ci: Install clang in version that matches LLVM version

LLVM bitcode is not backwards compatible and linking bitode objects
produced by different LLVM versions, even when it does not always
trigger any error in the backend, can lead to miscompilation (e.g.
#318).

Using clang 15 for a compile test  was "working" out of luck, because
the program was simple enough to not cause a miscompilation. But given
that it's wrong, ensure that clang version matches LLVM version

Author: vadorovsky

.github/workflows/ci.yml

@@ -96,28 +96,28 @@ jobs:
         run: cargo install btfdump
 
       - name: Install prerequisites
-        # ubuntu-22.04 comes with clang 13-15[0]; support for signed and 64bit
-        # enum values was added in clang 15[1] which isn't in `$PATH`.
-        #
         # gcc-multilib provides at least <asm/types.h> which is referenced by libbpf.
-        #
-        # [0] https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2204-Readme.md
-        #
-        # [1] https://github.com/llvm/llvm-project/commit/dc1c43d
         run: |
           set -euxo pipefail
           sudo apt update
           sudo apt -y install gcc-multilib
-          echo /usr/lib/llvm-15/bin >> $GITHUB_PATH
 
-      - name: Install LLVM
-        if: matrix.llvm-from == 'apt'
+      - name: Install clang
+        # We use clang in compiletests to test linking of bitcode produced by
+        # both C and Rust. The major version of clang must match the version of
+        # LLVM that bpf-linker is using.
         run: |
           set -euxo pipefail
           wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key | sudo tee /etc/apt/trusted.gpg.d/apt.llvm.org.asc
           echo -e deb http://apt.llvm.org/jammy/ llvm-toolchain-jammy-${{ matrix.llvm-version }} main | sudo tee /etc/apt/sources.list.d/llvm.list
 
           sudo apt update
+          sudo apt -y install clang-${{ matrix.llvm-version }}
+
+      - name: Install LLVM libraries and headers
+        if: matrix.llvm-from == 'apt'
+        run: |
+          set -euxo pipefail
           # TODO(vadorovsky): Remove the requirement of libpolly.
           #
           # Packages from apt.llvm.org are being built all at once, with one

tests/tests.rs

@@ -84,6 +84,14 @@ where
         .arg("-o")
         .arg(dst.as_ref())
         .arg(src.as_ref())
+        // `LD_LIBRARY_PATH` set to a custom LLVM build might mess up with
+        // clang, if it's dynamically linked to libLLVM that was build with
+        // larger configuration than the custom one, causing errors like:
+        // ```
+        // /usr/bin/clang-21: symbol lookup error: /usr/lib/llvm-21/bin/../lib/libclang-cpp.so.21.1:
+        //   undefined symbol: _ZTIN4llvm5MachO13RecordVisitorE, version
+        // ```
+        .env_remove("LD_LIBRARY_PATH")
         .output()
         .expect("failed to execute clang");