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, 47 insertions, 0 deletions

diff --git a/configure b/configure
index 285a653..f30c31a 100755
--- a/configure
+++ b/configure
@@ -664,6 +664,7 @@ INSTALL_PROGRAM
 sysnames
 submachine
 multi_arch
+libc_cv_textrel_ifunc
 no_stack_protector
 stack_protector
 libc_cv_ssp
@@ -4052,6 +4053,52 @@ fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_indirect_function" >&5
 $as_echo "$libc_cv_gcc_indirect_function" >&6; }
 
+# Check if linker supports textrel relocation with ifunc (used on elf/tests).
+# Note that it relies on libc_cv_ld_gnu_indirect_function test above.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the linker supports textrels along with ifunc" >&5
+$as_echo_n "checking whether the linker supports textrels along with ifunc... " >&6; }
+if ${libc_cv_textrel_ifunc+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat > conftest.S <<EOF
+.type foo,%gnu_indirect_function
+foo:
+.globl _start
+_start:
+.globl __start
+__start:
+.data
+#ifdef _LP64
+.quad foo
+#else
+.long foo
+#endif
+.text
+.globl address
+address:
+#ifdef _LP64
+.quad address
+#else
+.long address
+#endif
+EOF
+libc_cv_textrel_ifunc=no
+if test $libc_cv_ld_gnu_indirect_function = yes; then
+   if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS -nostartfiles -nostdlib $no_ssp -pie -o conftest conftest.S'
+  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
+  (eval $ac_try) 2>&5
+  ac_status=$?
+  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; }; then
+     libc_cv_textrel_ifunc=yes
+   fi
+fi
+rm -f conftest*
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_textrel_ifunc" >&5
+$as_echo "$libc_cv_textrel_ifunc" >&6; }
+
+
 # Check if gcc warns about alias for function with incompatible types.
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
 $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }