setup-build-env: Fix cross-compilation dependencies

setup-build-env/install_cross_compilation_toolchain.sh

@@ -46,14 +46,95 @@ fi
 
 sudo dpkg --add-architecture "$DEB_ARCH"
 sudo apt-get update -y
+
 sudo apt-get install -y --no-install-recommends    \
      binfmt-support qemu-user-static               \
      "gcc-${GCC_VERSION}-${TARGET_ARCH}-linux-gnu" \
-     "g++-${GCC_VERSION}-${TARGET_ARCH}-linux-gnu" \
-     "linux-libc-dev:${DEB_ARCH}"                  \
-     "libelf-dev:${DEB_ARCH}"                      \
-     "libssl-dev:${DEB_ARCH}"                      \
-     "zlib1g-dev:${DEB_ARCH}"
+     "g++-${GCC_VERSION}-${TARGET_ARCH}-linux-gnu"
+
+# Target libraries are downloaded and unpacked rather than installed as
+# :${DEB_ARCH} multiarch packages, so that they never enter the host dpkg
+# database. Being in it would subject them to Multi-Arch: same version equality
+# against the host-arch copies, and to
+#     libc6-dev:${DEB_ARCH} Breaks libc6-dev-${DEB_ARCH}-cross (<< <glibc>~)
+# which makes the multiarch and cross package sets mutually uninstallable
+# whenever the archive's glibc moves ahead of src:cross-toolchain-base.
+# See https://bugs.debian.org/1144098 and https://bugs.debian.org/1130544.
+#
+# ${SYSROOT} is already on the cross gcc's default search path, so the build
+# needs no -I/-L/--sysroot. selftests/bpf shares one $(EXTRA_CFLAGS) between the
+# host and target bpftool sub-makes, so a target-only flag would have nowhere to
+# live anyway.
+#
+# Both shared objects and static archives are needed: test binaries link
+# dynamically, but CI passes EXTRA_LDFLAGS=-static and the cross bpftool is
+# linked statically.
+SYSROOT="/usr/${TARGET_ARCH}-linux-gnu"
+TARGET_LIBS=(
+     "libelf1t64:${DEB_ARCH}"  "libelf-dev:${DEB_ARCH}"
+     "zlib1g:${DEB_ARCH}"      "zlib1g-dev:${DEB_ARCH}"
+     "libzstd1:${DEB_ARCH}"    "libzstd-dev:${DEB_ARCH}"
+     "libssl3t64:${DEB_ARCH}"  "libssl-dev:${DEB_ARCH}"
+)
+
+STAGE="$(mktemp -d)"
+trap 'rm -rf "${STAGE}"' EXIT
+(
+     cd "${STAGE}"
+     apt-get download "${TARGET_LIBS[@]}"
+     mkdir -p x
+     for deb in *.deb; do
+          dpkg-deb -x "${deb}" x
+     done
+)
+
+sudo mkdir -p "${SYSROOT}/include" "${SYSROOT}/lib"
+
+# Arch-specific headers are copied second so they win.
+if [ -d "${STAGE}/x/usr/include" ]; then
+     sudo cp -a "${STAGE}/x/usr/include/." "${SYSROOT}/include/"
+fi
+if [ -d "${STAGE}/x/usr/include/${TARGET_ARCH}-linux-gnu" ]; then
+     sudo cp -a "${STAGE}/x/usr/include/${TARGET_ARCH}-linux-gnu/." \
+                "${SYSROOT}/include/"
+fi
+
+for libdir in "usr/lib/${TARGET_ARCH}-linux-gnu" "lib/${TARGET_ARCH}-linux-gnu"; do
+     if [ -d "${STAGE}/x/${libdir}" ]; then
+          sudo cp -a "${STAGE}/x/${libdir}/." "${SYSROOT}/lib/"
+     fi
+done
+
+# -dev packages may ship .so as absolute symlinks, which dangle once relocated.
+for link in "${SYSROOT}"/lib/*; do
+     if [ ! -L "${link}" ]; then continue; fi
+     if [ -e "${link}" ]; then continue; fi
+     linktarget="$(basename "$(readlink "${link}")")"
+     if [ -e "${SYSROOT}/lib/${linktarget}" ]; then
+          sudo ln -sfn "${linktarget}" "${link}"
+     else
+          echo "WARNING: dangling symlink ${link} -> $(readlink "${link}")"
+     fi
+done
+
+# Surface a stale TARGET_LIBS here rather than as a link error much later.
+sysroot_missing=0
+for hdr in libelf.h gelf.h zlib.h openssl/evp.h; do
+     if [ ! -e "${SYSROOT}/include/${hdr}" ]; then
+          echo "ERROR: missing target header ${SYSROOT}/include/${hdr}"
+          sysroot_missing=1
+     fi
+done
+for lib in libelf.so libelf.a libz.so libz.a libzstd.a libcrypto.so libcrypto.a; do
+     if [ ! -e "${SYSROOT}/lib/${lib}" ]; then
+          echo "ERROR: missing target library ${SYSROOT}/lib/${lib}"
+          sysroot_missing=1
+     fi
+done
+if [ "${sysroot_missing}" -ne 0 ]; then
+     echo "Target sysroot assembly failed; TARGET_LIBS is probably stale."
+     exit 1
+fi
 
 sudo update-alternatives --install \
      /usr/bin/${TARGET_ARCH}-linux-gnu-gcc  \