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[committed v2 1/2] 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 e7feec374c63 ("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.
---
Hi Florian,
> > Index: glibc/elf/tst-absolute-zero-lib.c
> > ===================================================================
> > --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> > +++ glibc/elf/tst-absolute-zero-lib.c 2018-06-18 18:12:40.102324540 +0100
> > @@ -0,0 +1,25 @@
> > +/* BZ #xxxxx absolute zero symbol calculation shared module.
>
> Needs Bugzilla number.
>
> > Index: glibc/elf/tst-absolute-zero.c
> > ===================================================================
> > --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> > +++ glibc/elf/tst-absolute-zero.c 2018-06-18 18:12:40.147611912 +0100
> > @@ -0,0 +1,38 @@
> > +/* BZ #xxxxx absolute zero symbol calculation main executable.
>
> Likewise.
Thank you for your review. I have applied this change now, with these
issues fixed. I'm keeping BZ #23307 open for the outcome with 2/2.
Maciej
---
elf/Makefile | 8 ++++++--
elf/dl-lookup.c | 1 +
elf/tst-absolute-zero-lib.c | 25 +++++++++++++++++++++++++
elf/tst-absolute-zero-lib.lds | 1 +
elf/tst-absolute-zero.c | 38 ++++++++++++++++++++++++++++++++++++++
5 files changed, 71 insertions(+), 2 deletions(-)
glibc-elf-shn-abs-zero.diff
Index: glibc/elf/Makefile
===================================================================
--- glibc.orig/elf/Makefile 2018-06-18 18:12:19.911942887 +0100
+++ glibc/elf/Makefile 2018-06-18 18:12:40.092259983 +0100
@@ -186,7 +186,7 @@ tests += restest1 preloadtest loadfail m
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 testob
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
@@ -1456,6 +1456,10 @@ LDLIBS-tst-absolute-sym-lib.so = tst-abs
$(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)
Index: glibc/elf/dl-lookup.c
===================================================================
--- glibc.orig/elf/dl-lookup.c 2018-06-17 09:06:30.202407070 +0100
+++ glibc/elf/dl-lookup.c 2018-06-18 18:12:36.138114433 +0100
@@ -76,6 +76,7 @@ check_match (const char *const undef_nam
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))))
Index: glibc/elf/tst-absolute-zero-lib.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ glibc/elf/tst-absolute-zero-lib.c 2018-06-29 16:49:42.817333198 +0100
@@ -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;
+}
Index: glibc/elf/tst-absolute-zero-lib.lds
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ glibc/elf/tst-absolute-zero-lib.lds 2018-06-18 18:12:40.129495568 +0100
@@ -0,0 +1 @@
+"absolute" = 0;
Index: glibc/elf/tst-absolute-zero.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ glibc/elf/tst-absolute-zero.c 2018-06-29 16:49:50.893494243 +0100
@@ -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>