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<title>Gentoo Linux Documentation
--
The GNU Stack Quickstart</title>
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<body style="margin:0px;" bgcolor="#ffffff"><table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr><td valign="top" height="125" bgcolor="#45347b"><a href="http://www.gentoo.org/"><img border="0" src="http://www.gentoo.org/images/gtop-www.jpg" alt="Gentoo Logo"></a></td></tr>
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<td width="99%" class="content" valign="top" align="left">
<br><h1>The GNU Stack Quickstart</h1>
<form name="contents" action="http://www.gentoo.org">
<b>Content</b>:
<select name="url" size="1" OnChange="location.href=form.url.options[form.url.selectedIndex].value" style="font-family:sans-serif,Arial,Helvetica"><option value="#doc_chap1">1. Introduction</option>
<option value="#doc_chap2">2. Causes of executable stack markings</option>
<option value="#doc_chap3">3. Finding ELFs that ask for an executable stack</option>
<option value="#doc_chap4">4. What needs to be fixed</option>
<option value="#doc_chap5">5. How to fix the stack (in theory)</option>
<option value="#doc_chap6">6. How to fix the stack (in practice)</option>
<option value="#doc_chap7">7. Arch Status</option>
<option value="#doc_chap8">8. References</option></select>
</form>
<p class="chaphead"><a name="doc_chap1"></a><span class="chapnum">1.
</span>Introduction</p>
<p>
With the rise of mainstream consumer machines with hardware stack protection
(e.g. the <a href="http://en.wikipedia.org/wiki/NX_bit">NX bit</a> on
amd64), we developers have to be doubly sure that our packages build with the
correct stack settings. Keep in mind that stack protection is an issue for all
architectures, not just x86 or amd64.
</p>
<p>
The purpose of this document is to help package maintainers fix their packages
when they break. We will be focusing our attention on the GNU_STACK ELF
marking. ELF is simply a file format which all modern linux distros use. An
ELF can be an executable (say <span class="path" dir="ltr">/bin/ls</span>) or a library (say
<span class="path" dir="ltr">/lib/libncurses.so</span>). GNU_STACK is just an ELF program header
which tells the system how to control the stack when the ELF is loaded into
memory.
</p>
<p>
Before getting started, you should read through the Wikipedia entry on the
<a href="http://en.wikipedia.org/wiki/NX_bit">NX bit</a>. You can skip it
of course if you're already familiar with the concept of executable versus
non-executable stacks.
</p>
<p class="chaphead"><a name="doc_chap2"></a><span class="chapnum">2.
</span>Causes of executable stack markings</p>
<p>
ELF files end up with executable stack markings in one of three ways:
</p>
<ol>
<li>GCC generates code that uses executable stack</li>
<li>an object built from assembler source includes a marking indicating to
the linker that it needs an executable stack (the GNU-stack note set for
executable stack)</li>
<li>an object built from assembler source is missing the GNU-stack note;
a very common occurrence especially for code expected to work on
many platforms</li>
</ol>
<p>
GCC generates code to be executed on the stack when it implements a
<a href="http://gcc.gnu.org/onlinedocs/gccint/Trampolines.html">trampoline
for nested functions</a>. To remove the need for an executable stack in
this case, it is necessary to rewrite the code another way. Sometimes this
is relatively easy, other times not.
</p>
<p>
If an assembler source file includes a GNU-stack note that indicates it needs
an executable stack, presumably this is by design. Again, in order to remove
the need for an executable stack, the code probably needs to be rewritten.
</p>
<p>
If an assembler source contains no GNU-stack note, the system by default
assumes that an executable stack may be required. However, usually if there's
no GNU-stack note, this is simply because the author didn't include one,
rather than the code actually needing an executable stack.
</p>
<p>
In the first two cases above, the executable stack marking is correct, and
should only be removed by rewriting the code to eliminate the executable
stack requirement. Such rewriting has to be considered on a case-by-case
basis and is outside the scope of this document, at least for now. Here we
focus on the third case, where the upstream author has not indicated whether
the assembler object needs an executable stack; fixing this means adding the
GNU-stack note to the source to indicate an executable stack is not necessary.
</p>
<p class="chaphead"><a name="doc_chap3"></a><span class="chapnum">3.
