binfmt

I'd suspect a great deal of people are secretly benefiting from qemu when they do $(docker build --platform linux/{arm64,amd64}) courtesy of binfmt_misc and a static copy of qemu - https://github.com/moby/buildkit/blob/v0.23.2/docs/multi-pla... - https://github.com/moby/buildkit/blob/v0.23.2/Dockerfile#L16... - https://github.com/tonistiigi/binfmt/blob/buildkit/v9.2.2-54... - https://github.com/tonistiigi/binfmt/blob/buildkit/v9.2.2-54... and https://github.com/tonistiigi/binfmt/blob/buildkit/v9.2.2-54... and let me tell you from first-hand experience, that trying to swap in an updated version of the bundled qemu binary when the static version panics on some mis-emulated instruction is some whooooooo, boy

and its "oh, you want multi-arch, do you?" friend. While prosecuting this <https://gitlab.com/gitlab-org/gitlab/-/issues/339567> I learned that they bundle binfmt into the buildkit docker image <https://github.com/moby/buildkit/blob/v0.12.5/Dockerfile#L11...> which itself is compiled against a static copy of qemu <https://github.com/tonistiigi/binfmt/blob/deploy/v6.2.0-26/D...> with various /proc/sys/fs/binfmt_misc wizardry <https://github.com/tonistiigi/binfmt/blob/deploy/v6.2.0-26/c...> We ended up switching to using multiarch/qemu-user-static:7.2.0-1 <https://hub.docker.com/layers/multiarch/qemu-user-static/7.2...> which also mutates binfmt_misc in buildx's context in order to exec the static copy of qemu in it but I find their shell script much more legible about what's going on: <https://github.com/multiarch/qemu-user-static/blob/v7.

That's exactly what Docker does behind the scenes to run foreign arch images, see https://github.com/tonistiigi/binfmt