Add CONTRIBUTING.md doc

CONTRIBUTING.md

@@ -0,0 +1,21 @@
+# Contributing
+
+Thanks for your interest in contributing to rust-landlock!
+
+## Testing vs kernel ABI
+
+The Landlock functionality exposed differs between kernel versions.  In order
+to test all possible variations, the rust-landlock tests will run different
+subsets of tests based on the landlock support in the current kernel.
+
+In order to fully test a change, it should be verified against a range of
+kernel versions.  This is done by the github actions CI, but is currently
+challenging to do locally.  For local development, running `cargo test` will
+test against your currently running kernel version, which may result in some
+tests being skipped.
+
+The kernel to test against can be overridden using the LANDLOCK_CRATE_TEST_ABI
+environmental variable.  For more details see the comment in
+`compat.rs:current_kernel_abi()`.
+
+For more information about Landlock ABIs see https://landlock.io/rust-landlock/landlock/enum.ABI.html

src/compat.rs

@@ -146,7 +146,7 @@ lazy_static! {
                 panic!("Unknown ABI: {n}");
             }
         }
-        Err(std::env::VarError::NotPresent) => ABI::iter().last().unwrap(),
+        Err(std::env::VarError::NotPresent) => ABI::new_current(),
         Err(e) => panic!("Failed to read LANDLOCK_CRATE_TEST_ABI: {e}"),
     };
 }