Fix ifunc support with DT_TEXTREL segments (BZ#20480)

Currently, DT_TEXTREL is incompatible with IFUNC.  When DT_TEXTREL or
DF_TEXTREL is seen, the dynamic linker calls __mprotect on the segments
with PROT_READ|PROT_WRITE before applying dynamic relocations. It leads
to segfault when performing IFUNC resolution (which requires PROT_EXEC
as well for the IFUNC resolver).

This patch makes it call __mprotect with extra PROT_WRITE bit, which
will keep the PROT_EXEC bit if exists, and thus fixes the segfault.
FreeBSD rtld libexec/rtld-elf/rtld.c (reloc_textrel_prot) does the same.

Checked on x86_64-linux-gnu, i686-linux-gnu, aarch64-linux-gnu,
sparc64-linux-gnu, sparcv9-linux-gnu, and armv8-linux-gnueabihf.

	Adam J. Richte  <adam_richter2004@yahoo.com>
	Adhemerval Zanella  <adhemerval.zanella@linaro.org>
	Fangrui Song  <maskray@google.com>

	[BZ #20480]
	* config.h.in (CAN_TEXTREL_IFUNC): New define.
	* configure.ac: Add check if linker supports textrel relocation with
	ifunc.
	* elf/dl-reloc.c (_dl_relocate_object): Use all required flags on
	DT_TEXTREL segments, not only PROT_READ and PROT_WRITE.
	* elf/Makefile (ifunc-pie-tests): Add tst-ifunc-textrel.
	(CFLAGS-tst-ifunc-textrel.c): New rule.
	* elf/tst-ifunc-textrel.c: New file.

1 files changed, 45 insertions, 0 deletions

diff --git a/elf/tst-ifunc-textrel.c b/elf/tst-ifunc-textrel.c
new file mode 100644
index 0000000..d34c4db
--- /dev/null
+++ b/elf/tst-ifunc-textrel.c
@@ -0,0 +1,45 @@
+/* Check DT_TEXTREL/DF_TEXTREL support with ifunc.
+   Copyright (C) 2018 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <stdint.h>
+
+/* Force a text relocation in the object.  */
+static const uintptr_t
+address __attribute__((section(".text"))) = (uintptr_t) &address;
+
+static uintptr_t
+foo_impl (void)
+{
+  return address;
+}
+
+void *
+__attribute__((noinline))
+foo (void)
+{
+  return (void*) foo_impl;
+}
+__asm__ (".type foo, %gnu_indirect_function");
+
+static int
+do_test (void)
+{
+  return (uintptr_t) foo () != 0 ? 0 : 1;
+}
+
+#include <support/test-driver.c>