</span>Finding ELFs that ask for an executable stack</p>
<p>
Before you can start fixing something, you have to make sure it's broken first,
right? For this reason, we've developed a suite of tools named <a href="pax-utils.html">PaX Utilities</a>. If you are not
familiar with these utilities, you should read the <a href="pax-utils.html">PaX Utilities Guide</a> now. Gentoo users
can simply do <span class="code" dir="ltr">emerge pax-utils</span>. Non-Gentoo users should be able to
find a copy of the source tarball in the <span class="path" dir="ltr">distfiles</span> on a <a href="http://www.gentoo.org/main/en/mirrors.xml">Gentoo Mirror</a>. Once you have the PaX
Utilities setup on your system, we can start playing around with
<span class="code" dir="ltr">scanelf</span>.
</p>
<p>
Let's see if your system has any ELFs that want an executable stack.
</p>
<a name="doc_chap3_pre1"></a><table class="ntable" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr><td bgcolor="#7a5ada"><p class="codetitle">Code Listing3.1: Scan your system</p></td></tr>
<tr><td bgcolor="#eeeeff" align="left" dir="ltr"><pre>
$ <span class="code-input">scanelf -lpqe</span>
RWX --- --- /usr/lib/opengl/xorg-x11/lib/libGL.so.1.2
RWX --- --- /usr/lib/libcrypto.so.0.9.7
RWX --- --- /usr/lib/libmp.so.3.1.7
RWX --- --- /usr/lib/libSDL-1.2.so.0.7.2
RWX --- --- /usr/lib/libsmpeg-0.4.so.0.1.3
RWX --- --- /usr/lib/libImlib2.so.1.2.0
RWX --- --- /usr/lib/libOSMesa.so.4.0
RWX --- --- /usr/lib/libxvidcore.so.4.1
RWX --- --- /usr/lib/libgmp.so.3.3.3
RWX --- --- /usr/bin/mencoder
RWX --- --- /usr/bin/Xorg
RWX --- --- /usr/bin/mplayer
</pre></td></tr>
</table>
<p>
We really only need to look at the first column (which corresponds to the ELF
GNU_STACK markings). Most of the time, if we fix that field, all the others
fall into place. As we can see above, many files are marked with an
executable stack (<span class="emphasis">RWX</span>). We want to make sure all files are marked
with <span class="emphasis">RW-</span>. The large majority of the time this means the package was
compiled incorrectly, so not much will have to be done with patching up the
source code.
</p>
<p class="chaphead"><a name="doc_chap4"></a><span class="chapnum">4.
</span>What needs to be fixed</p>
<p>
We now know what files need to be fixed, but what source files are causing
this breakage? The only way to find this out is to compile the package and
analyze the object files before they are combined into the final executable or
library.
</p>
<p class="secthead"><a name="doc_chap4_sect2">Fixing smpeg</a></p>
<p>
So we first have to compile smpeg before we can analyze it.
</p>
<a name="doc_chap4_pre1"></a><table class="ntable" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr><td bgcolor="#7a5ada"><p class="codetitle">Code Listing4.1: Compiling smpeg</p></td></tr>
<tr><td bgcolor="#eeeeff" align="left" dir="ltr"><pre>
$ <span class="code-input">ebuild /usr/portage/media-libs/smpeg/smpeg-0.4.4-r6.ebuild clean unpack compile</span>
$ <span class="code-input">cd /var/tmp/portage/smpeg-0.4.4-r6/work/smpeg-0.4.4/</span>
</pre></td></tr>
</table>
<p>
Now we need to look at each object file and see if it has a
<span class="emphasis">.note.GNU-stack</span> ELF section. Chances are, the object which is causing
us trouble lacks this section completely. In that case, the compiler will
assume that the ELF should not be restricted at all and mark it as <span class="emphasis">RWX</span>.
The <span class="code" dir="ltr">scanelf</span> utility will display output slightly different when
presented with an object that is missing the ELF section. The <b>!WX</b>
below means that "Oh no, the GNU-stack is missing and write/execute permissions
will be used by default!"
