Advice for compiling for targets with old GLIBC

[myz-dev]

Hi,
I am trying to cross compile my application (using axum + tokio) that has a lot of dependencies for the i686-unknown-linux-gnu target.
The table in the overview README shows, that the docker container used for the target's cross compilation supports GLIBC 2.31 onward
I am targeting a device that has 2.20 on it.
I think I should try to build my own docker container that has GLIBC 2.20 installed, but I fear I have a bigger problem that would not be solved by this approach:
Testing the compilation with the older image for i686-unknown-linux-gnu (ghcr.io/cross-rs/i686-unknown-linux-gnu:0.2.2), that has GLIB version 2.23 installed, I get following errors:

error: failed to run custom build command for `openssl-sys v0.9.80`

Caused by:
  process didn't exit successfully: `/target/debug/build/openssl-sys-115ccf982b69f88a/build-script-main` (exit status: 1)
  --- stderr
  /target/debug/build/openssl-sys-115ccf982b69f88a/build-script-main: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.29' not found (required by /target/debug/build/openssl-sys-115ccf982b69f88a/build-script-main)
  /target/debug/build/openssl-sys-115ccf982b69f88a/build-script-main: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /target/debug/build/openssl-sys-115ccf982b69f88a/build-script-main)
  /target/debug/build/openssl-sys-115ccf982b69f88a/build-script-main: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.25' not found (required by /target/debug/build/openssl-sys-115ccf982b69f88a/build-script-main)
warning: build failed, waiting for other jobs to finish...

And many errors like this one:

error: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /target/debug/deps/libtokio_macros-7a0d36fad7badacc.so)
   --> /home/myz/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.23.0/src/lib.rs:586:13
    |
586 |     pub use tokio_macros::select_priv_declare_output_enum;
    |         

Does this mean that my dependencies inherently depend on GLIBC_2.28 and I will not be able to compile my project using these crates for my target?
Is there anything I could do to run my software on this target that does not involve not using all dependencies that show errors during the cross compilation?

Reading 90846 gave me some hope that I might be able to circumvent my issue with some kind of compilation parameter to use 'weak symbols', but to be honest I do not understand how to apply this information to my use case at all.
Is there anyone who would point me to the right direction on this issue?

[Emilgardis]

It seems you're hitting #724

Build-scripts are built and kept even if glibc changes, so a build-script compiled on the host can be carried over to the cross container with a lower glibc, causing #724

Now, for the glibc problem, you can solve this in various ways.

the simplest, is using cargo-zigbuild, integrated into our zig image and usable by

# Cross.toml
[target.i686-unknown-linux-gnu]
zig = "2.20"

another way, is piggy-backing on https://github.com/cross-rs/cross-toolchains and creating a image for i686-unknown-linux-gnu so that you can set glibc and even linux version. We don't currently have a config for it

[Emilgardis]

otherwise, I'd recommend using the vendored feature

[myz-dev]

Hi,

again, thank you so much for your help.
With the vendored option I could circumvent the libssl-dev problem, but I think I misunderstand something very fundamental.
I now get the same error for the libgit2-dev package. Of course in the pre-build hook I am installing the package with apt-get install -y libgit2-dev:i386 (also tried it without the :i386 architecture suffix). Also I have installed the pkg-config package (which was installed for the host architecture already, which I think is the only one needed. For good measure, I am also installing it with DEBIAN_FRONTEND=noninteractive apt install --assume-yes pkg-config:i386), but the openssl package and the git2 package are still not detected during the build step.
I have also tried passing environmental variables to the docker file that contain the directories the packages are installed to (checked by running the image interactively and running dpkg -L libgit2-dev for example). In the printed error messages of the failing build scripts I can see that the variables have been set and recognized, but the build scripts still can not locate the packages in question.
So I am very clueless to why this could be. Unfortunately I do not know much about this process, I am trying to learn still.

Forking the crate that uses gitlib2 (built) and enabling the vendored-libgit2 feature in its git2 dependency unfortunately did not solve the problem.
I will for now have to give up on the zig build and will try your second suggestion, creating my own image based on what I find here: https://github.com/cross-rs/cross-toolchains

Thanks

[Emilgardis]

hmm ok, unfortunate. This and maybe setting PKG_CONFIG_PATH=/usr/lib/i386-linux-gnu/pkgconfig/:${PKG_CONFIG_PATH} would maybe be of use, but not sure

Forking the crate that uses gitlib2 (built) and enabling the vendored-libgit2 feature in its git2 dependency unfortunately did not solve the problem.

just fyi, you could make your top crate depend (in [build-dependencies]) on git2 and then specify the feature, instead of forking built, since features are enabled for every crate dependency across the build

[myz-dev]

Thank you for the hint, setting the path did not work unfortunately. I will learn about PKG_CONFIG_PATH now and see if I can fix the problem that way :-)

[myz-dev]

And: Thanks, just setting the feature in my main crate is great advice :-)