ver_test_4.so ver_test_14.script
 	$(CXXLINK) -Bgcctestdir/ -Wl,--version-script,$(srcdir)/ver_test_14.script -Wl,-E -Wl,-R,. ver_test_main.o ver_test_1.so ver_test_2.so ver_test_4.so
 
+check_SCRIPTS += weak_as_needed.sh
+check_DATA += weak_as_needed.stdout
+weak_as_needed.stdout: weak_as_needed_a.so
+	$(TEST_READELF) -dW --dyn-syms $< >$@
+weak_as_needed_a.so: gcctestdir/ld weak_as_needed_a.o weak_as_needed_b.so weak_as_needed_c.so
+	gcctestdir/ld -shared -rpath . -o $@ weak_as_needed_a.o --as-needed weak_as_needed_b.so weak_as_needed_c.so
+weak_as_needed_b.so: gcctestdir/ld weak_as_needed_b.o weak_as_needed_b.script
+	gcctestdir/ld -shared -rpath . -o $@ --version-script $(srcdir)/weak_as_needed_b.script weak_as_needed_b.o
+weak_as_needed_c.so: gcctestdir/ld weak_as_needed_c.o weak_as_needed_c.script
+	gcctestdir/ld -shared -rpath . -o $@ --version-script $(srcdir)/weak_as_needed_c.script weak_as_needed_c.o
+weak_as_needed_a.o: weak_as_needed_a.c
+	$(COMPILE) -c -fpic -o $@ $<
+weak_as_needed_b.o: weak_as_needed_b.c
+	$(COMPILE) -c -fpic -o $@ $<
+weak_as_needed_c.o: weak_as_needed_c.c
+	$(COMPILE) -c -fpic -o $@ $<
+
 check_PROGRAMS += protected_1
 protected_1_SOURCES = \
 	protected_main_1.cc protected_main_2.cc protected_main_3.cc
diff --git a/gold/testsuite/Makefile.in b/gold/testsuite/Makefile.in
index e1b1762..14e4ce5 100644
--- a/gold/testsuite/Makefile.in
+++ b/gold/testsuite/Makefile.in
@@ -479,7 +479,8 @@ check_PROGRAMS = $(am__EXEEXT_1) $(am__EXEEXT_2) $(am__EXEEXT_3) \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_4.sh ver_test_5.sh \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_7.sh ver_test_8.sh \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_10.sh ver_test_13.sh \
-@GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_14.sh relro_test.sh \
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_14.sh weak_as_needed.sh \
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	relro_test.sh \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_matching_test.sh \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	script_test_3.sh \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	script_test_4.sh \
@@ -534,7 +535,9 @@ check_PROGRAMS = $(am__EXEEXT_1) $(am__EXEEXT_2) $(am__EXEEXT_3) \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_8_2.so.syms \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_10.syms \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_13.syms \
-@GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_14.syms protected_3.err \
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_test_14.syms \
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	weak_as_needed.stdout \
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	protected_3.err \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	relro_test.stdout \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	ver_matching_test.stdout \
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	script_test_3.stdout \
@@ -5802,6 +5805,13 @@ ver_test_14.sh.log: ver_test_14.sh
 	--log-file $$b.log --trs-file $$b.trs \
 	$(am__common_driver_flags) $(AM_LOG_DRIVER_FLAGS) $(LOG_DRIVER_FLAGS) -- $(LOG_COMPILE) \
 	"$$tst" $(AM_TESTS_FD_REDIRECT)
+weak_as_needed.sh.log: weak_as_needed.sh
+	@p='weak_as_needed.sh'; \
