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, 54 insertions, 0 deletions

diff --git a/lib/is_single_threaded.c b/lib/is_single_threaded.c
new file mode 100644
index 000000000..8c98b20bf
--- /dev/null
+++ b/lib/is_single_threaded.c
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Function to determine if a thread group is single threaded or not
+ *
+ * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ * - Derived from security/selinux/hooks.c
+ */
+#include <linux/sched/signal.h>
+#include <linux/sched/task.h>
+#include <linux/sched/mm.h>
+
+/*
+ * Returns true if the task does not share ->mm with another thread/process.
+ */
+bool current_is_single_threaded(void)
+{
+	struct task_struct *task = current;
+	struct mm_struct *mm = task->mm;
+	struct task_struct *p, *t;
+	bool ret;
+
+	if (atomic_read(&task->signal->live) != 1)
+		return false;
+
+	if (atomic_read(&mm->mm_users) == 1)
+		return true;
+
+	ret = false;
+	rcu_read_lock();
+	for_each_process(p) {
+		if (unlikely(p->flags & PF_KTHREAD))
+			continue;
+		if (unlikely(p == task->group_leader))
+			continue;
+
+		for_each_thread(p, t) {
+			if (unlikely(t->mm == mm))
+				goto found;
+			if (likely(t->mm))
+				break;
+			/*
+			 * t->mm == NULL. Make sure next_thread/next_task
+			 * will see other CLONE_VM tasks which might be
+			 * forked before exiting.
+			 */
+			smp_rmb();
+		}
+	}
+	ret = true;
+found:
+	rcu_read_unlock();
+
+	return ret;
+}