Enable fatal errors by default

Ignore errors emitted on calls to builtins which are spurious. These
builtins end up being lowered to libcalls to hidden functions, which are
totally OK. See https://reviews.llvm.org/D155894 and
aya-rs/aya#698 for some additional discussion.

There doesn't seem to be a silver bullet here; BPF is fundamentally
adversarial to these memory intrinsics. We can revisit this once we have
DI and can more easily locate the source location that results in their
inclusion.

src/bin/bpf-linker.rs

@@ -141,7 +141,7 @@ struct CommandLine {
     export: Vec<String>,
 
     /// Whether to treat LLVM errors as fatal.
-    #[clap(long, action = clap::ArgAction::Set, default_value_t = false)]
+    #[clap(long, action = clap::ArgAction::Set, default_value_t = true)]
     fatal_errors: bool,
 
     // The options below are for wasm-ld compatibility

src/linker.rs

@@ -516,8 +516,24 @@ impl Linker {
 
 impl llvm::LLVMDiagnosticHandler for Linker {
     fn handle_diagnostic(&mut self, severity: llvm_sys::LLVMDiagnosticSeverity, message: &str) {
+        // TODO(https://reviews.llvm.org/D155894): Remove this when LLVM no longer emits these
+        // errors.
+        //
+        // See https://github.com/rust-lang/compiler-builtins/blob/a61823f/src/mem/mod.rs#L22-L68.
+        const MATCHERS: &[&str] = &[
+            "A call to built-in function 'memcpy' is not supported.\n",
+            "A call to built-in function 'memmove' is not supported.\n",
+            "A call to built-in function 'memset' is not supported.\n",
+            "A call to built-in function 'memcmp' is not supported.\n",
+            "A call to built-in function 'bcmp' is not supported.\n",
+            "A call to built-in function 'strlen' is not supported.\n",
+        ];
+
         match severity {
             llvm_sys::LLVMDiagnosticSeverity::LLVMDSError => {
+                if MATCHERS.iter().any(|matcher| message.ends_with(matcher)) {
+                    return;
+                }
                 self.has_errors = true;
 
                 error!("llvm: {}", message)

tests/assembly/auxiliary/dep.rs

@@ -5,4 +5,4 @@
 #[no_mangle]
 fn some_dep() -> u8 {
     42
-}
+}

tests/assembly/auxiliary/loop-panic-handler.rs

@@ -6,4 +6,3 @@
 fn panic_impl(_: &core::panic::PanicInfo) -> ! {
     loop {}
 }
-

tests/assembly/elf-sections.rs

@@ -22,4 +22,4 @@ static mut COUNTER: u32 = 0;
 
 // CHECK: .section "uprobe/connect","ax"
 // CHECK: .section "uprobe/dep","ax"
-// CHECK: .section "maps/counter","aw"
+// CHECK: .section "maps/counter","aw"

tests/assembly/re-export-symbols.rs

@@ -15,4 +15,4 @@ fn connect() {
 }
 
 // CHECK: .globl connect
-// CHECK: .globl some_dep
+// CHECK: .globl some_dep