+	b='weak_as_needed.sh'; \
+	$(am__check_pre) $(LOG_DRIVER) --test-name "$$f" \
+	--log-file $$b.log --trs-file $$b.trs \
+	$(am__common_driver_flags) $(AM_LOG_DRIVER_FLAGS) $(LOG_DRIVER_FLAGS) -- $(LOG_COMPILE) \
+	"$$tst" $(AM_TESTS_FD_REDIRECT)
 relro_test.sh.log: relro_test.sh
 	@p='relro_test.sh'; \
 	b='relro_test.sh'; \
@@ -8712,6 +8722,20 @@ uninstall-am:
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	$(TEST_OBJDUMP) -T $< | $(TEST_CXXFILT) >$@
 @GCC_TRUE@@NATIVE_LINKER_TRUE@ver_test_14: gcctestdir/ld ver_test_main.o ver_test_1.so ver_test_2.so ver_test_4.so ver_test_14.script
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	$(CXXLINK) -Bgcctestdir/ -Wl,--version-script,$(srcdir)/ver_test_14.script -Wl,-E -Wl,-R,. ver_test_main.o ver_test_1.so ver_test_2.so ver_test_4.so
+@GCC_TRUE@@NATIVE_LINKER_TRUE@weak_as_needed.stdout: weak_as_needed_a.so
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	$(TEST_READELF) -dW --dyn-syms $< >$@
+@GCC_TRUE@@NATIVE_LINKER_TRUE@weak_as_needed_a.so: gcctestdir/ld weak_as_needed_a.o weak_as_needed_b.so weak_as_needed_c.so
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	gcctestdir/ld -shared -rpath . -o $@ weak_as_needed_a.o --as-needed weak_as_needed_b.so weak_as_needed_c.so
+@GCC_TRUE@@NATIVE_LINKER_TRUE@weak_as_needed_b.so: gcctestdir/ld weak_as_needed_b.o weak_as_needed_b.script
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	gcctestdir/ld -shared -rpath . -o $@ --version-script $(srcdir)/weak_as_needed_b.script weak_as_needed_b.o
+@GCC_TRUE@@NATIVE_LINKER_TRUE@weak_as_needed_c.so: gcctestdir/ld weak_as_needed_c.o weak_as_needed_c.script
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	gcctestdir/ld -shared -rpath . -o $@ --version-script $(srcdir)/weak_as_needed_c.script weak_as_needed_c.o
+@GCC_TRUE@@NATIVE_LINKER_TRUE@weak_as_needed_a.o: weak_as_needed_a.c
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	$(COMPILE) -c -fpic -o $@ $<
+@GCC_TRUE@@NATIVE_LINKER_TRUE@weak_as_needed_b.o: weak_as_needed_b.c
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	$(COMPILE) -c -fpic -o $@ $<
+@GCC_TRUE@@NATIVE_LINKER_TRUE@weak_as_needed_c.o: weak_as_needed_c.c
+@GCC_TRUE@@NATIVE_LINKER_TRUE@	$(COMPILE) -c -fpic -o $@ $<
 
 @GCC_TRUE@@NATIVE_LINKER_TRUE@protected_1.so: gcctestdir/ld protected_1_pic.o protected_2_pic.o protected_3_pic.o
 @GCC_TRUE@@NATIVE_LINKER_TRUE@	$(CXXLINK) -Bgcctestdir/ -shared protected_1_pic.o protected_2_pic.o protected_3_pic.o
diff --git a/gold/testsuite/weak_as_needed.sh b/gold/testsuite/weak_as_needed.sh
new file mode 100755
index 0000000..05fdc75
--- /dev/null
+++ b/gold/testsuite/weak_as_needed.sh
@@ -0,0 +1,62 @@
+#!/bin/sh
+
+# weak_as_needed.sh -- a test case for version handling with weak symbols
+# and --as-needed libraries.
+
+# Copyright (C) 2018 Free Software Foundation, Inc.
+# Written by Cary Coutant <ccoutant@gmail.com>.
+
+# This file is part of gold.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+
+# This program 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 General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
+# MA 02110-1301, USA.
+
+# This test verifies that a weak reference is properly bound to
+# an as-needed library, when it is first resolved to a symbol in
+# a library that ends up being not needed.
