Document that the pinned .NET 11 preview SDK cannot run on Bionic-only hosts #1226

What would you like to see in the repo docs? The pinned .NET 11 preview SDK cannot be installed on a Bionic-only host, and I could not find that written down anywhere. Raising it because I hit it hard while trying to build this repo on a phone, and the failure mode is misleading.

The constraint

global.json pins:

{ "sdk": { "version": "11.0.100-preview.3.26207.106", "rollForward": "latestMajor" } }

The official SDK tarballs and dotnet-install.sh are glibc binaries. On a host whose loader is Bionic they install cleanly and then do not run.

What I verified on an aarch64 Termux/Android host

So the install succeeds and the failure only appears on first run, which reads like a corrupt download rather than an ABI mismatch.

What does not help

• patchelf-ing the muxer to the Bionic loader. The glibc runtime itself is absent, so there is nothing to point at.
• rollForward in global.json. That selects SDK versions, not the ABI.
• Installing a different SDK version. dotnet-sdk-10.0 runs and builds net10.0, but it does not satisfy this global.json, so the repo still will not build.

Environments that do work

• GitHub Codespaces / Actions — actions/setup-dotnet@v5 with global-json-file: global.json. This is what CI uses and it is the green path.
• Docker — mcr.microsoft.com/dotnet/sdk:11.0-preview is glibc-based and resolves the pinned preview.
• WSL 2 — real glibc userspace, the preview runs.
• Any glibc Linux, x64 or arm64, real or VM.

For a phone, the route that worked for me was a glibc guest under PRoot (Ubuntu 24.04, glibc 2.39), which needs three settings because PRoot blocks .NET's default virtual-address reservation:

export DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1   # no libicu in a minimal rootfs
export DOTNET_GCHeapHardLimit=134217728          # 128 MiB hard GC limit
ulimit -v 8388608                                # cap virtual memory at 8 GiB

Without them the runtime aborts with GC: Reserving 274877906944 bytes ... failed / 0x8007000E.

The suggestion

Not a new docs/ file — @AbhitejJohn pointed out on #1036 that platform notes belong in the README or contributing docs. My instinct is a short subsection in CONTRIBUTING.md under development setup, because the reader who hits this is a contributor trying to build, not someone reading the README. Something like a "supported build environments" note plus the one-line fact that the pinned preview is glibc-only.

Happy to send a small PR against main for whatever shape you prefer, and I am happy to drop the PRoot detail entirely if it is too niche for a shared doc — the useful part is really just "the pinned SDK is glibc-only; use Codespaces, Docker, WSL 2 or a glibc VM".