objtool: Validate disassembler headers in libopcodes probegrafted

commit 3f2de814c059 ("objtool: Fix libopcodes linking with static libraries")
tested for libopcodes availability by linking a test snippet with a forward
declaration of disassemble_init_for_target().

However, testing symbol linkage with an extern declaration only verifies
the presence of the library (.so/.a) and bypasses checking for development
headers (binutils-dev). On systems where libopcodes is present without
development headers installed, the probe succeeds, enabling BUILD_DISAS.
Subsequent compilation of objtool then fails:

  fatal error: 'bfd.h' file not found
    113 | #include <bfd.h>

Additionally, the probe invokes $(HOSTCC) without $(HOSTCFLAGS), ignoring
any sysroot or include flags specified for the host compiler.

Fix this by including <bfd.h> and <dis-asm.h> directly in the test snippet,
passing $(HOSTCFLAGS) so host compiler options are respected, and defining
PACKAGE="objtool" to satisfy the configuration check in <bfd.h>.

Fixes: 3f2de814c059 ("objtool: Fix libopcodes linking with static libraries")
Fixes: 436326bc525d ("objtool: fix build failure due to missing libopcodes check")
Reported-by: Alice Ryhl <aliceryhl@google.com>
Assisted-by: Antigravity:Gemini-Next
Signed-off-by: Ulises Mendez Martinez <umendez@google.com>
Link: https://patch.msgid.link/20260904150710.2997558-1-umendez@google.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

1 files changed, 77 insertions, 0 deletions

diff --git a/rust/proc-macro2/detection.rs b/rust/proc-macro2/detection.rs
new file mode 100644
index 000000000..3de448cb2
--- /dev/null
+++ b/rust/proc-macro2/detection.rs
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use core::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Once;
+
+static WORKS: AtomicUsize = AtomicUsize::new(0);
+static INIT: Once = Once::new();
+
+pub(crate) fn inside_proc_macro() -> bool {
+    match WORKS.load(Ordering::Relaxed) {
+        1 => return false,
+        2 => return true,
+        _ => {}
+    }
+
+    INIT.call_once(initialize);
+    inside_proc_macro()
+}
+
+pub(crate) fn force_fallback() {
+    WORKS.store(1, Ordering::Relaxed);
+}
+
+pub(crate) fn unforce_fallback() {
+    initialize();
+}
+
+#[cfg(not(no_is_available))]
+fn initialize() {
+    let available = proc_macro::is_available();
+    WORKS.store(available as usize + 1, Ordering::Relaxed);
+}
+
+// Swap in a null panic hook to avoid printing "thread panicked" to stderr,
+// then use catch_unwind to determine whether the compiler's proc_macro is
+// working. When proc-macro2 is used from outside of a procedural macro all
+// of the proc_macro crate's APIs currently panic.
+//
+// The Once is to prevent the possibility of this ordering:
+//
+//     thread 1 calls take_hook, gets the user's original hook
+//     thread 1 calls set_hook with the null hook
+//     thread 2 calls take_hook, thinks null hook is the original hook
+//     thread 2 calls set_hook with the null hook
+//     thread 1 calls set_hook with the actual original hook
+//     thread 2 calls set_hook with what it thinks is the original hook
+//
+// in which the user's hook has been lost.
+//
+// There is still a race condition where a panic in a different thread can
+// happen during the interval that the user's original panic hook is
+// unregistered such that their hook is incorrectly not called. This is
+// sufficiently unlikely and less bad than printing panic messages to stderr
+// on correct use of this crate. Maybe there is a libstd feature request
+// here. For now, if a user needs to guarantee that this failure mode does
+// not occur, they need to call e.g. `proc_macro2::Span::call_site()` from
+// the main thread before launching any other threads.
+#[cfg(no_is_available)]
+fn initialize() {
+    use std::panic::{self, PanicInfo};
+
+    type PanicHook = dyn Fn(&PanicInfo) + Sync + Send + 'static;
+
+    let null_hook: Box<PanicHook> = Box::new(|_panic_info| { /* ignore */ });
+    let sanity_check = &*null_hook as *const PanicHook;
+    let original_hook = panic::take_hook();
+    panic::set_hook(null_hook);
+
+    let works = panic::catch_unwind(proc_macro::Span::call_site).is_ok();
+    WORKS.store(works as usize + 1, Ordering::Relaxed);
+
+    let hopefully_null_hook = panic::take_hook();
+    panic::set_hook(original_hook);
+    if sanity_check != &*hopefully_null_hook {
+        panic!("observed race condition in proc_macro2::inside_proc_macro");
+    }
+}