+
+# Ref: https://stackoverflow.com/questions/50751421/undefined-behavior-in-shared-lib-using-libpthread-but-not-having-it-in-elf-as-d
+
+check()
+{
+    if ! grep -q "$2" "$1"
+    then
+	echo "Did not find expected output in $1:"
+	echo "   $2"
+	echo ""
+	echo "Actual output below:"
+	cat "$1"
+	exit 1
+    fi
+}
+
+check_missing()
+{
+    if grep -q "$2" "$1"
+    then
+	echo "Found unexpected output in $1:"
+	echo "   $2"
+	echo ""
+	echo "Actual output below:"
+	cat "$1"
+	exit 1
+    fi
+}
+
+check weak_as_needed.stdout "WEAK .* UND  *bar@v2"
+check weak_as_needed.stdout "NEEDED.*weak_as_needed_c\.so"
+check_missing weak_as_needed.stdout "NEEDED.*weak_as_needed_b\.so"
+
+exit 0
diff --git a/gold/testsuite/weak_as_needed_a.c b/gold/testsuite/weak_as_needed_a.c
new file mode 100644
index 0000000..2e4ff8f
--- /dev/null
+++ b/gold/testsuite/weak_as_needed_a.c
@@ -0,0 +1,10 @@
+extern void bar(void) __attribute__ (( weak ));
+extern void t4(void);
+
+void foo(void);
+
+void foo(void)
+{
+  bar();
+  t4();
+}
diff --git a/gold/testsuite/weak_as_needed_b.c b/gold/testsuite/weak_as_needed_b.c
new file mode 100644
index 0000000..6a1fbf8
--- /dev/null
+++ b/gold/testsuite/weak_as_needed_b.c
@@ -0,0 +1,23 @@
+#include <stdio.h>
+
+__asm__ (".symver bar_v1, bar@v1");
+__asm__ (".symver bar_v2, bar@@v2");
+
+void bar_v1(void);
+void bar_v2(void);
+void baz(void);
+
+void bar_v1(void)
+{
+  printf("weak_as_needed_b: bar_v1\n");
+}
+
+void bar_v2(void)
+{
+  printf("weak_as_needed_b: bar_v2\n");
+}
+
+void baz(void)
+{
+  printf("weak_as_needed_b: baz\n");
+}
diff --git a/gold/testsuite/weak_as_needed_b.script b/gold/testsuite/weak_as_needed_b.script
new file mode 100644
index 0000000..23ce2fa
--- /dev/null
+++ b/gold/testsuite/weak_as_needed_b.script
@@ -0,0 +1,11 @@
+v1 {
+  global:
+   bar;
+};
+v2 {
+  global:
+   bar;
+   baz;
+  local:
+   *;
+} v1;
diff --git a/gold/testsuite/weak_as_needed_c.c b/gold/testsuite/weak_as_needed_c.c
new file mode 100644
index 0000000..4c16bac
--- /dev/null
+++ b/gold/testsuite/weak_as_needed_c.c
@@ -0,0 +1,29 @@
+#include <stdio.h>
+
+__asm__ (".symver bar_v1, bar@v1");
+__asm__ (".symver bar_v2, bar@@v2");
+
+void bar_v1(void);
+void bar_v2(void);
+void baz(void);
+void t4(void);
+
+void bar_v1(void)
+{
+  printf("weak_as_needed_c: bar_v1\n");
+}
+
+void bar_v2(void)
+{
+  printf("weak_as_needed_c: bar_v2\n");
+}
+
+void baz(void)
+{
+  printf("weak_as_needed_c: baz\n");
+}
+
+void t4(void)
+{
+  printf("weak_as_needed_c: t4\n");
+}
diff --git a/gold/testsuite/weak_as_needed_c.script b/gold/testsuite/weak_as_needed_c.script
new file mode 100644
index 0000000..206d407
--- /dev/null
+++ b/gold/testsuite/weak_as_needed_c.script
@@ -0,0 +1,12 @@
+v1 {
+  global:
+   bar;
+};
+v2 {
+  global:
+   bar;
+   baz;
+   t4;
+  local:
+   *;
+} v1;