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.

3 files changed, 57 insertions, 12 deletions

diff --git a/elf/Makefile b/elf/Makefile
index 4a4ca84..037f681 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -340,6 +340,9 @@ test-internal-extras += $(ifunc-test-modules)
 ifeq (yes,$(have-fpie))
 ifunc-pie-tests = ifuncmain1pie ifuncmain1vispie ifuncmain1staticpie \
 		  ifuncmain5pie ifuncmain6pie ifuncmain7pie
+ifeq (yes,$(have-textrel_ifunc))
+ifunc-pie-tests += tst-ifunc-textrel
+endif
 tests-internal += $(ifunc-pie-tests)
 tests-pie += $(ifunc-pie-tests)
 endif
@@ -1269,6 +1272,7 @@ CFLAGS-ifuncmain1staticpie.c += $(pie-ccflag)
 CFLAGS-ifuncmain5pie.c += $(pie-ccflag)
 CFLAGS-ifuncmain6pie.c += $(pie-ccflag)
 CFLAGS-ifuncmain7pie.c += $(pie-ccflag)
+CFLAGS-tst-ifunc-textrel.c += $(pic-ccflag)
 
 $(objpfx)ifuncmain1pie: $(objpfx)ifuncmod1.so
 $(objpfx)ifuncmain1staticpie: $(objpfx)ifuncdep1pic.o
diff --git a/elf/dl-reloc.c b/elf/dl-reloc.c
index 053916e..164f4ef 100644
--- a/elf/dl-reloc.c
+++ b/elf/dl-reloc.c
@@ -200,17 +200,6 @@ _dl_relocate_object (struct link_map *l, struct r_scope_elem *scope[],
 	    newp->start = PTR_ALIGN_DOWN (ph->p_vaddr, GLRO(dl_pagesize))
 			  + (caddr_t) l->l_addr;
 
-	    if (__mprotect (newp->start, newp->len, PROT_READ|PROT_WRITE) < 0)
-	      {
-		errstring = N_("cannot make segment writable for relocation");
-	      call_error:
-		_dl_signal_error (errno, l->l_name, NULL, errstring);
-	      }
-
-#if (PF_R | PF_W | PF_X) == 7 && (PROT_READ | PROT_WRITE | PROT_EXEC) == 7
-	    newp->prot = (PF_TO_PROT
-			  >> ((ph->p_flags & (PF_R | PF_W | PF_X)) * 4)) & 0xf;
-#else
 	    newp->prot = 0;
 	    if (ph->p_flags & PF_R)
 	      newp->prot |= PROT_READ;
@@ -218,7 +207,14 @@ _dl_relocate_object (struct link_map *l, struct r_scope_elem *scope[],
 	      newp->prot |= PROT_WRITE;
 	    if (ph->p_flags & PF_X)
 	      newp->prot |= PROT_EXEC;
-#endif
+
+	    if (__mprotect (newp->start, newp->len, newp->prot|PROT_WRITE) < 0)
+	      {
+		errstring = N_("cannot make segment writable for relocation");
+	      call_error:
+		_dl_signal_error (errno, l->l_name, NULL, errstring);
+	      }
+
 	    newp->next = textrels;
 	    textrels = newp;
 	  }
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>