</p>
<a name="doc_chap4_pre2"></a><table class="ntable" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr><td bgcolor="#7a5ada"><p class="codetitle">Code Listing4.2: Locate missing .note.GNU-stack sections</p></td></tr>
<tr><td bgcolor="#eeeeff" align="left" dir="ltr"><pre>
$ <span class="code-input">scanelf -qeR .</span>
!WX --- --- ./video/mmxflags_asm.o
!WX --- --- ./video/mmxflags_asm.lo
!WX --- --- ./video/mmxidct_asm.o
!WX --- --- ./video/mmxidct_asm.lo
</pre></td></tr>
</table>
<p>
Sure enough, these objects lack the <span class="emphasis">.note.GNU-stack</span> ELF section and
they are linked into the final <span class="path" dir="ltr">libsmpeg.so</span> library. If we were
to patch the source files <span class="path" dir="ltr">video/mmxflags_asm.S</span> and
<span class="path" dir="ltr">video/mmxidct_asm.S</span> so that they contain <span class="emphasis">.note.GNU-stack</span>,
everything would be peachy.
</p>
<p class="secthead"><a name="doc_chap4_sect3">Check objects by hand</a></p>
<p>
For fun, lets see how we could use the more common <span class="code" dir="ltr">readelf</span> utility
(which is part of the <span class="emphasis">binutils</span> package).
</p>
<a name="doc_chap4_pre3"></a><table class="ntable" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr><td bgcolor="#7a5ada"><p class="codetitle">Code Listing4.3: Using readelf</p></td></tr>
<tr><td bgcolor="#eeeeff" align="left" dir="ltr"><pre>
<span class="code-comment">This is what the output should look like, notice the .note.GNU-stack line</span>
$ <span class="code-input">readelf -S plaympeg.o</span>
There are 12 section headers, starting at offset 0x256c:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS 00000000 000040 001ede 00 AX 0 0 16
[ 2] .rel.text REL 00000000 0030c0 000728 08 10 1 4
[ 3] .data PROGBITS 00000000 001f20 000000 00 WA 0 0 4
[ 4] .bss NOBITS 00000000 001f20 000000 00 WA 0 0 4
[ 5] .rodata.str1.4 PROGBITS 00000000 001f20 0003db 01 AMS 0 0 4
[ 6] .rodata.str1.1 PROGBITS 00000000 0022fb 0001c9 01 AMS 0 0 1
<span class="code-input">[ 7] .note.GNU-stack PROGBITS 00000000 0024c4 000000 00 0 0 1</span>
[ 8] .comment PROGBITS 00000000 0024c4 00003e 00 0 0 1
[ 9] .shstrtab STRTAB 00000000 002502 000067 00 0 0 1
[10] .symtab SYMTAB 00000000 00274c 0005e0 10 11 9 4
[11] .strtab STRTAB 00000000 002d2c 000394 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
<span class="code-comment">Notice how there is no .note.GNU-stack section here</span>
$ <span class="code-input">readelf -S video/mmxidct_asm.o</span>
There are 8 section headers, starting at offset 0x738:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS 00000000 000034 0005ee 00 AX 0 0 4
[ 2] .rel.text REL 00000000 000a4c 0000f0 08 6 1 4
[ 3] .data PROGBITS 00000000 000630 0000d8 00 WA 0 0 16
[ 4] .bss NOBITS 00000000 000708 000000 00 WA 0 0 4
[ 5] .shstrtab STRTAB 00000000 000708 000030 00 0 0 1
[ 6] .symtab SYMTAB 00000000 000878 000120 10 7 17 4
[ 7] .strtab STRTAB 00000000 000998 0000b1 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
</pre></td></tr>
</table>
<p class="chaphead"><a name="doc_chap5"></a><span class="chapnum">5.
</span>How to fix the stack (in theory)</p>
<p>
When you compile source code normally, gcc takes care of adding the GNU_STACK
markings so that the final object code is not marked with an executable
stack unless it actually needs it. However, if you compile assembly code,
gcc will not automatically add GNU_STACK markings. So, the most common source
of executable stacks in ELF binaries are packages which include raw assembly
code. Note that we're not talking about inline assembly code, but rather
files like <span class="emphasis">.S</span> which are written in pure assembler.
</p>
<p>
We can either patch each source file written in assembler and send the fixes
upstream, or we can be lazy and simply force the package build system to
assemble the source files with the GNU as option <span class="emphasis">--noexecstack</span> (but
this is highly discouraged).
