ci(rust-musl): check that musl targets link aws-lc

.github/Dockerfile-rust-musl

@@ -0,0 +1,34 @@
+# Checks that the rust-musl image links a C dependency for both musl targets.
+# aws-lc-sys compiled against glibc >= 2.38 headers emits __isoc23_* symbols
+# that musl lacks; calling into it keeps the linker from dropping its archive.
+# CI passes the freshly built rust-musl image so the check tests this PR's image.
+ARG RUST_MUSL_IMAGE=ghcr.io/linkerd/dev:ci-rust-musl
+FROM ${RUST_MUSL_IMAGE}
+# aws-lc-rs pulls in aws-lc-sys, a C library sensitive to which libc headers it sees.
+ARG AWS_LC_RS_VERSION=1.18.2 # repo=aws/aws-lc-rs,prefix=v
+WORKDIR /musl-link
+RUN cargo init --quiet --name musl-link && \
+    cargo add --quiet "aws-lc-rs@=${AWS_LC_RS_VERSION}"
+# Calls into aws-lc so the linker must resolve its symbols; an unused dependency would be dropped.
+COPY <<EOF src/main.rs
+fn main() {
+    let d = aws_lc_rs::digest::digest(&aws_lc_rs::digest::SHA256, b"musl");
+    println!("{:02x?}", d.as_ref());
+}
+EOF
+# just-cargo sets the musl CC, linker and flags, as the consuming repos' builds do.
+RUN for target in x86_64-unknown-linux-musl aarch64-unknown-linux-musl ; do \
+        just-cargo target="$target" build || exit 1 ; \
+    done
+# The musl headers must come from the musl release Rust links, or C dependencies
+# can declare symbols the toolchain's libc.a lacks. Runs after the link so a
+# header mismatch with the host libc is reported first.
+RUN headers=$(cat /usr/local/musl/VERSION) && \
+    for target in x86_64-unknown-linux-musl aarch64-unknown-linux-musl ; do \
+        # musl embeds its release string in libc.a; this is the version Rust links.
+        toolchain=$(strings "$(rustc --print target-libdir --target "$target")/self-contained/libc.a" | grep -m1 -xE '[0-9]+\.[0-9]+\.[0-9]+') && \
+        if [ "$headers" != "$toolchain" ]; then \
+            echo "musl headers $headers do not match $target libc.a $toolchain" >&2 ; \
+            exit 1 ; \
+        fi ; \
+    done

.github/workflows/devcontainer.yml

@@ -4,7 +4,9 @@ on:
   pull_request:
     paths:
       - .devcontainer/**
+      - .github/Dockerfile-rust-musl
       - .github/workflows/devcontainer.yml
+      - bin/just-cargo
       - Dockerfile
       - justfile
 
@@ -19,6 +21,16 @@ jobs:
       - uses: ./actions/setup-tools
       - run: just build
 
+  # Builds the rust-musl image, then links a C dependency with it for both musl
+  # targets (see .github/Dockerfile-rust-musl).
+  dev-musl-link:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1
+      - uses: ./actions/setup-tools
+      - run: just load=true version=ci targets=rust-musl build
+      - run: docker build --build-arg RUST_MUSL_IMAGE=ghcr.io/linkerd/dev:ci-rust-musl -f .github/Dockerfile-rust-musl .github
+
   dev-shellcheck:
     runs-on: ubuntu-latest
     steps:

Dockerfile

@@ -347,6 +347,40 @@ ENV CARGO_INCREMENTAL=0 \
     RUSTUP_MAX_RETRIES=10
 ENTRYPOINT ["/usr/local/bin/just-cargo"]
 
+# Builds musl C headers that match the musl release this Rust toolchain links.
+#
+# Why: musl targets compile C (e.g. `aws-lc-sys`) with clang, which otherwise
+# uses the host glibc headers. Since glibc 2.38 those redirect `sscanf`/`strtol`
+# to `__isoc23_*` symbols that musl lacks, so links fail. Debian's `musl-dev`
+# doesn't help: clang ignores its include dir and its release can differ from
+# the musl Rust links (trixie 1.2.5 vs Rust 1.90's 1.2.3).
+#
+# Caveats:
+# - The version is read with `strings` from the toolchain's `libc.a`; if musl
+#   stops embedding it, `grep` finds no match and the build fails before the fetch.
+# - Headers are fetched from musl.libc.org at build time, like other tools here.
+# - Both musl targets are assumed to ship the same musl release.
+FROM rust as rust-musl-patch
+# Only needed to read the bundled musl version; both targets ship the same release.
+RUN rustup target add x86_64-unknown-linux-musl
+# musl embeds its release string (e.g. 1.2.3) in libc.a; there is no rustc flag for it.
+# Saved to /musl/VERSION for the steps below and for CI to check against libc.a.
+# grep exits non-zero if no version is found, so a format change fails here.
+RUN musl_target_libdir=$(rustc --print target-libdir --target x86_64-unknown-linux-musl) && \
+    mkdir -p /musl && \
+    strings "$musl_target_libdir/self-contained/libc.a" | grep -m1 -xE '[0-9]+\.[0-9]+\.[0-9]+' > /musl/VERSION
+# Fetch the musl source for that release.
+RUN musl_version=$(cat /musl/VERSION) && \
+    scurl -o /musl.tar.gz "https://musl.libc.org/releases/musl-${musl_version}.tar.gz"
+RUN mkdir -p /src && \
+    tar xzf /musl.tar.gz --strip-components=1 -C /src
+# Installs only headers (copy plus sed-generated bits/*.h); nothing is compiled.
+RUN make -C /src ARCH=x86_64 prefix=/musl/x86_64 install-headers
+# bits/*.h are generated per arch under obj/; clear x86_64's so aarch64 regenerates its own.
+RUN rm -rf /src/obj
+# Installs the aarch64 headers.
+RUN make -C /src ARCH=aarch64 prefix=/musl/aarch64 install-headers
+
 FROM rust as rust-musl
 RUN rustup target add \
         aarch64-unknown-linux-musl \
@@ -359,6 +393,11 @@ RUN --mount=type=cache,from=apt-base,source=/etc/apt,target=/etc/apt,sharing=loc
         g++-aarch64-linux-gnu \
         gcc-aarch64-linux-gnu \
         libc6-dev-arm64-cross
+COPY --link --from=rust-musl-patch /musl /usr/local/musl
+# cc-rs reads these per-target flags; `-isystem` puts musl's headers ahead of glibc's (see rust-musl-patch).
+ENV CFLAGS_x86_64_unknown_linux_musl="-isystem /usr/local/musl/x86_64/include"
+# Same for aarch64, with the aarch64 musl headers.
+ENV CFLAGS_aarch64_unknown_linux_musl="-isystem /usr/local/musl/aarch64/include"
 
 ##
 ## Devcontainer