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Most applications' build systems are not well-tested against static linking, so you will often run into situations where you need to tweak them a little bit.
Each application's build system is different, so each application requires different tweaks. The general idea is that your compilation should should modify the application's build system in such a way that it works. This tutorial serves as an example: we will show you how to tweak Nginx's build system.
One of the problems in the Nginx build system is that it is able to detect OpenSSL's libcrypto library (for cryptographic algorithms), but fails to detect its libssl library (for SSL/TLS support). Because of this, it is not possible to compile Nginx with HTTPS support. Consider what happens if we were to try to run Nginx's configure script (inside Holy Build Box) with --with-http_ssl_module:
+ ./configure --without-http_rewrite_module --with-http_ssl_module
...
checking for OpenSSL library ... not found
./configure: error: SSL modules require the OpenSSL library.
You can either do not enable the modules, or install the OpenSSL library
into the system, or build the OpenSSL library statically from the source
with nginx by using --with-openssl=<path> option.
The best way to analyze the problem is by entering the Holy Build Box container, running the compilation script, then check whether the build system left any logs files that we can use to further analyze the problem.
Let's first update compile.sh so that the configure script is called with --with-http_ssl_module:
./configure --without-http_rewrite_module --with-http_ssl_module
Next, enter the container, invoke the compilation script and verify that the Nginx configure script bails out with an error. We assume that nginx-1.8.0.tar.gz from tutorial 3 is still in the current working directory.
$ docker run -t -i --rm \
-v `pwd`:/io \
phusion/holy-build-box-64:latest \
bash
container# bash /io/compile.sh
...
./configure: error: ...
container#
Most build systems log the reason why a configure check failed. Autotools log all checks to config.log. Nginx's build system is not written in autotools, but if you look around inside the Nginx source directory (/nginx-1.8.0) you will eventually find a file objs/autoconf.err. That is where the Nginx build system logs to. Let's look inside the file.
container# cd nginx-1.8.0
container# cat objs/autoconf.err
Towards the end of the file, we encounter a bunch of errors:
checking for OpenSSL library
/hbb_exe/lib/libcrypto.a(c_zlib.o): In function `bio_zlib_free':
(.text+0x6f): undefined reference to `inflateEnd'
...
/hbb_exe/lib/libcrypto.a(dso_dlfcn.o): In function `dlfcn_globallookup':
(.text+0x30): undefined reference to `dlopen'
...
----------
#include <sys/types.h>
#include <unistd.h>
#include <openssl/ssl.h>
int main() {
SSL_library_init();
return 0;
}
----------
cc -O2 -fvisibility=hidden -I/hbb_exe/include -L/hbb_exe/lib -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -o objs/autotest objs/autotest.c -lssl -lcrypto
In the above snippet, we see that the Nginx configure script tried to compile a small program in order to autodetect whether OpenSSL is available. However, the compilation of that small program failed because of undefined symbol references.
The fact that the inflateEnd symbol is undefined clearly indicates that the program should have been linked to zlib (-lz), but wasn't. Similarly, dlopen indicates that the program needed to be linked to the dl library (-ldl).
Don't know which library a symbol belongs to? Try Google. If you search for
inflateEndyou should see hints it is related to zlib.
The file inside the Nginx build system that is responsible for autodetect OpenSSL is auto/lib/openssl/conf. Inside that file, we see this:
OPENSSL=NO
ngx_feature="OpenSSL library"
ngx_feature_name="NGX_OPENSSL"
ngx_feature_run=no
ngx_feature_incs="#include <openssl/ssl.h>"
ngx_feature_path=
ngx_feature_libs="-lssl -lcrypto"
ngx_feature_test="SSL_library_init()"
. auto/feature
These are the lines responsible for running the OpenSSL check. As you can see, the Nginx configure script tries to link to OpenSSL with -lssl -lcrypto, although -lz -ldl are also required.
Linking to -lssl -lcrypto is enough if OpenSSL is compiled dynamically, because the OpenSSL dynamic is itself linked to zlib and dl. But this doesn't work with static libraries: they do not contain information about which further dependencies are required, so it is up to us to specify all dependencies.