</p>
<p>
The advantage to patching the code is that it's easy to do, it's portable,
and we can usually convince upstream to add it to their packages with little
fuss. The disadvantage to patching is that we may have to patch many many
files.
</p>
<p>
The advantage to just using <span class="emphasis">--noexecstack</span> is that you can simply add it
to your ebuild and be done. The disadvantage is that the option isn't very
portable (it won't work with non-GNU systems, and it probably won't even
work with all GNU systems), and we can't really convince upstream to make this
change. Thus, the only people who see the benefit here is Gentoo users. You
gotta think big baby!
</p>
<p class="chaphead"><a name="doc_chap6"></a><span class="chapnum">6.
</span>How to fix the stack (in practice)</p>
<p class="secthead"><a name="doc_chap6_sect1">Patching</a></p>
<p>
The great thing about patching is that you can copy and paste this stuff
everywhere. Just make sure the code will be preprocessed (e.g. the source
file is named with <span class="emphasis">.S</span> and not <span class="emphasis">.s</span>). Stick these code snippets
at the end of the source file, recompile, and do a jig.
</p>
<a name="doc_chap6_pre1"></a><table class="ntable" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr><td bgcolor="#7a5ada"><p class="codetitle">Code Listing6.1: Stack markings for GNU as (arch-independent)</p></td></tr>
<tr><td bgcolor="#eeeeff" align="left" dir="ltr"><pre>
#if defined(__linux__) && defined(__ELF__)
.section .note.GNU-stack,"",%progbits
#endif
</pre></td></tr>
</table>
<a name="doc_chap6_pre2"></a><table class="ntable" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr><td bgcolor="#7a5ada"><p class="codetitle">Code Listing6.2: Stack markings for NASM/YASM (x86/amd64-only)</p></td></tr>
<tr><td bgcolor="#eeeeff" align="left" dir="ltr"><pre>
%ifidn __OUTPUT_FORMAT__,elf
section .note.GNU-stack noalloc noexec nowrite progbits
%endif
%ifidn __OUTPUT_FORMAT__,elf32
section .note.GNU-stack noalloc noexec nowrite progbits
%endif
%ifidn __OUTPUT_FORMAT__,elf64
section .note.GNU-stack noalloc noexec nowrite progbits
%endif
</pre></td></tr>
</table>
<p class="secthead"><a name="doc_chap6_sect2">Compiling with --noexecstack</a></p>
<p>
Often times you only need to add the following code in your ebuild. You must
first be sure that the code does not actually require an executable stack as
forcing this flag will break the package otherwise.
</p>
<table class="ncontent" width="100%" border="0" cellspacing="0" cellpadding="0"><tr><td bgcolor="#ffffbb"><p class="note"><b>Important: </b>
Please rethink patching before using this option. Patching source code
benefits a lot more people which is the goal of OSS.
</p></td></tr></table>
<a name="doc_chap6_pre3"></a><table class="ntable" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr><td bgcolor="#7a5ada"><p class="codetitle">Code Listing6.3: Using --noexecstack</p></td></tr>
<tr><td bgcolor="#eeeeff" align="left" dir="ltr"><pre>
<span class="code-comment"># This line goes at the top of your ebuild</span>
inherit flag-o-matic
<span class="code-comment"># This line goes before CFLAGS is used (either by the ebuild or by econf/emake)</span>
append-flags -Wa,--noexecstack
</pre></td></tr>
</table>
<p>
On the off chance that you cannot assemble the files, you can tell the linker
to disable execstack stack.
</p>
<a name="doc_chap6_pre4"></a><table class="ntable" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr><td bgcolor="#7a5ada"><p class="codetitle">Code Listing6.4: Using -z noexecstack</p></td></tr>
<tr><td bgcolor="#eeeeff" align="left" dir="ltr"><pre>
<span class="code-comment"># This line goes at the top of your ebuild</span>
inherit flag-o-matic
<span class="code-comment"># This line goes before LDFLAGS is used (either by the ebuild or by econf/emake)</span>
append-ldflags -Wl,-z,noexecstack
</pre></td></tr>
</table>
<p class="secthead"><a name="doc_chap6_sect3">If all else fails ...</a></p>
<p>
If all else fails, ask around on #gentoo-dev on the irc server
irc.freenode.net. Or send an e-mail to the <a href="http://www.gentoo.org/main/en/lists.xml">gentoo-dev mailing list</a>.
