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improve support for out-of-tree kernel modules #1220

Merged
merged 7 commits into from
Dec 1, 2020
Merged

improve support for out-of-tree kernel modules #1220

merged 7 commits into from
Dec 1, 2020

Commits on Dec 1, 2020

  1. kernel: enable zstd compression for squashfs 

    We use squashfs archives for files that must be included, but which
are rarely or never accessed on most running systems. zstd offers
compression ratios similar to xz, and decompression speeds like lz4.
This saves space while keeping reads fast.

Signed-off-by: Ben Cressey <bcressey@amazon.com>
    @bcressey
    bcressey committed Dec 1, 2020
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  2. kernel: overhaul kernel development sources 

    Previously, we included host programs like `objtool` which are built
with the default `gcc` compiler and not our cross-compiler toolchain.

This works as long as the running system matches our build host, but
would break if we began building x86_64 images on an aarch64 system.
The reverse is not true today, but only because `objtool` is not yet
required for the arm64 target.

Ideally, we'd be able to cross-compile these host programs, but that
isn't supported by the kernel's build system, and would be hard to
implement. For example, `fixdep` is both a tool we'd want to ship,
meaning it would need to be cross-compiled, and a tool that's used
to build `objtool`, meaning it couldn't be cross-compiled and still
run on the build host.

Instead we push the problem out to the downstream consumer, who can
be relied on to have a compiler that can build native versions of the
host programs. This requires shipping all the headers, tools, and
scripts needed to run `make prepare`.

For compatibility with solutions like DKMS, which do not expect to
run anything but the module build, we add a minimal prepare target
to this path so that the host programs will be automatically rebuilt.
We also make some edits and exclude some files to avoid dependencies
on bison, flex, and OpenSSL.

Signed-off-by: Ben Cressey <bcressey@amazon.com>
    @bcressey
    bcressey committed Dec 1, 2020
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  3. kernel: package archived development sources 

    The squashfs filesystem is meant to be used on a running host, while
a tarball is easier to work with when assembling a combined archive
that also includes our toolchain.

Signed-off-by: Ben Cressey <bcressey@amazon.com>
    @bcressey
    bcressey committed Dec 1, 2020
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  4. build: set options for license squashfs 

    Apply the same options we use for the kernel-devel squashfs.

Signed-off-by: Ben Cressey <bcressey@amazon.com>
    @bcressey
    bcressey committed Dec 1, 2020
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  5. release: enable writes to /usr/src/kernels 

    With the changes to our packaging of kernel development sources, any
out-of-tree module builds will need to run `make prepare` in order to
compile dependencies like `objtool`. These binaries need to land in
the same directory tree as the other development files we ship.

Using an overlayfs mount allows writes to the otherwise read-only
content from the squashfs. We purge the upper directory on reboot so
changes do not persist across system upgrades.

Signed-off-by: Ben Cressey <bcressey@amazon.com>
    @bcressey
    bcressey committed Dec 1, 2020
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  6. host-ctr: allow writes to /usr/src/kernels 

    This directory needs to be writable in order to build out-of-tree
modules inside a superpowered container.

Signed-off-by: Ben Cressey <bcressey@amazon.com>
    @bcressey
    bcressey committed Dec 1, 2020
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  7. build: add target for kmod kit 

    To support compiling out-of-tree modules ahead of time, rather than
on a running Bottlerocket host, we need to provide two things: the
kernel development sources, such as headers and Makefiles; and the
toolchain we use to build our kernel.

Our toolchain is built separately as part of our cross-compiling SDK,
and it's possible, if unlikely, that we would ship two releases with
the same kernel version built with a different GCC.

It's also possible that variants will use different kernels, so we
cannot have just one development kit per release. This is not yet
supported, but we need the ecosystem to anticipate the requirement
for a per-variant, per-architecture kit.

The build target combines the archives from the toolchain matching
the SDK we used to build the kernel, and kernel development sources
from the most recent build. This produces a single artifact that
can be uploaded for later retrieval by a consumer that knows the
variant, architecture, and version that they are targeting.

Signed-off-by: Ben Cressey <bcressey@amazon.com>
    @bcressey
    bcressey committed Dec 1, 2020
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