elf: Accept absolute (SHN_ABS) symbols whose value is zero [BZ #23307]

We have this condition in `check_match' (in elf/dl-lookup.c):

  if (__glibc_unlikely ((sym->st_value == 0 /* No value.  */
                         && stt != STT_TLS)
                        || ELF_MACHINE_SYM_NO_MATCH (sym)
                        || (type_class & (sym->st_shndx == SHN_UNDEF))))
    return NULL;

which causes all !STT_TLS symbols whose value is zero to be silently
ignored in lookup.  This may make sense for regular symbols, however not
for absolute (SHN_ABS) ones, where zero is like any value, there's no
special meaning attached to it.

Consequently legitimate programs fail, for example taking the
`elf/tst-absolute-sym' test case, substituting 0 for 0x55aa in
`elf/tst-absolute-sym-lib.lds' and then trying to run the resulting
program we get this:

$ .../elf/tst-absolute-sym
.../elf/tst-absolute-sym: symbol lookup error: .../elf/tst-absolute-sym-lib.so: undefined symbol: absolute
$

even though the symbol clearly is there:

$ readelf --dyn-syms .../elf/tst-absolute-sym-lib.so | grep '\babsolute\b'
     7: 00000000     0 NOTYPE  GLOBAL DEFAULT  ABS absolute
$

The check for the zero value has been there since forever or commit
d66e34cd4234/08162fa88891 ("Implemented runtime dynamic linker to
support ELF shared libraries.") dating back to May 2nd 1995, and the
problem triggers regardless of commit e7feec374c ("elf: Correct
absolute (SHN_ABS) symbol run-time calculation [BZ #19818]") being
present or not.

Fix the issue then, by permitting `sym->st_value' to be 0 for SHN_ABS
symbols in lookup.

	[BZ #23307]
	* elf/dl-lookup.c (check_match): Do not reject a symbol whose
	`st_value' is 0 if `st_shndx' is SHN_ABS.
	* elf/tst-absolute-zero.c: New file.
	* elf/tst-absolute-zero-lib.c: New file.
	* elf/tst-absolute-zero-lib.lds: New file.
	* elf/Makefile (tests): Add `tst-absolute-zero'.
	(modules-names): Add `tst-absolute-zero-lib'.
	(LDLIBS-tst-absolute-zero-lib.so): New variable.
	($(objpfx)tst-absolute-zero-lib.so): New dependency.
	($(objpfx)tst-absolute-zero: New dependency.

ChangeLog

@@ -1,3 +1,17 @@
+2018-06-29  Maciej W. Rozycki  <macro@mips.com>
+
+	[BZ #23307]
+	* elf/dl-lookup.c (check_match): Do not reject a symbol whose
+	`st_value' is 0 if `st_shndx' is SHN_ABS.
+	* elf/tst-absolute-zero.c: New file.
+	* elf/tst-absolute-zero-lib.c: New file.
+	* elf/tst-absolute-zero-lib.lds: New file.
+	* elf/Makefile (tests): Add `tst-absolute-zero'.
+	(modules-names): Add `tst-absolute-zero-lib'.
+	(LDLIBS-tst-absolute-zero-lib.so): New variable.
+	($(objpfx)tst-absolute-zero-lib.so): New dependency.
+	($(objpfx)tst-absolute-zero: New dependency.
+
 2018-06-29  Zack Weinberg  <zackw@panix.com>
 
 	* configure.ac: New command-line option --disable-crypt.

elf/Makefile

@@ -186,7 +186,7 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 tst-tlsalign tst-tlsalign-extern tst-nodelete-opened \
 	 tst-nodelete2 tst-audit11 tst-audit12 tst-dlsym-error tst-noload \
 	 tst-latepthread tst-tls-manydynamic tst-nodelete-dlclose \
-	 tst-debug1 tst-main1 tst-absolute-sym tst-big-note
+	 tst-debug1 tst-main1 tst-absolute-sym tst-absolute-zero tst-big-note
 #	 reldep9
 tests-internal += loadtest unload unload2 circleload1 \
 	 neededtest neededtest2 neededtest3 neededtest4 \
@@ -273,7 +273,7 @@ modules-names = testobj1 testobj2 testobj3 testobj4 testobj5 testobj6 \
 		tst-latepthreadmod $(tst-tls-many-dynamic-modules) \
 		tst-nodelete-dlclose-dso tst-nodelete-dlclose-plugin \
 		tst-main1mod tst-libc_dlvsym-dso tst-absolute-sym-lib \
-		tst-big-note-lib
+		tst-absolute-zero-lib tst-big-note-lib
 
 ifeq (yes,$(have-mtls-dialect-gnu2))
 tests += tst-gnu2-tls1
@@ -1467,6 +1467,10 @@ LDLIBS-tst-absolute-sym-lib.so = tst-absolute-sym-lib.lds
 $(objpfx)tst-absolute-sym-lib.so: $(LDLIBS-tst-absolute-sym-lib.so)
 $(objpfx)tst-absolute-sym: $(objpfx)tst-absolute-sym-lib.so
 
+LDLIBS-tst-absolute-zero-lib.so = tst-absolute-zero-lib.lds
+$(objpfx)tst-absolute-zero-lib.so: $(LDLIBS-tst-absolute-zero-lib.so)
+$(objpfx)tst-absolute-zero: $(objpfx)tst-absolute-zero-lib.so
+
 # Both the main program and the DSO for tst-libc_dlvsym need to link
 # against libdl.
 $(objpfx)tst-libc_dlvsym: $(libdl)

elf/dl-lookup.c

@@ -76,6 +76,7 @@ check_match (const char *const undef_name,
   unsigned int stt = ELFW(ST_TYPE) (sym->st_info);
   assert (ELF_RTYPE_CLASS_PLT == 1);
   if (__glibc_unlikely ((sym->st_value == 0 /* No value.  */
+			 && sym->st_shndx != SHN_ABS
 			 && stt != STT_TLS)
 			|| ELF_MACHINE_SYM_NO_MATCH (sym)
 			|| (type_class & (sym->st_shndx == SHN_UNDEF))))

elf/tst-absolute-zero-lib.c

@@ -0,0 +1,25 @@
+/* BZ #23307 absolute zero symbol calculation shared module.
+   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/>.  */
+
+extern char absolute;
+
+void *
+get_absolute (void)
+{
+  return &absolute;
+}

elf/tst-absolute-zero-lib.lds

@@ -0,0 +1 @@
+"absolute" = 0;

elf/tst-absolute-zero.c

@@ -0,0 +1,38 @@
+/* BZ #23307 absolute zero symbol calculation main executable.
+   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 <support/check.h>
+#include <support/support.h>
+#include <support/test-driver.h>
+
+void *get_absolute (void);
+
+static int
+do_test (void)
+{
+  void *ref = (void *) 0;
+  void *ptr;
+
+  ptr = get_absolute ();
+  if (ptr != ref)
+    FAIL_EXIT1 ("Got %p, expected %p\n", ptr, ref);
+
+  return 0;
+}
+
+#include <support/test-driver.c>