Setup 1: glibc without dedicated GCC
This setup might work and is quick as it does not recompile the whole GCC toolchain, just glibc.
The only problem I have with this setup is that I haven't found a nice way to use runtime objects such as crt1.o, crti.o, and crtn.o provided by our glibc, and I'm using the host ones for now. This is mentioned at: https://sourceware.org/glibc/wiki/Testing/Builds?action=recall&rev=21#Compile_against_glibc_in_an_installed_location Those objects do early setup that glibc relies on, so I wouldn't be surprised if things crashed in wonderful and awesomely subtle ways. See attempts at solving this below.
Build glibc and install locally:
git clone git://sourceware.org/git/glibc.git
cd glibc
git checkout glibc-2.32
mkdir build
cd build
export glibc_install="$(pwd)/install"
../configure --prefix "$glibc_install"
make -j `nproc`
make install -j `nproc`
Setup 1: verify the build
test_glibc.c
#define _GNU_SOURCE
#include <assert.h>
#include <gnu/libc-version.h>
#include <stdatomic.h>
#include <stdio.h>
#include <threads.h>
atomic_int acnt;
int cnt;
int f(void* thr_data) {
for(int n = 0; n < 1000; ++n) {
++cnt;
++acnt;
}
return 0;
}
int main(int argc, char **argv) {
/* Basic library version check. */
printf("gnu_get_libc_version() = %s
", gnu_get_libc_version());
/* Exercise thrd_create from -pthread,
* which is not present in glibc 2.27 in Ubuntu 18.04.
* https://stackoverflow.com/questions/56810/how-do-i-start-threads-in-plain-c/52453291#52453291 */
thrd_t thr[10];
for(int n = 0; n < 10; ++n)
thrd_create(&thr[n], f, NULL);
for(int n = 0; n < 10; ++n)
thrd_join(thr[n], NULL);
printf("The atomic counter is %u
", acnt);
printf("The non-atomic counter is %u
", cnt);
}
Compile and run with test_glibc.sh:
#!/usr/bin/env bash
set -eux
gcc
-L "${glibc_install}/lib"
-I "${glibc_install}/include"
-Wl,--rpath="${glibc_install}/lib"
-Wl,--dynamic-linker="${glibc_install}/lib/ld-linux-x86-64.so.2"
-std=c11
-o test_glibc.out
-v
test_glibc.c
-pthread
;
ldd ./test_glibc.out
./test_glibc.out
Command adapted from https://sourceware.org/glibc/wiki/Testing/Builds?action=recall&rev=21#Compile_against_glibc_in_an_installed_location
The program outputs the expected:
gnu_get_libc_version() = 2.32
The atomic counter is 10000
The non-atomic counter is 8674
ldd output confirms that the ldd and libraries that we've just built are actually being used as expected:
+ ldd test_glibc.out
linux-vdso.so.1 (0x00007ffe4bfd3000)
libpthread.so.0 => /home/ciro/glibc/build/install/lib/libpthread.so.0 (0x00007fc12ed92000)
libc.so.6 => /home/ciro/glibc/build/install/lib/libc.so.6 (0x00007fc12e9dc000)
/home/ciro/glibc/build/install/lib/ld-linux-x86-64.so.2 => /lib64/ld-linux-x86-64.so.2 (0x00007fc12f1b3000)
The gcc compilation debug output shows that my host runtime objects were used, which is bad as mentioned previously, but I don't know how to work around it, e.g. it contains:
COLLECT_GCC_OPTIONS=/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/crt1.o
Setup 1: modify glibc
Now let's modify glibc with:
diff --git a/nptl/thrd_create.c b/nptl/thrd_create.c
index 113ba0d93e..b00f088abb 100644
--- a/nptl/thrd_create.c
+++ b/nptl/thrd_create.c
@@ -16,11 +16,14 @@
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
+#include <stdio.h>
+
#include "thrd_priv.h"
int
thrd_create (thrd_t *thr, thrd_start_t func, void *arg)
{
+ puts("hacked");
_Static_assert (sizeof (thr) == sizeof (pthread_t),
"sizeof (thr) != sizeof (pthread_t)");
Then recompile and re-install glibc, and recompile and re-run our program:
cd glibc/build
make -j `nproc`
make -j `nproc` install
./test_glibc.sh
and we see hacked printed a few times as expected.
This further confirms that we actually used the glibc that we compiled and not the host one.
Tested on Ubuntu 20.10.