If no one can seem to answer your question, give me a poke either on irc
(nickname SpanKY/vapier) or via <a href="mailto:vapier@gentoo.org">e-mail</a>.
</p>
<p class="chaphead"><a name="doc_chap7"></a><span class="chapnum">7.
</span>Arch Status</p>
<table class="ntable">
<tr>
<td class="infohead"><b>Arch</b></td> <td class="infohead"><b>Status</b></td>
</tr>
<tr>
<td class="tableinfo">alpha</td> <td class="tableinfo">fully supported (gcc-4.4.x/glibc-2.11)</td>
</tr>
<tr>
<td class="tableinfo">amd64</td> <td class="tableinfo">fully supported</td>
</tr>
<tr>
<td class="tableinfo">arm</td> <td class="tableinfo">fully supported (gcc-4.1.x/glibc-2.5)</td>
</tr>
<tr>
<td class="tableinfo">blackfin</td> <td class="tableinfo">fully supported (gcc-4.3+)</td>
</tr>
<tr>
<td class="tableinfo">hppa</td> <td class="tableinfo">gcc-3.4.x does not generate .note.GNU-stack</td>
</tr>
<tr>
<td class="tableinfo">ia64</td> <td class="tableinfo">fully supported (gcc-3.4.4+)</td>
</tr>
<tr>
<td class="tableinfo">m68k</td> <td class="tableinfo">fully supported (gcc-3.4.x)</td>
</tr>
<tr>
<td class="tableinfo">mips</td> <td class="tableinfo">gcc-3.4.x does not generate .note.GNU-stack</td>
</tr>
<tr>
<td class="tableinfo">ppc</td> <td class="tableinfo">fully supported (gcc-4.4.x/glibc-2.11)</td>
</tr>
<tr>
<td class="tableinfo">ppc64</td> <td class="tableinfo">fully supported (gcc-4.4.x/glibc-2.11)</td>
</tr>
<tr>
<td class="tableinfo">s390</td> <td class="tableinfo">fully supported</td>
</tr>
<tr>
<td class="tableinfo">s390x</td> <td class="tableinfo">fully supported</td>
</tr>
<tr>
<td class="tableinfo">sh</td> <td class="tableinfo">fully supported (gcc-3.4.x/glibc-2.5)</td>
</tr>
<tr>
<td class="tableinfo">sparc</td> <td class="tableinfo">fully supported</td>
</tr>
<tr>
<td class="tableinfo">x86</td> <td class="tableinfo">fully supported</td>
</tr>
</table>
<p class="chaphead"><a name="doc_chap8"></a><span class="chapnum">8.
</span>References</p>
<ul>
<li>thanks to the <a href="http://pax.grsecurity.net/">PaX team</a> for holding my hand</li>
<li>Roland McGrath's <a href="http://www.redhat.com/archives/fedora-devel-list/2003-November/msg00838.html">brain dump</a>
</li>
<li>
<a href="http://en.wikipedia.org/wiki/NX_bit">NX bit</a> Wikipedia entry</li>
</ul>
<br><br>
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<tr><td class="topsep" align="center"><p class="alttext">Page updated June 11, 2011</p></td></tr>
<tr><td class="topsep" align="left"><p class="alttext"><b>Summary: </b>Handbook for proper GNU Stack management in ELF systems</p></td></tr>
<tr><td align="left" class="topsep"><p class="alttext">
<a href="mailto:vapier@gentoo.org" class="altlink"><b>Mike Frysinger</b></a>
<br><i>Author</i><br><br>
<a href="mailto:solar@gentoo.org" class="altlink"><b>solar</b></a>
<br><i>Author</i><br><br>
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<br><i>Contributor</i><br><br>
<a href="mailto:g2@kevquinn.com" class="altlink"><b>Kevin F. Quinn</b></a>
<br><i>Contributor</i><br><br>
<a href="mailto:klondike@gentoo.org" class="altlink"><b>klondike</b></a>
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