Now that you are done analyzing the problem, you can exit the container shell.
container# exit
We fix this problem by modifying auto/lib/openssl/conf so that it compiles the test program with -lz -ldl. The best place to do this is in the compilation script, right after extracting the Nginx source code. Before the ./configure invocation, insert:
sed -i 's|-lssl -lcrypto|-lssl -lcrypto -lz -ldl|' auto/lib/openssl/conf
Now test the script:
docker run -t -i --rm \
-v `pwd`:/io \
phusion/holy-build-box-64:latest \
bash /io/compile.sh
You should see that the configure script succeeds in detecting OpenSSL this time:
+ ./configure --without-http_rewrite_module --with-http_ssl_module
...
checking for OpenSSL library ... found
Verify that the compiled Nginx binary works and that it is compiled with SSL support:
$ ./nginx -V
nginx version: nginx/1.8.0
built with OpenSSL 1.0.2d 9 Jul 2015
TLS SNI support enabled
Finally, verify that it is not dynamically linked to any non-essential libraries:
$ ldd nginx
linux-vdso.so.1 => (0x00007ffddb4fd000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fa531b61000)
libcrypt.so.1 => /lib/x86_64-linux-gnu/libcrypt.so.1 (0x00007fa531928000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fa531724000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa53135f000)
/lib64/ld-linux-x86-64.so.2 (0x00007fa531d7f000)
Another problem is that the Nginx build system does not respect $LDFLAGS.
If you look closely at the output of the compilation script, you will see that the Nginx build system invokes the compiler like this:
cc -c -O2 -fvisibility=hidden -I/hbb_exe/include -L/hbb_exe/lib -I src/core -I src/event -I src/event/modules -I src/os/unix -I objs \
-o objs/src/core/ngx_log.o \
src/core/ngx_log
Notice the -O2 -fvisibility=hidden -I/hbb_exe/include -L/hbb_exe/lib part. This is the value of the $CFLAGS environment variable (as you have seen in tutorial 1), so the Nginx build system passed that environment variable to the compiler, just as we wanted.
However, if you look a bit further at how Nginx links the executable, that doesn't look so good:
cc -o objs/nginx \
objs/src/core/nginx.o \
...
objs/ngx_modules.o \
-lpthread -lcrypt -lssl -lcrypto -lz -ldl -ldl -lz
Nginx did not pass the value of $LDFLAGS -- which is -L/hbb_exe/lib -- to the linker at all!
Until now, this hasn't been much of a problem. $LDFLAGS only contains a -L parameter. The Holy Build Box environment also sets the LIBRARY_PATH environment variable, so the linker can still find the Holy Build Box static libraries. However, as you will learn in tutorial 5, we provide alternative library variants where $LDFLAGS can contain much more than just -L. So we need to find a way to make Nginx's build system pass our $LDFLAGS.
One way to fix this is by editing the Makefile. However this should be considered the "nuclear option" -- something to be done when you have no other choice. If Nginx provides a way for us to pass additional linker flags, then we should use that.
If we run the Nginx configure script with --help, we find just what we need:
$ ./configure --help
...
--with-ld-opt=OPTIONS set additional linker options
...
So we modify compile.sh and change the configure invocation to:
./configure --without-http_rewrite_module --with-http_ssl_module --with-ld-opt="$LDFLAGS"
Notice the quotes around $LDFLAGS. This ensures that Bash considers $LDFLAGS to be a single argument, despite the fact that it contains spaces.
Now test the script:
docker run -t -i --rm \
-v `pwd`:/io \
phusion/holy-build-box-64:latest \
bash /io/compile.sh
You should see that linker flags are now passed properly:
cc -c -O2 -fvisibility=hidden -I/hbb_exe/include -L/hbb_exe/lib -I src/core -I src/event -I src/event/modules -I src/os/unix -I objs \
-o objs/ngx_modules.o \
objs/ngx_modules.c
...
objs/ngx_modules.o \
-L/hbb_exe/lib -lpthread -lcrypt -lssl -lcrypto -lz -ldl -ldl -lz
#!/bin/bash
set -e
# Activate Holy Build Box environment.
source /hbb_exe/activate
set -x
# Extract and enter source
tar xzf /io/nginx-1.8.0.tar.gz
cd nginx-1.8.0
# Compile
sed -i 's|-lssl -lcrypto|-lssl -lcrypto -lz -ldl|' auto/lib/openssl/conf
./configure --without-http_rewrite_module --with-http_ssl_module --with-ld-opt="$LDFLAGS"
make
make install
# Copy result to host
cp /usr/local/nginx/sbin/nginx /io/You have now seen an example of how a program's build system can be tweaked to allow proper static linking. Next up, we will introduce you to the different library variants that Holy Build Box provides.