aya: Improve Extension Docs

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>

aya/src/programs/extension.rs

@@ -61,8 +61,11 @@ impl Extension {
     /// Prepares the code included in the extension to replace the code of the function
     /// `func_name` within the eBPF program represented by the `program` file descriptor.
     /// This requires that both the [`Extension`] and `program` have had their BTF
-    /// loaded into the kernel as the verifier must check that the function signatures
+    /// loaded into the kernel.
-    /// match.
+    ///
+    /// The BPF verifier requires that we specify the target program and function name
+    /// at load time, so it can identify that the program and target are BTF compatible
+    /// and to enforce this constraint when programs are attached.
     ///
     /// The extension code will be loaded but inactive until it's attached.
     /// There are no restrictions on what functions may be replaced, so you could replace
@@ -79,7 +82,8 @@ impl Extension {
 
     /// Attaches the extension.
     ///
-    /// Attaches the extension effectively replacing the original target function.
+    /// Attaches the extension to the program and function name specified at load time,
+    /// effectively replacing the original target function.
     ///
     /// The returned value can be used to detach the extension and restore the
     /// original function, see [Extension::detach].
@@ -96,10 +100,14 @@ impl Extension {
         self.data.links.insert(ExtensionLink(FdLink::new(link_fd)))
     }
 
-    /// Attaches the extension to another BTF-compatible program.
+    /// Attaches the extension to another program.
+    ///
+    /// Attaches the extension to a program and/or function other than the one provided
+    /// at load time. You may only attach to another program/function if the BTF
+    /// type signature is identical to that which was verified on load. Attempting to
+    /// attach to an invalid program/function will result in an error.
     ///
-    /// Attaches the extension effectively replacing the original target function.
+    /// Once attached, the extension effectively replaces the original target function.
-    /// program.
     ///
     /// The returned value can be used to detach the extension and restore the
     /// original function, see [Extension::detach].
@@ -140,23 +148,30 @@ impl Extension {
     }
 }
 
+/// Retrieves the FD of the BTF object for the provided `prog_fd` and the BTF ID of the function
+/// with the name `func_name` within that BTF object.
 fn get_btf_info(prog_fd: i32, func_name: &str) -> Result<(RawFd, u32), ProgramError> {
+    // retrieve program information
     let info =
         sys::bpf_obj_get_info_by_fd(prog_fd).map_err(|io_error| ProgramError::SyscallError {
             call: "bpf_obj_get_info_by_fd".to_owned(),
             io_error,
         })?;
 
+    // btf_id refers to the ID of the program btf that was loaded with bpf(BPF_BTF_LOAD)
     if info.btf_id == 0 {
         return Err(ProgramError::ExtensionError(ExtensionError::NoBTF));
     }
 
+    // the bpf fd of the BTF object
     let btf_fd =
         sys::bpf_btf_get_fd_by_id(info.btf_id).map_err(|io_error| ProgramError::SyscallError {
             call: "bpf_btf_get_fd_by_id".to_owned(),
             io_error,
         })?;
 
+    // we need to read the btf bytes into a buffer but we don't know the size ahead of time.
+    // assume 4kb. if this is too small we can resize based on the size obtained in the response.
     let mut buf = vec![0u8; 4096];
     let btf_info = match sys::btf_obj_get_info_by_fd(btf_fd, &mut buf) {
         Ok(info) => {