Setup 1: attempts at using the correct crt* objects
https://sourceware.org/glibc/wiki/Testing/Builds?action=recall&rev=21#Compile_against_glibc_in_an_installed_location suggests adding --sysroot to the gcc command but:
- it does not really change the objects to ours according to logs
- and it makes compilation fail with
/usr/bin/ld: cannot find libgcc_s.so.1 presumably because the sysroot gets used for this GCC provided object, which we don't have at that sysroot because we only built glibc
At https://stackoverflow.com/a/66634184/895245 ZeZNiQ provides a workaround that is likely correct, by passing:
-nostartfiles
followed by all the objects. You just need to extract the correct objects from the full command with -nostartfiles and pass them manually.
For example, on my amd64 machine, the objects used were different from his 32-bit command, so this is a bit fiddly.
Bibliography:
Setup 2: crosstool-NG pristine setup
This is an alternative to setup 1, and it is the most correct setup I've achieved far: everything is correct as far as I can observe, including the C runtime objects such as crt1.o, crti.o, and crtn.o.
In this setup, we will compile a full dedicated GCC toolchain that uses the glibc that we want.
The only downside to this method is that the build will take longer. But I wouldn't risk a production setup with anything less.
crosstool-NG is a set of scripts that downloads and compiles everything from source for us, including GCC, glibc and binutils.
Yes the GCC build system is so bad that we need a separate project for that.
This setup is only not perfect because crosstool-NG does not support building the executables without extra -Wl flags, which feels weird since we've built GCC itself. But everything seems to work, so this is only an inconvenience.
Get crosstool-NG, configure and build it:
git clone https://github.com/crosstool-ng/crosstool-ng
cd crosstool-ng
git checkout a6580b8e8b55345a5a342b5bd96e42c83e640ac5
export CT_PREFIX="$(pwd)/.build/install"
export PATH="/usr/lib/ccache:${PATH}"
./bootstrap
./configure --enable-local
make -j `nproc`
./ct-ng x86_64-unknown-linux-gnu
./ct-ng menuconfig
env -u LD_LIBRARY_PATH time ./ct-ng build CT_JOBS=`nproc`
The build takes about thirty minutes to two hours.
The only mandatory configuration option that I can see, is making it match your host kernel version to use the correct kernel headers. Find your host kernel version with:
uname -a
which shows me:
4.15.0-34-generic
so in menuconfig I do:
so I select:
4.14.71
which is the first equal or older version. It has to be older since the kernel is backwards compatible.
Setup 2: optional configurations
The .config that we generated with ./ct-ng x86_64-unknown-linux-gnu has:
CT_GLIBC_V_2_27=y
To change that, in menuconfig do:
C-libraryVersion of glibc
save the .config, and continue with the build.
Or, if you want to use your own glibc source, e.g. to use glibc from the latest git, proceed like this:
Paths and misc options
Try features marked as EXPERIMENTAL: set to true
C-library
Source of glibc
Custom location: say yesCustom location
Custom source location: point to a directory containing your glibc source
where glibc was cloned as:
git clone git://sourceware.org/git/glibc.git
cd glibc
git checkout glibc-2.28
Setup 2: test it out
Once you have built he toolchain that you want, test it out with:
#!/usr/bin/env bash
set -eux
install_dir="${CT_PREFIX}/x86_64-unknown-linux-gnu"
PATH="${PATH}:${install_dir}/bin"
x86_64-unknown-linux-gnu-gcc
-Wl,--dynamic-linker="${install_dir}/x86_64-unknown-linux-gnu/sysroot/lib/ld-linux-x86-64.so.2"
-Wl,--rpath="${install_dir}/x86_64-unknown-linux-gnu/sysroot/lib"
-v
-o test_glibc.out
test_glibc.c
-pthread
;
ldd test_glibc.out
./test_glibc.out
Everything seems to work as in Setup 1, except that now the correct runtime objects were used:
COLLECT_GCC_OPTIONS=/home/ciro/crosstool-ng/.build/install/x86_64-unknown-linux-gnu/bin/../x86_64-unknown-linux-gnu/sysroot/usr/lib/../lib64/crt1.o
Setup 2: failed efficient glibc recompilation attempt
It does not seem possible with crosstool-NG, as explained below.
If you just re-build;
env -u LD_LIBRARY_PATH time ./ct-ng build CT_JOBS=`nproc`
then your changes to the custom glibc source location are taken into account, but it builds everything from scratch, making it unusable for iterative development.
If we do:
./ct-ng list-steps
it gives a nice overview of the build steps:
Available build steps, in order:
- companion_tools_for_build
- companion_libs_for_build
- binutils_for_build
- companion_tool
